The Facts On File

DICTIONARYof

BOTANY

The Facts On File

DICTIONARYof

BOTANY

Edited byJill Bailey

The Facts On File Dictionary of Botany

Copyright © 2003 by Market House Books Ltd

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PREFACE

This dictionary is one of a series covering the terminology and concepts usedin important branches of science. The Facts On File Dictionary of Botany isplanned as an additional source of information for students taking Ad-vanced Placement (AP) Science courses in high schools, but will also be help-ful to older students taking introductory college courses.

This volume covers the whole area of pure and applied plant science includ-ing anatomy and plant morphology, plant physiology, biochemistry, cell bi-ology, genetics, evolution, and ecology. It also covers the taxonomy andclassification of plants, with entries for the higher-ranking taxa. The defini-tions are intended to be clear and informative and, where possible, we haveprovided helpful diagrams and examples. The book also has a selection ofshort biographical entries for people who have made important contribu-tions to the field. The appendices include lists of webpages and an informa-tive bibliography.

The book will be a helpful additional source of information for anyonestudying AP Biology, notably the sections on Organisms and Populations,Structure and Function of Plants and Animals, and Molecules and Cells.However, we have not restricted the content to this syllabus. Modern plantscience is a subject of considerable importance and we hope that this bookwill be useful to anyone interested in the subject.

ACKNOWLEDGMENTS

Consultant

Andrew Lack B.Sc., Pd.D.

Contributors

Eve Daintith B.Sc.Elizabeth Tootill B.Sc.

CONTENTS

Entries A to Z 1

Appendixes

I. SI Units 248

II. Webpages 249

Bibliography 250

ABA See abscisic acid.

abaxial In structures such as a leavesand petals, the side facing away from themain axis, i.e. the lower surface. In lateralorgans such as leaves, abaxial is synony-mous with the underside. Compare adax-ial.

abiotic environment The nonlivingfactors of the environment that influenceecological systems. Abiotic factors includeclimate, chemical pollution, geographicalfeatures, etc.

abscisic acid (ABA) A plant hormone,that functions chiefly as an inhibitor ofgrowth and cell elongation. Abscisic acidhas a variety of effects related to seed dor-mancy and stress responses: it regulatesprotein expression in seed developmentleading to dormancy and is one of the hor-mones involved in bud dormancy; it regu-lates stress responses by, for example,closing stomata in times of water shortageand increasing the ability of roots to carrywater. It can also promote root growth andinhibit shoot growth. Despite its name itdoes not directly promote abscission, butonly indirectly through increasing ethyleneproduction. Formerly it was known as ab-scisin II or dormin.

abscission The organized loss of part ofa plant, usually a leaf, fruit, or unfertilizedflower. An abscission zone occurs at thebase of the organ. Here a separation layer(abscission layer) is formed by breakdownor separation of cells and final severanceoccurs when the vascular bundles are bro-ken mechanically, e.g. by wind or rain. Theabscission layer is activated by increasinglevels of ethylene and a decreasing concen-tration of auxin

absolute humidity See humidity.

absolute pollen frequency (APF) Seepollen analysis.

absorption 1. The uptake of liquid bycells and organs. In plants, water and min-eral salts are absorbed mainly by the roothairs, just behind the root tips.2. The capture of radiant energy by plantpigments. About 80% of the visible lightfalling on a leaf is absorbed, and about10% of the infrared radiation.

absorption spectrum A plot of the ab-sorbance by a substance of radiation at dif-ferent wavelengths, usually of ultraviolet,visible, or infrared radiation. It can give in-formation about the identity or quantity ofa substance. Chlorophylls, for example,have absorption peaks in the red and blue(and therefore reflect green light). Com-pare action spectrum.

accessory cell See subsidiary cell.

accessory chromosome See B chromo-some.

accessory pigment See photosyntheticpigments.

1

A

Abscisic acid

acellular

2

acellular Denoting relatively large tis-sues or organisms that are not composed ofdiscrete cells and are, in effect, unicellular.In flowering plants, for instance, the earlystages of endosperm development are oftenacellular. Other examples include aseptatefungal hyphae and certain green algae,such as Acetabularia. The term is used inpreference to unicellular to distinguishsuch structures (which are often multinu-cleate) from conventional cells and showtheir equivalence to multicellular struc-tures. See also coenocyte.

Aceraceae A family of temperate andtropical trees and shrubs that includes themaples and sycamores.

acetaldehyde (ethanal) An aldehyde,CH3CHO, that is an intermediate in theconversion of pyruvic acid to ethanol dur-ing the final stage of glycolysis duringanaerobic respiration in plants. It is in-volved in the synthesis and breakdown ofthe amino acid threonine.

acetic acid (ethanoic acid) A carboxylicacid, CH3COOH, obtained by the oxida-tion of ethyl alcohol. Acetic acid is a com-ponent of vinegar (which is obtained bybacterial oxidation of wine waste). It isused as an alternative carbon source by cer-tain green algae. When combined withCOENZYME A to form ACETYL COA, it plays akey role in AEROBIC RESPIRATION.

acetocarmine A stain used to colorchromosomes deep reddish-black for view-ing with a light microscope. Tissues arefixed in acetic acid before applying aceto-carmine. See fixation; staining.

acetyl CoA (acetyl coenzyme A) A com-pound made up of acetyl and coenzyme Alinked by a sulfur bridge. Acetyl CoA playsa key role in metabolism, being a precursorof the KREBS CYCLE and GLYOXYLATE CYCLE,and the starting point for synthesis of fattyacids, terpenes, and some amino acids. Thesynthesis of acetyl CoA is a high-energyprocess, requiring energy from ATP, whichis converted to AMP.

achene A dry indehiscent fruit formedfrom an ovary with a single carpel contain-ing a single seed, e.g. oak (Quercus). Dif-ferent types of achenes include theCARYOPSIS, CYPSELA, NUT, and SAMARA.

acicular Needle-shaped.

acid A substance that gives rise to hy-drogen ions (or H3O+) when dissolved inwater. An acid in aqueous solution willhave a pH below 7. Lowry–Brønstedtheory defines an acid as a substance thatexhibits a tendency to release a proton, anda base as a substance that tends to accept aproton. Strong acids (e.g. HNO3) reactcompletely with water to give H3O+. Weakacids (e.g. CH3COOH) are only partly dis-sociated because H3O+ is a stronger acidthan the free acids.

acidic stain See staining.

acid rain The deposition of acids by nat-ural precipitation, mainly by rain but alsoby snow and fog. Acids are formed by re-action of gaseous waste products, particu-larly sulfur dioxide, and also nitrogenoxides, with moisture in the air to form sul-furic and nitric acids. The subsequent pre-cipitation has led to raised acidity in forestsand lakes, especially in Scandinavia andsome other parts of northern Europe, dam-aging the environment. Acid rain is oftenused more loosely for any atmospheric pol-lutant that dissolves in precipitation caus-ing environmental damage, such as carbonmonoxide or ozone, and these may interactwith the acids.

The most serious pollutant is sulfurdioxide, which comes mainly from burningcoal and is, consequently, less serious nowthan in the twentieth century. Sulfuric acidin soils may lead to the formation of am-monium sulfate, which causes the releaseof toxic aluminum and heavy metal ionsthat inhibit metabolic activity. In water-ways these can damage the gills of fish. Un-polluted rain is normally slightly acidicwith a pH of 5.0–5.6.

acid soil A soil with a pH less than 6.0.Such soils usually form in areas of heavy

rainfall, which causes leaching of limefrom the surface layers, or over acid sub-strata such as granite or sand. In the acidicconditions, decomposition of organic ma-terial in the soil is slow. Acid soils oftencontain substantial concentrations of ironand aluminum hydroxides. See brownearth; podsol.

Acrasiomycota (cellular slime molds) Aclass of protoctists in the phylum Rhi-zopoda. Formerly considered to be fungi,they are made up of independently livingamebas that feed on bacteria, which theyingest by phagocytosis. Under conditionsof food shortage or other stresses, theycome together to form a slug-like aggrega-tion of amebas called a pseudoplasmo-dium, which acts like an individualorganism.

acrocarpous Describing mosses inwhich the reproductive organs are borne atthe top of the main axis, which is usuallyerect, so that subsequent growth is sympo-dial.

acrocentric See centromere.

acropetal Describing a process (such asgrowth or development) that progressesfrom the base or point of attachment, sothat the oldest parts are at the base and theyoungest are at the tip. Compare basipetal.

actin A globular contractile protein (G-actin) that makes up 10–15% of the totalcell protein in eukaryotic cells. It is a majorcomponent of the CYTOSKELETON. It alsocontrols motility, cellular movements, and cyclosis. G-actin can polymerize intohelical strands that coil together to formmicrofilaments called fibrous actin (F-actin).

Actinobacteria (ray fungi; actinomycetes)A phylum of Eubacteria that contains thetrue actinobacteria and the coryneformbacteria. The true actinobacteria formbranching filaments that resemble smallfungal mycelia, with hyphae usually lessthan 1.5 m m in diameter; the coryne-

form bacteria are rodlike structures. TheActinobacteria are distinguished by pro-ducing actinospores: entire cells encystedin thick walls to form resistant spores.

actinodromous (palmate; digitate) De-scribing a form of leaf VENATION in whichthree or more primary veins radiate fromthe base of the lamina toward the margin,as in sycamore (Acer pseudoplatanus), re-sulting in a leaf that has several large lobesor several leaflets all originating at thesame point.

actinomorphy See radial symmetry.

actinomycetes See Actinobacteria.

actinostele A type of protostele inwhich the xylem is star shaped and thephloem lies between the points of the star. Protosteles are found in the primaryroots of higher plants, and in some spe-cies of Lycopodium and Psilotum. Seestele.

action spectrum A graph showing theeffect of different wavelengths of radiation,usually light, on a given process. It is oftensimilar to the absorption spectrum of thesubstance that absorbs the radiation andcan therefore be helpful in identifying thatsubstance. For example, the action spec-trum of photosynthesis is similar to the ab-sorption spectrum of chlorophyll. It showsthe net assimilation plotted against wave-length. See absorption spectrum. See illus-tration overleaf.

activation energy The minimum extraenergy that must be put into a system to en-able a reaction to occur. It increases the en-ergy levels of participating molecules, andhence raises their reactivity, often by rais-ing electrons to an excited state. ENZYMES

enhance reaction rates by lowering the ac-tivation energy. This is often achieved bythe attraction of the reactants to the en-zyme’s active site, thus coming into closerproximity, or by the enzyme causing con-formational or electrostatic changes in oneor more of the reactants.

3

activation energy

activator 1. A metal ion that acts to-gether with an enzyme or its substrate tobring about a reaction.2. A protein that positively regulates thetranscription of a gene.

active site The particular part of an en-zyme molecule that combines with and actson the substrate. The active site consists ofamino acids arranged in a configurationspecific to a particular substrate or type ofsubstrate. Binding of an inhibiting com-pound elsewhere on the enzyme moleculemay change this configuration and hence

the efficiency of the enzyme activity. See al-losteric site.

active transport The transport of mol-ecules or ions across a cell membraneagainst a concentration gradient, with theexpenditure of energy, usually in the formof ATP. It is probably an attribute of allcells. Anything that interferes with the pro-vision of energy will interfere with activetransport. The mechanism typically in-volves a carrier protein that spans the cellmembrane and transfers substances in orout of the cell by changing shape.

activator

4

Action spectrum of photosynthesis

Absorption spectrum of chlorophylls

acuminate Describing a structure thatgradually narrows to a point, such as cer-tain leaves.

acylglycerol (glyceride) An ester ofglycerol and one or more fatty acids. Theymay be mono-, di-, or triacylglycerolsaccording to the number of –OH groupsesterified. The fat stores of the body consistmainly of triacylglycerols (triglycerides).These can form a source of energy whencarbohydrate levels are low, being brokendown by lipases into fatty acids, which can enter metabolic pathways. Glyco-lipids (glycosyldiacylglycerols) are diacyl-glycerols with a sugar attached to the unes-terified hydroxyl group. See also lipid.

adaptation The extent to which an or-ganism, or a physiological or structuralcharacteristic of an organism, is suited to aparticular environment. Specialist organ-isms that have become highly adapted toone environment are then often not soadaptable as less specialized organisms and are at a disadvantage in a changing en-vironment (adaptation versus adaptabil-ity).

adaptive enzyme See inducible enzyme.

adaptive radiation The formationthrough evolution of a number of differentvarieties or species from a common ances-tor. It is often seen most clearly on isolatedoceanic islands, e.g. the many forms oftrees, shrubs, vines, and herbs of the genus

Hedyotis on the Hawaiian Islands, but theterm can apply to the range of species inany genus or family.

adaxial In structures such as a leavesand petals, the side facing toward the mainaxis, i.e. the upper surface. Compare abax-ial.

adder’s tongue ferns See Ophioglos-sales.

adenine A nitrogenous base (6-amino-purine) found in DNA and RNA. It is alsoa constituent of certain coenzymes, e.g.NAD and FAD, and when combined withthe sugar ribose it forms the nucleosideADENOSINE found in AMP, ADP, and ATP.Adenine has a purine ring structure.

adenosine (adenine nucleoside) A nu-cleoside formed from adenine linked to D-ribose with a b-glycosidic bond. Adenosinetriphosphate (ATP) is a nucleotide derivedfrom adenosine.

adenosine diphosphate See ADP.

adenosine monophosphate See AMP.

adenosine triphosphate See ATP.

Adiantaceae (maidenhair ferns) A fam-ily of ferns (Filicinophyta) found through-out the world, especially in the moistAmerican tropics.

5

ADP

Active site

ADP (adenosine diphosphate) A nu-cleotide consisting of adenine and ribosewith two phosphate groups attached. Thesecond phosphate is attached to the first bya high-energy bond.

adventitious Describing plant organsthat arise in unexpected places, for exam-ple the development of adventitious rootsfrom stems, and adventitious buds fromleaves.

aerenchyma A plant tissue containinglarge intercellular air spaces, usuallyformed as a consequence of the death ofparenchyma cells. Aerenchyma is typical ofthe stems of many aquatic plants whoseleaves float at the surface of ponds andlakes, giving added buoyancy to the tis-sues.

aerial root A root that arises above soillevel. Examples include the roots of epi-phytes and climbers, which hang down inthe air or stick to a trunk or branch. Theroots of many orchids and other epiphytesdeveloped a sheath of dead cells, the vela-men, which helps to absorb water from theatmosphere.

aerobe An organism that can live andgrow only in the presence of free oxygen,i.e. it respires aerobically (see aerobic res-piration). All plants and most bacteria andfungi are aerobes. Compare anaerobe.

aerobic respiration Respiration inwhich free oxygen is used to oxidize or-ganic substrates to carbon dioxide andwater, with a high yield of energy. The re-action overall is:

C6H12O6 + 6O2 = 6CO2 + 6H2O +energy

It occurs in a number of stages, the first ofwhich (GLYCOLYSIS) also occurs in anaero-bic respiration in the cell cytoplasm. Withglucose as the substrate, a sequence of re-actions results in the formation of pyru-vate. The remaining stages, which do notoccur in anaerobic respiration, take placein the mitochondria. Pyruvate is convertedto ACETYL COA, which enters a cyclic seriesof reactions, the KREBS CYCLE, with the pro-duction of carbon dioxide and hydrogenatoms. These and other hydrogen atomsproduced at earlier stages are now passedto the ELECTRON-TRANSPORT CHAIN (involv-ing CYTOCHROMES and FLAVOPROTEINS).Here they combine with atoms of free oxy-gen to form water. Energy released at each

adventitious

6

Aerobic respiration

stage of the chain is used to form ATP dur-ing a coupling process (see oxidative phos-phorylation). There is a net production of38 ATPs (2930 kJ) per molecule of glucoseduring aerobic respiration, a yield of about19 times that of anaerobic respiration (300kJ), and the mechanism of the majority oforganisms. Compare anaerobic respira-tion.

aerotaxis (aerotactic movement) ATAXIS in response to an oxygen concentra-tion gradient. For instance, motile aerobicbacteria are positively aerotactic, whereasmotile obligate anaerobic bacteria are neg-atively aerotactic. Some photosynthetic eu-karyotic unicells are also aerotactic.

aerotropism A TROPISM in which theorientating stimulus is oxygen.

afforestation The establishment of for-est on land not previously forested, eitherby natural succession or by planting.

aflatoxin One of a group of carcino-genic toxins produced by fungi of the genusAspergillus, especially A. flavus. Thisspecies is a common contaminant of crops,such as peanuts. Contaminated feed causesserious outbreaks of disease among live-stock.

after-ripening The collective name forprocesses that are necessary before germi-nation can take place in certain seeds, eventhough external conditions may be suit-able. For example, a period of dormancymay be imposed on the seed, preventingpremature germination before an unfavor-able season such as winter. After-ripeningis common in areas with marked seasonal-ity.

agamospermy See apomixis.

agar A gelling agent prepared from sea-weed (normally a red alga, Gelidium), usedto set liquid nutrients. It can withstandsterilization at high temperatures and is re-sistant to attack by most bacteria. Agar

gels are extensively used for growingmicroorganisms and tissue cultures.

Agaricales (agarics) An order of basid-iomycetes that contains the mushroomsand toadstools. The cap of the fleshy fruit-ing body – the mushroom or toadstool –bears a series of parallel gills on its lowersurface that greatly increase its surfacearea. These are covered in the spore-bearing layer (hymenium). Agarics arecommon in leaf litter and rotting wood,where they live as saprobes.

aggregate fruit (compound fruit) A fruit-like structure that is made up of several in-dividual fruits derived from the carpels of asingle flower, e.g. strawberry (Fragaria),blackberry (Rubus fruticosus). These maybe achenes, follicles, berries, or drupelets.An aggregate fruit is sometimes called anetaerio, but this term may also be restrictedto an aggregate of drupelets. A fruit thatdevelops from a group of flowers is termeda multiple fruit, e.g. pineapple (Ananas co-mosus) and hop (Humulus lupulus).

Agrobacterium A genus of soil bacte-ria, the species A. tumefaciens being thecausative agent of CROWN GALL, a type oftumor in plants. A segment of DNA (trans-ferred DNA, T-DNA) from a PLASMID inthe bacterium is transferred into the hostDNA and induces tumor formation. Sincethe plasmid is capable of independentreplication in host cells of many dicotyle-donous plants, it has been used as a cloningvector in GENETIC ENGINEERING.

agroforestry A system of cultivationcommon in many parts of the tropics, es-pecially in rainforest regions, in whichforestry and arable farming are mixed.This reduces soil degradation, as freshlytilled soils are shaded and protected fromheavy rains and run-off by the trees. Thetrees help to draw nutrients up from thedeeper soil layers into the zone occupied bythe roots of the crop plants.

agronomy A branch of agriculture deal-

7

air bladder

ing with soil management and crop pro-duction.

air bladder 1. A structure on the pollengrains of some conifers, such as Pinus, thataids the wind dispersal of the pollen. Itconsists of an air-filled protuberance oneach side of the pollen grain developed bythe separation of the exine layers, givingthe pollen a characteristic winged appear-ance.2. An air-filled chamber in the thallus ofmany Phaeophyta (brown algae) that in-creases the buoyancy of the thallus, en-abling it to float toward the surface whenunderwater to increase light absorptionand gaseous exchange for photosynthesis.Air bladders are highly developed in cer-tain wracks, such as the bladder wrack(Fucus vesiculosus).

air plant See epiphyte.

alanine A simple AMINO ACID, one of theearly products of photosynthesis. Alanineis formed by transamination when anamino group is donated by gutamine topyruvic acid. It may be deaminated back topyruvate for use in the KREBS CYCLE.

albumin One of a group of simple, low-molecular-weight proteins found in plants,for example in the endosperm of barleyand wheat seeds. Albumins are water-soluble and coagulate when heated.

albuminous cell 1. A vertically elon-gated parenchyma cell, found in groups inthe rays of the secondary phloem in gym-nosperms, where they are associated withsieve cells. Unlike the companion cells ofangiosperms, they are not derived from thesame mother cell as the sieve cell.2. An albumin-containing cell found in cer-tain seeds.

alcohol A type of organic compound ofthe general formula ROH, where R is a hy-drocarbon group. Examples of simple alco-hols are methanol (CH3OH) and ethanol(C2H5OH). Ethanol is a product of anaer-obic metabolism.

alcohol dehydrogenase An enzymethat converts ethanol into acetaldehyde(ethanal). It is important in a plant’s abilityto withstand waterlogging. Ethanol is aproduct of anaerobic respiration in plants,and waterlogged roots suffer from a short-age of oxygen. Accumulation of ethanol isharmful to a plant.

alcoholic fermentation A form ofANAEROBIC RESPIRATION in which glucose isbroken down to form ethanol and carbondioxide. It is carried out by yeasts andsome other fungi and certain bacteria.Alcoholic fermentation is catalyzed byenzymes of the zymase complex, which aresecreted by the cells or released after thecells die. The process is self-limiting, sincethe organisms usually die once the alcoholconcentration in the medium exceeds 15%.Alcoholic fermentation is the basis of thepreparation of alcoholic beverages, and ofbread making (in which the carbon dioxide

air plant

8

Alcoholic fermentation

released causes the dough to rise). Duringalcoholic fermentation acetaldehyde actsas a hydrogen acceptor instead of oxygen.The pyruvic acid formed by GLYCOLYSIS isbroken down into acetaldehyde and car-bon dioxide. The acetaldehyde is furtherreduced by NADH to form ethanal, releas-ing only about a tenth of the energy thatwould be released by aerobic respiration.

aldehyde A type of organic compoundwith the general formula RCHO, wherethe –CHO group (the aldehyde group) con-sists of a carbonyl group attached to a hy-drogen atom. Simple examples of

aldehydes are methanal (formaldehyde,HCHO) and ethanal (acetaldehyde,CH3CHO).

aldose A SUGAR containing an aldehyde(CHO) or potential aldehyde group. Ex-amples include the sugars ribose (with five

carbon atoms) and glucose (with six car-bon atoms).

aleurone grain (aleurone body) A mod-ified vacuole found in the embryo and en-dosperm of seeds and containing mostlyreserve proteins, but also phytic acid andvarious enzymes associated with mobiliza-tion (digestion) of these reserves. The pro-teins and phytic acid are present incrystalline form in the dormant seed.

aleurone layer The outermost protein-rich layer of the endosperm of grass fruits(e.g. cereal grains). At germination, the em-bryo produces gibberellin, which stimu-lates the aleurone layer to synthesizeenzymes, especially amylase. The lattercauses hydrolysis of the starch in the en-dosperm. The enzymes are synthesizedfrom the amino acids supplied by break-down of ALEURONE GRAINs.

algae (sing. alga) A large mixed group ofphotosynthesizing eukaryotic organisms,now usually placed in the kingdom Protoc-tista. They often resemble plants and arefound mainly in marine or fresh-waterhabitats, although some algae are terres-trial. Algae differ from plants in lackingany real differentiation of leaves, stems,and roots, in having no layer of sterile cellsaround the reproductive organs,and in nothaving an embryo stage in their life cycle.Algae can be unicellular (e.g. Chlamy-domonas), colonial (e.g. Volvox), filamen-tous (e.g. Spirogyra), or thalloid (e.g.Fucus). All algae contain chlorophyll butthis may be masked by various accessorypigments, these being one of the majorcharacteristics used to divide the algae into their various phyla. Other charactersused to classify the algae are the nature of storage products, the type of cell wall,the form and number of undulipodia (flagella), ultrastructural cell details, typeof food reserves, and reproductive pro-cesses.

alkaline soil See calcareous soil.

alkaloid One of a group of organic com-

9

alkaloid

Aldose

pounds found in plants, which are poiso-nous insoluble crystalline compounds.They contain nitrogen and usually occur assalts of acids such as citric, malic, and suc-cinic acids. Their function in plants re-mains obscure, but it is suggested that theymay be nitrogenous end-products of me-tabolism, or they may have a protectivefunction against herbivores, since theytaste bitter. Important examples in humanuse are quinine, nicotine, atropine, opium,morphine, codeine, and strychnine. Theyoccur mainly in the poppy family, the but-tercup family, and the nightshade family ofplants.

allele (allelomorph) One of the possibleforms of a given gene. The alleles of a par-ticular gene occupy the same positions(loci) on homologous chromosomes. Agene is said to be homozygous if the twoloci have identical alleles and heterozygouswhen the alleles are different. When twodifferent alleles are present, one (the domi-nant allele) usually masks the effect of theother (the recessive allele). The allele deter-mining the normal form of the gene is usu-ally dominant while mutant alleles areusually recessive. Thus most mutationsshow in the phenotype only when they arehomozygous. In some cases one allele is notcompletely dominant or recessive to an-other allele (INCOMPLETE DOMINANCE).Thus an intermediate phenotype will beproduced in the heterozygote. See also in-complete dominance; multiple allelism.

allelomorph See allele.

allelopathy Inhibition of the germina-tion, growth, or reproduction of an organ-ism effected by a chemical substancereleased from another organism. It is prob-ably an anticompetition mechanism inplants; for example, barley secretes an al-kaloid substance from its roots to inhibitcompeting weeds, but it is not known howimportant it is generally.

allogamy Cross-fertilization in plants.This promotes genetic variation in thepopulation especially in plants that are

DIOECIOUS and MONOECIOUS. Most plantsare hermaphrodite, but have mechan-isms such as self-INCOMPATIBILITY thatpromote allogamy. Compare autogamy.See also heterostyly; chasmogamy; di-chogamy.

allopatric species See species.

allopolyploidy A type of polyploidy in-volving the combination of chromosomesfrom two or more different species. Al-lopolyploids usually arise from the dou-bling of chromosomes of a hybrid betweenspecies, the doubling often making the hy-brid fertile. The properties of the hybrid,such as greater vigor and adaptability, areretained in the allopolyploid in subsequentgenerations and such organisms are oftenhighly successful. Many plant species havebeen derived originally from allopoly-ploidy, e.g. cultivated wheat. Compareautopolyploidy.

allosteric enzyme An enzyme whosecatalytic activity can be modified by thenoncovalent binding of a particularmetabolite (a modulator) at a site (the AL-LOSTERIC SITE) other than the active site. Itmay either inhibit or enhance the enzymeactivity. Allosteric enzymes may have morethan one modulator.

allosteric site A part of an enzyme towhich a specific effector or modulator canbe attached. This attachment is reversible.Allosteric enzymes possess an allosteric sitein addition to their ACTIVE SITE.

allotetraploid (amphidiploid) An allo-polyploid whose chromosomes are derivedfrom two different species and whichtherefore has four times the haploid num-ber of chromosomes, e.g. Spartina anglica,derived from the diploids S. alternifloraand S. maritima. See allopolyploidy.

alluvial soil A type of soil formed onriver floodplains and deltas, where newsediment is deposited on the land duringfloods.

allele

10

alpha helix A highly stable structure inwhich peptide chains are coiled to form aspiral. Each turn of the spiral contains ap-proximately 3.6 amino acid residues. TheR group of these amino acids extends out-ward from the helix. Hydrogen bondingbetween successive coils holds the helix to-gether. If the alpha helix is stretched thehydrogen bonds are broken but reform onrelaxation. The alpha helix is found inmuscle protein and keratin.

alpha-naphthol test (Molisch’s test) Atest for detecting the presence of carbohy-drates in solution. The test solution isplaced in a test tube, and a small amount ofalpha-naphthol added; concentrated sulfu-ric acid is then trickled slowly down theside of the tube. If carbohydrate is present,a violet ring will form at the junction of theliquids.

alpine Describing a BIOME (regionalcommunity) of plants above the treelineand below the snowline on high moun-tains. The lower limit of the alpine zonevaries in different mountain regions, ac-cording to the rainfall and other climaticand topographic factors, from 100 metersabove sea level in parts of Scotland to 3700meters in the western Himalayas. There areoften considerable differences between thenature and distribution of vegetation onnorth- and south-facing slopes and be-tween windward and leeward slopes.Alpine vegetation is often similar to TUN-DRA vegetation, being adapted to harsh cli-mate conditions, including high windspeeds.

alternate Describing a leaf arrangementin which there is only one leaf at each node,as in hazel. This is the commonest form ofleaf arrangement. See phyllotaxis.

alternate host Any host other than the main (most common) one. For exam-ple, many rusts overwinter on alternatehosts. Some parasites and pests are able tolive on more than one host, and some need a second host to complete their lifecycle.

alternation of generations The occur-rence of two, or occasionally more, gener-ations during the life cycle of an organism.In all plants and some algae there is an al-ternation between sexual haploid andasexual diploid stages. They usually differmarkedly in morphology. The haploidplant produces gametes mitotically and isthus termed the gametophyte while thediploid plant produces spores meioticallyand is called the sporophyte. The gametesfuse to form a zygote, which develops intothe sporophyte, and the spores germinateand produce the gametophyte, so forminga cycle. In bryophytes (mosses, liverwortsand hornworts) the haploid gametophyte isthe dominant phase of the life cycle and thesporophyte is represented only by the cap-sule, seta, and foot. In vascular plants thediploid sporophyte is the dominant phaseand in the ferns, for example, the gameto-phyte is a small prothallus. The concept ofan alternation of generations can be ex-tended to the flowering plants, in which theembryo sac and pollen represent the muchreduced female and male gametophyte gen-erations respectively.

ameboid Describing an organism thatresembles an ameba in shape and move-ment.

amensalism An association betweentwo different species, at either the level ofthe individual organism or the populationlevel, in which one is harmed and the otheris unaffected. Compare commensalism;mutualism.

amino acids Compounds containingboth carboxylic acid and amino groups intheir molecules. The amino acids have thegeneral formula RCHNH2–COOH. Here,the group R ranges from a simple hydrogenatom to complex ring structures. All aminoacids are white, crystalline, soluble inwater, and with the sole exception of thesimplest member, glycine, all are opticallyactive.

Amino acids are the basic componentsof proteins, which consist of chains ofamino acids. Plants also contain some 200

11

amino acids

amino acids that do not occur in proteins,some of which have protective or storageroles, while others are intermediates in thesynthesis of commoner amino acids.

Amino acids contain both acids (car-boxylic acids) and bases (amino group), sothey react with both acids and bases. Thusthe charge on the amino acid depends on

amino acid sequencing

12

Amino acids

the pH. This feature is used to separate

amino acids for analysis by chromatogra-

phy and other methods. Each amino acid

has its own characteristic isoelectric point,

where the net charge on the molecule is

zero. See illustration overleaf.

13

amitosis

Amino acids

amino acid sequencing The determina-tion of the amino acid sequence of a pro-tein or peptide. This sequence is the primarystructure of a protein; it influences sec-ondary, tertiary, and quaternary structure.

amino sugar A monosaccharaide sugarin which a hydroxyl group (OH) has beenreplaced by an amino group (NH2). Glu-cosamine (from glucose) is one of thecommonest amino sugars. Galactosamine(from galactose) is a major component ofglycolipids. Amino sugars are importantcomponents of bacterial cell walls.

amitosis Nuclear division characterizedby the absence of a nuclear spindle andleading to the production of daughter nu-clei with unequal sets of chromosomes.The ordered process of division, duplica-tion of chromosomes, dissolution of nu-clear membrane, and production of aspindle as in mitosis is apparently absent.Cells produced amitotically inherit vari-able numbers of chromosomes. Thechances of a daughter cell lacking essentialgenes are less than may be expected sincemany cells that characteristically divideamitotically are polyploid, e.g. the en-dosperm nucleus in angiosperms. Compareendomitosis; mitosis.

AMP (adenosine monophosphate) Anucleotide consisting of adenine, ribose,and phosphate. AMP has an importantrole in the regulation of glycolysis, pro-moting the production of fructose bisphos-phate from fructose 6-phosphate. See ATP;cyclic AMP.

amphidiploid See allotetraploid.

amphimixis True sexual reproductionby fusion of gametes. Compare apomixis.

amylase An enzyme, found widely inplants, animals, and microorganisms, thathydrolyzes starch or glycogen to the sugarsmaltose, glucose, or dextrin. Amylase hy-drolyzes the a(1-4)glycosidic bonds instarch. It occurs in two forms, a and b. b-amylase attacks the nonreducing ends of

the starch molecule, while a-amylase canattack bonds within the starch molecule.Both a- and b-amylases occur in plants, thelatter particularly in malt (being used in thebrewing industry), but only b-amylase isfound in animals, having an important rolein digestion. During seed germination a-amylase breaks down the starch of theendosperm, the transcription of the a-amylase gene being greatly enhanced bygibberellic acid produced by the embryo.a-amylase is also involved in the release ofstored starch from plastids.

amylopectin The water-insoluble frac-tion of STARCH. It is a branching polymer ofglucose units.

amyloplast A plastid that synthesizesand stores starch grains. Amyloplasts arecommon in storage organs, e.g. the potatotuber. They have a physiological role in theroot cap and elsewhere, where the starchgrains act as statoliths.

amylose The water-soluble fraction ofSTARCH. It is an unbranched polymer ofglucose units.

anabolism Metabolic reactions inwhich molecules are linked together toform more complex compounds. Thus, an-abolic reactions are concerned with build-ing up structures, storage compounds, andcomplex metabolites in the cell. Starch,fats, and proteins are all products of ana-bolic pathways. Anabolic reactions gener-ally require an input of energy, usuallyprovided by ATP produced by catabolism.An example is the Calvin cycle. See alsometabolism. Compare catabolism.

anaerobe An organism that can live andgrow in the absence of free oxygen, i.e. itrespires anaerobically (see anaerobic respi-ration). Anaerobes can be facultative, inthat they usually respire aerobically butcan switch to anaerobic respiration whenfree oxygen is in short supply, as are mostyeasts; or obligate, in that they neverrespire aerobically and may even be poi-soned by free oxygen, as are denitrifying

AMP

14

bacteria and lactic acid bacteria. Compareaerobe.

anaerobic respiration Respiration inwhich oxygen is not involved, found in al-most all organisms. The organic substrateis not completely oxidized and the energyyield is low. The glycolytic pathway,whereby glucose is degraded to pyruvate,with the production of a small amount ofenergy, is the same as in aerobic respira-tion, but after that, in plants and fungi,ethanol and carbon dioxide are producedas the end products of the process, nor-mally known as FERMENTATION. In animalslactic acid is produced and normally reme-tabolized with oxygen at a later stage. Onlytwo molecules of ATP are produced by thisprocess. See anaerobe; glycolysis. Compareaerobic respiration.

analogous Describing structures thatare apparently similar (structurally orfunctionally) but have a different evolu-tionary origin, and thus a different embry-ological origin and structure. Phyllodes areanalogous to leaf blades, but are derivedfrom petioles (leaf stalks). See also ho-mologous.

anaphase The stage in mitosis or meio-sis when chromatids are pulled toward op-posite poles of the nuclear spindle. Inmitosis each of the chromatids moving to-ward the poles represents a single completechromosome. During anaphase I of meiosispairs of chromatids still connected at theircentromere move to the spindle poles. Dur-ing anaphase II the centromeres divide andsingle chromatids are drawn toward thepoles, thus sister chromatids are separated.

anatomy The organization of the partsof the body and the structural relationshipsbetween them, including the arrangementof tissues and their component cells.

anatropous See ovule.

androdioecious Describing species inwhich there are two forms, one bearingonly male flowers, the other bearing only

hermaphrodite flowers (or occasionallybearing male and female flowers, i.e. MO-NOECIOUS). Compare gynodioecious.

androecium The collective name inhigher plants for the male parts of a plant,i.e. the STAMENS. It is denoted in the floralformula by a letter A.

andromonoecious Describing speciesin which both male and hermaphroditeflowers are borne on the same plant. Com-pare gynomonoecious.

anemophily Pollination by wind. Plantspollinated in this manner (e.g. grasses) usu-ally have insignificant unscented flowerswith large, often feathery stigmas.

aneuploidy The condition, resultingfrom nondisjunction of homologous chro-mosomes at meiosis, in which one or morechromosomes are missing from or added tothe normal somatic chromosome number.If both of a pair of homologous chromo-somes are missing, nullisomy results. Mono-somy and trisomy are the conditions inwhich one or three homologs occur respec-tively, instead of the normal two. Poly-somy, which includes trisomy, is thecondition in which one or more chromo-somes are represented more than twice inthe cell. See nondisjunction.

aneuspory The production of an un-usual number of spores during meiosis of aspore mother cell. In dandelion (Tarax-acum), for example, the chromosomes ofthe megaspore mother cell remain in onecell after the first meiotic division, forminga restitution nucleus. The second meioticdivision thus gives rise to two cells with thediploid number of chromosomes, one ofwhich divides parthogenetically to form anembryo. This is a form of APOMIXIS. Sincecrossing over can occur during the firstmeiotic division, it does allow for some ge-netic variation in the offspring. See par-thenogenesis.

angiosperms The flowering plants con-stituting by far the largest phylum (Antho-

15

aniline stains

phyta or Angiospermophyta) of vascularseed plants. They differ from conifers andother gymnosperms by having the ovuleenclosed within an ovary, which after fer-tilization develops into a fruit. The femalegametophyte is represented by the embryosac, the archegonia being absent. The con-tents of the pollen grain represent the malegametophyte. The pollen grain germinateson a special extension of the carpel, thestigma, and double fertilization results in adiploid zygote and triploid (usually) en-dosperm nucleus. Angiosperms are dividedinto two major classes, the monocoty-ledons (MONOCOTYLEDONAE) and dicoty-ledons (DICOTYLEDONAE), with a smallbasal group known as primitive dicotyle-dons.

angstrom Symbol: Å A former unit oflength equal to 10–10 meter (one thou-sandth of a micrometer, one tenth of ananometer). It is still used occasionally formeasurements of wavelength or inter-atomic distance.

aniline stains A group of stains withvarious properties that are derived fromaniline. For example, aniline hydro-choloride and aniline sulfate will stainlignin yellow. See staining.

anion A negatively charged ion, formedby addition of electrons to atoms or mol-ecules. In electrolysis anions are attractedto the positive electrode (the anode). Com-pare cation.

anisogamy (heterogamy) The sexual fu-sion of nonidentical gametes. Anisogamygrades from situations in which the ga-metes differ only in size to the extreme ofoogamy, in which one gamete is a largenonmotile ovum and the other a smallmotile sperm. Compare isogamy. Seeoogamy.

annual A plant that completes its lifecycle within a year. Examples are the com-mon field poppy (Papaver rhoeas) and thesunflower (Helianthus). Compare biennial;ephemeral; perennial.

annual ring (growth ring) The annualincrease in girth of the stems or roots ofwoody plants, as a result of cambial activ-ity. The annual rings of plants growing intemperate climates can be seen in cross-section as two consecutive rings of light-and dark-colored xylem tissue. These areformed from a zone containing larger ves-sel elements produced by the cambium inthe spring (lighter layer), followed by azone containing smaller vessel elements(darker layer) produced during the latesummer. This process is repeated annuallyso that the number of light or dark rings in-dicate the age of that part of the plant.

annular thickening Rings of thickeninglaid down on the inner wall of protoxylemvessels and tracheids. Such thickening al-lows extension of the XYLEM between therings so that it is not ruptured as the sur-rounding tissues grow.

annulus 1. The ring of tissue surround-ing the stalk of the mature fruiting body(mushroom or toadstool) of the basid-iomycete fungi. The annulus is all that re-mains of the veil, which joined the rim ofthe pileus (cap) to the stalk in the immaturemushroom or toadstool. It is sometimestermed the velum.2. A special arc or ring of cells in the spo-rangia of ferns that constitutes the mecha-nism for spore dispersal. The cells of theannulus are thickened except on the outerwall, so that they contract on drying out.This causes the stomium to rupture, expos-ing the spores within the capsule. The cap-sule wall gradually bends back as the waterin the annulus cells continues to evaporateuntil a point when the remaining water inthe cells suddenly turns to vapor. Thisresults in the wall springing back to itsoriginal position, the sudden movementdispersing any remaining spores.3. A ring of large cells separating the epi-dermis from the operculum in certain bryo-phytes (e.g. Funaria).

anterior Designating the part of aflower or axillary bud facing away fromthe inflorescence axis or stem, respectively.

anion

16

anther The part of the stamen that pro-duces the pollen. The anther is usuallyjoined to the tip of the filament (stalk) andis made up of two lobes. Each lobe con-tains two pollen sacs that produce verylarge quantities of small pollen grains. Thepollen is released when the lobes split openlongitudinally.

The anther is made up of an outer epi-dermis, a middle fibrous layer, and aninner nutritive layer, the tapetum. The hap-loid pollen cells develop in the tapetal zonefrom spore mother cells.

anther culture (pollen culture) The gen-eration of haploid plants from immaturepollen grains or intact excised anthers. Theresultant plants are generally smaller thantheir diploid counterparts.

antheridial cell The cell from which theantheridium develops. In seed plants it isthe generative cell in the pollen grain,which divides in the pollen tube to producetwo sperm cells.

antheridium The male sex organ of thealgae, mosses, liverworts, hornworts,ferns, horsetails, clubmosses, and fungi. Itmay be made up of one cell, or one ormany layers of cells. It produces gametesthat are usually motile. Compare archego-nium.

antherozoid (spermatozoid) The malegamete of algae, some gymnosperms, andthe non-seed-bearing plants. It is mo-tile and is produced in an antheridium,except in certain gymnosperms (e.g. Cycas,Ginkgo) in which antherozoids developfrom the generative cells of the pollen tube.

Anthocerophyta (hornworts) The smallphylum of nonvascular plants. Like thal-lose liverworts, the hornworts have analternation of generations in which thegametophyte is a dorsiventral flattenedgreen thallus and the sporophyte is an up-right structure that has a foot embedded inthe gametophyte, from which it draws nu-trition.

anthocyanin One of a group of water-soluble pigments found dissolved in the sapof higher plant cell vacuoles. Anthocyaninsare red, purple, and blue and are widelydistributed, particularly in flowers andfruits where they are important in attract-ing insects, birds, etc. They also occur inbuds and sometimes contribute to theautumn colors of leaves. They are naturalpH indicators, often changing from red toblue as pH increases, i.e. acidity decreases.Color may also be modified by traces ofiron and other metal salts and organic sub-stances, for example cyanin is red in rosesbut blue in the cornflower. See flavonoid.

Anthophyta See angiosperms.

antibiotic One of a group of organiccompounds, varying in structure, that areproduced by microorganisms and can killor inhibit the activities of other micro-organisms. One of the best-known exam-ples is penicillin, which is produced by themold Penicillium notatum. Another exam-ple is streptomycin, from the actinobac-terium Streptomyces griseus. Antibioticsare also commonly used in medicine tocombat bacterial infections. Antibiotics arewidely used in research to inhibit proteinsynthesis. Some antibiotics selectively in-hibit protein synthesis by the ribosomes ofprokaryotes, but not by those of eukary-otes (excepting the ribosomes of chloro-plasts and mitochondria), lending supportto the theory that these organelles evolvedfrom symbiotic microorganisms. Antibi-otic-resistant genes are used as markers ingenetic-engineering experiments.

anticlinal Describing something that isat right angles to a surface. An anticlinaldivision results in the formation of anticli-nal walls between daughter cells, allowinga tissue to increase its circumference. Theanticlinal wall of a cell is perpendicular tothe surface of the plant body. In cylindricalorgans, such as stems and roots, the termradial is often used instead of anticlinal.Compare periclinal.

anticodon A nucleotide triplet on trans-

17

antiport

fer RNA that is complementary to andbonds with the corresponding codon(triplet) of messenger RNA in the ribo-somes during protein synthesis, thus ensur-ing the correct sequence of amino acids isbonded together in the final peptide. Seetransfer RNA.

antioxidant A substance that slows orinhibits oxidation reactions, especially inbiological materials or within cells, therebyreducing spoilage or preventing damage.Natural antioxidants include vitamin Eand b-carotene.

antipodal cells The three haploid cellsfound in the embryo sac of seed-bearingplants that migrate to the chalazal end ofthe sac farthest from the micropyle. Thesenuclei arise as a result of the three meioticdivisions that produce the egg cell, syn-ergid cells, and polar nuclei, but they donot themselves take part in the fertilizationprocess. They are eventually absorbed bythe developing embryo, and their functionis uncertain. They do not occur in all flow-ering plants.

antiport A membrane transport proteinin which the energy liberated by the passivemovement of H+ ions across the membraneis coupled to the active transport of an-other solute, e.g. Na+, in the opposite di-rection but against its own electrochemicalpotential gradient.

antisense DNA The DNA strand that isnot transcribed. In transcription the DNAdouble helix unwinds and only one of thestrands acts as the template for messengerRNA synthesis.

antitranspirant A chemical that re-duces transpiration. Antitranspirants maywork by depositing a waterproof film overthe stomata or by inducing stomatal clo-sure.

ant plant A plant that is adapted to pro-vide a home for ants. Such plants usuallyhave one or more domatia, cavities instems, leaves or roots that house ants.These may be elaborate systems of cavities

with many chambers, e.g. Myrmecodia, orsimple hollows in internodes, leaf veins(Coffea arabica), petioles (Tococa guya-nensis) or stipular spines, e.g. whistlingthorns (Acacia sphaerocephala).

APF See pollen analysis.

aphid (greenfly; blackfly; plant louse) Abug of the family Aphididae of the insectorder Hemiptera. Aphids feed by suckingplant juices. Many occur in such numbersthat they are serious pests of crops andgreenhouse plants. Others carry plantviruses.

aphotic zone The lower part of a bodyof water where light intensity is insufficientto support photosynthesis. Comparephotic zone.

Aphyllophorales See bracket fungi.

aphyllous Without leaves.

Apiaceae (umbellifers; carrot family) Afamily of dicotyledons distinguished by itsINFLORESCENCES, which are umbels. Mostspecies are herbaceous, but a few areshrubs and trees. The plants have a charac-teristic odor and some species, such asparsley, fennel, and dill, are used as herbs;the seeds of many others are popularspices, e.g. aniseed, caraway, coriander,cumin. Many species develop swollen tap-roots, and the family includes several im-portant root vegetables, e.g. carrot andparsnip; the leaf stalks of celery are alsoeaten.

apical dominance The phenomenon inwhich the presence of a growing apical budon a plant inhibits the growth of lateralbuds. It is controlled by the interactions ofplant hormones, particularly AUXIN (pro-duced by the shoot tip) and ABSCISIC ACID.

apical meristem The actively dividingcells constituting the growing point at thetip of the root or stem in vascular plants.New cells are cut off on the lower side toform new stem tissue at the stem apex, andon both sides in the root apex to form root

antisense DNA

18

tissue and a protective root cap. The apicalmeristems in the lower plants consist ofone cell only, as in the ferns, but becomemore complex and consist of groups ofcells in the higher plants. See histogentheory; tunica–corpus theory; meristem.

aplanospore A nonmotile spore, char-acteristic of the pin molds and certaingreen algae of the phyla Clorophyta andChrysomonada. It is an asexual sporeformed in a sporangium and is usuallythick walled.

apocarpous See carpel.

apocarpy An ovary made up of unfusedcarpels, as in the buttercup (Ranunuculus).Compare syncarpy.

apoenzyme An ENZYME whose cofactoror prosthetic group has been removed (e.g.via dialysis) rendering it catalytically inac-tive. It is the protein part of a conjugate en-zyme. When combined with its prostheticgroup (coenzyme) it forms a complete en-zyme (holoenzyme). Examples include thecytochromes, which require ferrous or fer-ric ions for activity.

apogamy In certain ferns, algae, andfungi, the development of the sporophytedirectly from a cell of the gametophyte, sofusion of gametes is bypassed. It frequentlyoccurs in gametophytes that have beenproduced aposporously and are thusdiploid. The term also describes the devel-opment of an unfertilized female gameteinto the sporophyte, a phenomenon de-scribed as PARTHENOGENESIS. See apospory;apomixis.

apomixis Any form of asexual repro-duction. The term is sometimes used in arestricted sense to describe a modified formof reproduction by plants in which seedsare formed without fusion of gametes, aprocess also termed agamospermy. A plantthat reproduces in this way is called anapomict, e.g. many species of Rubus.Apomicts are often polyploids that wouldhave difficulty reproducing sexually.

Apomixis is comparable to the conditionsof apogamy and apospory, which are seenin many ferns. It includes the processwhereby a diploid cell of the nucellus de-velops into an embryo giving a diploid seedwith a genetic constitution identical to theparent. Another form of apomixis in whichseeds develop from unfertilized gametescan also be termed parthenogenesis. Seedsproduced in this way may be either haploidor diploid depending on whether or not themegaspore mother cell undergoes meiosis.Often, in the process termed pseudogamy,entry of the male gamete is required tostimulate the development of the femalegamete, even though nuclear fusion doesnot occur. Such cases of apomixis are diffi-cult to distinguish from true sexual repro-duction. Compare amphimixis. See alsoapogamy; apospory; parthenogenesis; sex-ual reproduction.

apoplast The system of cell walls and in-tercellular material extending through aplant body and along which water contain-ing mineral salts, etc. can move passively. Itis an important pathway for movement ofthese substances outside the xylem, for ex-ample across the root cortex. Comparesymplast.

apospory The development of the ga-metophyte directly from the cell of a sporo-phyte, thus bypassing meiosis and sporeproduction. Gametophytes produced inthis manner are thus diploid instead ofhaploid. If such gametophytes produce fer-tile gametes, the resulting sporophyte isthen tetraploid, and large polyploid seriesmay subsequently be developed. Aposporyis found in some mosses and ferns. In an-giosperms, a diploid embryo may developfrom a cell of the nucellus or chalaza; thisis also apospory. See also apogamy;apomixis.

apothecium See ascoma.

apposition The deposition of successivelayers of cellulose on the inner surface of aplant cell wall after elongation is com-pleted, resulting in an increase in thickness

19

Archaea

of the wall. These extra layers are calledthe secondary cell wall. Compare intussus-ception.

appressorium (pl. appressoria) In manyparasitic fungi, a flattened hyphal structurethat presses closely against the host epider-mis. From its undersurface a narrow infec-tion hypha or penetration tube pushesthrough the cell wall or intercellular spacesand develops into a series of hyphae or aHAUSTORIUM.

aquaporin (water pore) A protein chan-nel in the plasma membrane that facilitateswater movement across the membrane.Aquaporins can open or close to increaseor decrease water flow.

Archaea (archaebacteria) In the FiveKingdoms classification scheme, a sub-kingdom of the BACTERIA, but in most mod-ern classifications, a DOMAIN in its ownright. It contains bacteria-like organismswith distinctive ribosomal RNA whose nu-cleotide sequences differ significantly fromthose of other prokaryotes. Because RNAis considered to evolve very slowly, thesedifferences are considered important in as-signing the Archaea the status of domain.The shape and structure of their ribosomesare more similar to those of eukaryotesthan to bacterial ribosomes. Whether theyare more closely related to the eukaryotesthan to the bacteria is hotly debated. Theyappear to have a mixture of prokaryoticand eukaryotic genes. Like eukaryotes, butunlike bacteria, they have introns in theirtransfer RNA, but like bacteria they havepolycistronic operons (gene regulators).Archaean cell walls lack murein, a com-mon component of bacterial cell walls, andtheir lipids have ether links rather thanester links.

The Archaea contain many organismsof extreme environments, such as hy-drothermal vents, hot springs, saline sedi-ments, volcanic craters and boiling muds.They include methane-generating organ-isms (methanogens), sulfate reducers, andextremophiles.

archaebacteria See Archaea.

Archean (Archeozoic) See Precambrian.

archegonium The female sex organ ofthe mosses, ferns, clubmosses, horsetails,most gymnosperms, and some red andbrown algae. It is a multicellular flask-shaped structure made up of a narrow neckand a swollen base (venter) that containsthe female gamete. Compare antheridium.

archesporium The single cell or groupof plant cells in the sporophyte from whichspore mother cells may eventually developin a sporangium.

Arecaceae (Palmae; palms) A family ofmonocotyledonous plants found mainly intropical regions. The typical growth formis an evergreen tree with unbranched trunkand a crown of large spirally arrangedleaves at the apex. The trunk is oftensheathed in old leaf bases. Some palms, e.g. the doum palms (Hyphaene), branchdichotomously. Many palms grow in theunderstorey of rainforests in Asia and theAmericas.

arginine An AMINO ACID found espe-cially in the histone proteins that surroundDNA in chromosomes.

aril A brightly colored fleshy or hairyoutgrowth from the funicle at the base ofthe ovule that may partly or completelycover the seed. The mace-yielding out-growth around the fruit of the nutmeg is anexample, as is the red fleshy cup surround-ing a yew seed (Taxus baccata). See alsocaruncle.

artificial parthenogenesis See partheno-genesis.

ascocarp See ascoma.

ascogonium (pl. ascogonia) The femalegametangium of certain fungi of the phy-lum Ascomycota (e.g. Erysiphe, Eu-rotium).

archaebacteria

20

ascoma (ascocarp) The fruiting body ofmost fungi of the phylum Ascomycota, inwhich the asci are borne.

ascomycete See Ascomycota.

Ascomycota (sac fungi; ascomycetes) Alarge phylum of fungi characterized bytheir distinctive reproductive structure, theASCUS. In some species the asci are cylindri-cal, and the ascospores are discharged vio-lently as a result of hydrostatic pressurebuilding up inside the ascus. In others, theasci are globular, and the ascospores are re-leased passively. The tip of the ascus mayopen through a simple pore, or it may havea cap. Most ascomycotes form branchingseptate hyphae, but some are yeastlike.

Ascomycotes are found in a wide rangeof habitats. Some are saprobes, such asblue and green molds (e.g. Neurospora),morels, and truffles, which are importantdecomposers. There are also parasites,such as ergot of rye (Claviceps purpurea),peach leaf curl (Taphrina), the powderymildews (Erysiphales), and Fusarium wilt.A few are symbionts in lichens and mycor-rhizas. The yeasts are economically impor-tant in fermentation processes such asbrewing (Saccharomyces cerevisiae), winemaking, and bread making. Some yeastsare parasites of humans, e.g. Candida albi-cans; others, such as Penicillium, aresources of antibiotics.

ascorbic acid See vitamins.

ascospore One of (usually) eight hap-loid spores produced inside an ASCUS bymembers of the phylum Ascomycota.

ascus (pl. asci) The spore-producing cellof fungi of the phylum ASCOMYCOTA. It is asaclike structure that is formed eithersingly or in large numbers in ascomata.After meiosis the ascus contains four oreight haploid ASCOSPORES that are liberatedthrough a pore at the end of the sac.

asexual reproduction The formationof new individuals from a single parentwithout the production of gametes or spe-

cial reproductive structures. It occurs inmany plants, usually by vegetative propa-gation or spore formation; in unicellularorganisms usually by fission or budding;and in some algae by fragmentation. Such‘offspring’ are genetically identical clonesof their parents. See apomixis.

asparagine An AMINO ACID formed bythe addition of ammonia to aspartic acid.This is a way of removing ammonia, whichis highly toxic, from living cells. As-paragine can later be broken down and thenitrogen used in amino acid synthesis.

aspartic acid An AMINO ACID formed bytransamination, in which the amino groupof glutamic acid is transferred to ox-aloacetic acid. Aspartic acid can be brokendown in the KREBS CYCLE. It is an importantprecursor of several amino acids, and alsoacts as a source of the amino group in var-ious metabolic reactions.

association A climax plant communitynamed according to the dominant type ofspecies. Examples are heath associationsand coniferous forest associations. See alsoconsociation.

assortative mating Sexual reproduc-tion in which the pairing of male andfemale is not random. If similar pheno-types (e.g. plants with similar flowershapes and color that attract the same in-sects) breed together, this a positive assor-tative mating; it may lead to INBREEDING.This is likely in insect-pollinated plants,where the insect looks for similar colorsand guide markings. Negative assortativemating occurs when dissimilar phenotypesbreed together, leading to OUTBREEDING.

aster See centriole.

Asteraceae (Compositae) A large familyof flowering plants, with some 21 000species found worldwide and including al-most every kind of growth form, butmainly herbs. The leaves, which may besimple or compound, are usually alternate,occasionally opposite or whorled, and

21

ATP

often in basal rosettes. Latex is present insome genera, e.g. lettuce (Lactuca). Tap-roots are common. Cultivated compositesinclude sunflower (Helianthus annuus), asource of oil and seeds; Jerusalem arti-choke (H. tuberosus), grown for food;globe artichoke (Cynara scolymus), whoseyoung flower heads are eaten; salsify(Tragopogon porrifolius), whose roots areeaten; chicory (Cichorium intybus), used asa substitute for coffee; lettuce (Lactucasativa); and Chrysanthemum cinerari-ifolium and C. coccineum, which aresources of the insecticide pyrethrum.

astrosclereid An irregularly branchedSCLEREID found in the mesophyll of leavesof certain dicotyledons.

ATP (adenosine triphosphate) The uni-versal energy carrier of living cells. Energyfrom respiration or, in photosynthesis,from sunlight is used to make ATP fromADP. It is then reconverted to ADP in var-ious parts of the cell, the energy releasedbeing used to drive cell reactions. ATP is anucleotide consisting of adenine and ribosewith three phosphate groups attached. Hy-drolysis of the terminal phosphate bond re-leases energy (30.6 kJ mol–1) and iscoupled to an energy-requiring process.Further hydrolysis of ADP to AMP some-times occurs, releasing more energy. Thepool of ATP is small, but the faster it isused, the faster it is replenished.

ATPase (adenosine triphosphatase) Anenzyme that catalyzes the hydrolysis ofATP to ADP and inorganic phosphate.Found in every cell, it is responsible for therelease of energy to power metabolic reac-tions and active transport. In oxidativephosphorylation, it forms part of the en-zyme complex ATP synthetase, catalyzingthe synthesis of ATP from ADP and inor-ganic phosphate using energy from theproton gradient generated by the electron-transport chain.

atropous (orthotropous) See ovule.

autecology The study of the interac-

tions of an individual organism or a singlespecies with the living and nonliving com-ponents of its environment. Compare syn-ecology.

autodiploid 1. A diploid plant producedby doubling the chromosomes of a haploidplant. Such a plant is homozygous forevery allele.2. A gamete containing a diploid numberof chromosomes, rather than the usualhaploid number. Such a gamete may beproduced by tetraploid tissues, or it may bethe result of faulty meiosis. Two diploidgametes may fuse to give a tetraploid zy-gote, or gamete may develop directly byparthenogenesis.

autoecious Denoting rust fungi that re-quire only one host species to complete thevarious stages of their life cycle. Examplesare Puccinium antirrhini found on antir-rhinum and the mint rust (Puccinia men-thae). Compare heteroecious.

autogamy Self-fertilization in plants.This restricts genetic variation but allowsisolated individuals to reproduce. It isfound particularly in pioneer weed speciesand in ecosystems such as the tundra,where insect vectors are rare. Very fewplants rely exclusively on autogamy. Com-pare allogamy.

autolysis The self-destruction of cells bydigestive enzyme activity. It is the finalstage of cell senescence resulting in com-plete digestion of all cell components. Seelysosome.

autonomic movements (autogenic move-ments; spontaneous movements) Move-ments of plants in response to internalrather than external stimuli. Examples are cytoplasmic streaming, chromosomemovement during nuclear division, andgrowth itself. Compare paratonic move-ments.

autophagy The process by which worn-out cell organelles are enclosed in vacuoles,then broken down by hydrolytic enzymes

ATPase

22

secreted into vacuole, the breakdownproducts being reabsorbed afterward. Seealso lysosome. Compare autolysis.

autopolyploidy A type of polyploidyinvolving the multiplication of chromo-some sets from only one species. Autopoly-ploids may arise from the fusion of diploidgametes that have resulted from theNONDISJUNCTION of chromosomes at meio-sis. Alternatively, like allopolyploids, theymay arise by the nondisjunction of chro-matids during the mitotic division of a zy-gote. Autopolyploids often have largercells than the parent plants but are mor-phologically very similar. They may, how-ever, be at least partially isolated from eachother reproductively and occupy differentecological niches. Compare allopolyploidy.

autoradiography A technique wherebya thin slice of tissue containing a radioac-tive isotope is placed in contact with a pho-tographic plate. The image obtained ondevelopment shows the distribution of theisotope in the tissue.

autosomes Paired somatic chromo-somes that play no part in sex determina-tion. Compare sex chromosomes.

autotetraploid An autopolyploid thathas four times the haploid number of chro-mosomes. See autopolyploidy.

autotrophism A type of nutrition inwhich the principal source of carbon isinorganic (carbon dioxide or carbonate).Organic materials are synthesized from in-organic starting materials. The processmay occur by use of light energy (photo-autotrophism) or chemical energy (chemo-autotrophism). Autotrophic organisms(autotrophs) are important ecologically asprimary producers, their activities ulti-mately supplying the carbon requirementsof all heterotrophic organisms. Compareheterotrophism. See chemotrophism; pho-totrophism.

auxanometer An instrument designedto measure the increase in length of a plantpart. A growing plant is attached by threadto the end of a lever that magnifies anygrowth movement. The opposite end of thelever is used to record a trace on a slowlyrotating drum.

auxin Any of a group of plant hor-mones, the most common naturally occur-ring one being indole acetic acid, IAA.Auxins are made continually in growingshoot and root tips. They are activelymoved away from the tip, having variouseffects along their route before being inac-tivated. They regulate the rate of extensionof cells in the growing region behind theshoot tip and are involved in phototropicand gravitropic curvature responses ofshoot tips (but probably not root tips)moving laterally away from light and to-ward gravity.

Auxins stimulate cell enlargement,probably by stimulating excretion of pro-tons leading to acid-induced wall looseningand thus wall extension. They help main-tain apical dominance by inhibiting lateralbud development. Root initiation may bestimulated by auxin from the shoot, andauxins have been shown to move towardthe root tips. Pollen tube growth is stimu-lated by auxin and its production by devel-oping seeds stimulates fruit set andpericarp growth in fleshy fruits. It interactssynergistically with gibberellins and cy-tokinins in stimulating cell division anddifferentiation in the cambium. A highauxin:cytokinin ratio stimulates rootgrowth but inhibits regeneration of buds intobacco pith callus. It is antagonistic to ab-scisic acid in abscission.

awn In grass flowers (Poaceae), a stiffbristlelike structure formed from a contin-uation of the central nerves of the LEMMAS

or GLUMES.

axenic culture A culture that consists ofonly one type of organism or cell, i.e. it is

23

Azotobacter

Bacillariophyta See Diatoms.

bacillus (pl. bacilli) Any rod-shaped bac-terium. Bacilli may occur singly (e.g.Pseudomonas), in pairs, or in chains (e.g.Lactobacillus). Some are motile.

backcross A cross between an individ-ual and an individual of the original parentgeneration. If a homozygous dominant AAis crossed with a homozygous recessive aa,the F2 generation obtained by selfing the F1would be 25% AA, 50% Aa, and 25% aa.To distinguish between the phenotypicallyidentical AA and Aa, these can be back-crossed to the homozygous recessive par-ent aa (a test cross). The offspring from AA´ aa will all be identical (Aa) whereas in thecross Aa ́ aa, 50% will have the dominantphenotype (Aa) and 50% will show the re-cessive character (aa).

Bacteria A kingdom containing all theprokaryotic organisms, formerly calledMonera or Prokaryotae. This is a large anddiverse group of organisms, which, interms of numbers and variety of habitats,includes the most successful life forms. Innature, bacteria are important in the nitro-gen and carbon cycles, and some are usefulto humans in various industrial processes,especially in the food industry, and in tech-niques of genetic engineering (see alsobiotechnology). However, there are alsomany harmful parasitic bacteria that causediseases such as botulism and tetanus. Oth-ers are sources of antibiotics such as strep-tomycin and erythromycin. Some bacteriarequire oxygen for respiration, while oth-ers can survive without oxygen. There maybe at least ten times more bacteria living incracks in the rocks deep inside the earththan in the surface layers of the planet, in-

cluding the soil. Some derive their carbonfrom inorganic sources, and their hydrogenfrom hydrogen sulfide or other inorganiccompounds, while other bacteria may bephotosynthetic or saprobic.

Bacterial cells are simpler than those ofanimals and plants. They lack well-definedmembrane-bound nuclei and histone-coated chromosomes, and do not containcomplex organelles such as chloroplastsand mitochondria. Their DNA is usuallylocated in a specific part of the cell, calledthe nucleoid. Bacteria may divide every 20minutes and can thus reproduce veryrapidly. They also form resistant spores.The Bacteria are divided into two maingroups or subkingdoms: the ARCHAEA, orarchaebacteria, which often occur in ex-treme conditions, such as hot springs; andthe EUBACTERIA, which include the vast ma-jority of bacteria. Some taxonomists con-sider these groups to be full kingdoms intheir own right. See Actinobacteria;Cyanobacteria; myxobacteria; sulfur bac-teria. See also prokaryote.

bacterial chromosome The nakedDNA of bacteria and Archaea. Bacteria donot have true CHROMOSOMES – linear piecesof DNA complexed with histone proteins.Their DNA is circular and is not com-plexed with proteins. It lacks the cen-tromere, telomere, and other structures ofthe eukaryote chromosome.

bacterial transformation See transfor-mation.

bactericidal Used to describe a com-pound that has a lethal effect on bacteria.A bactericidal compound may act by inter-fering with a vital biochemical pathway, or

24

B

by destroying the molecular structure ofthe cell.

bacteriochlorophyll Any of severaltypes of chlorophyll found in photosyn-thetic bacteria, such as the purple bacteria.There are seven forms, designated bacteri-ochlorophylls a–g. All are structurally sim-ilar to chlorophyll a of plants. Thebacteriochlorophylls absorb light at longerwavelengths than chlorophyll a enablingfar-red and infrared light to be used inphotosynthesis. See photosynthetic pig-ments.

bacteriophage (phage) A virus that in-fects bacteria. Phages usually have com-plex capsids (protein coats) composed of apolyhedral head, containing the nucleicacid (DNA or RNA), and a helical tail,through which nucleic acid is injected intothe host. Phages are usually specific to par-ticular species or even strains of bacteria.Infection by a phage may or may not leadto the death of the bacterium, dependingon the phage and on the conditions. AllRNA phages cause the death of the hostcell. After reproduction of the viral nucleicacid the host cell usually undergoes lysis(LYSOGENY). Some DNA phages, calledTEMPERATE PHAGES, may become integratedinto the host DNA and are replicated whenit replicates. Some of these phages arecapable of transferring genes between bac-teria in a process called TRANSDUCTION. InGENETIC ENGINEERING, nonviral DNA canbe inserted into a phage, which is then usedas a cloning vector.

bacteriostatic Used to describe a com-pound that prevents reproduction of bacte-ria, but does not kill them.

bacteroid A modified bacterial cell in aroot nodule, typically a cell of the bac-terium Rhizobium in the root nodule of aleguminous plant (family Fabaceae).

Banks, Sir Joseph (1743–1820) Britishbotanist. Banks used his considerable for-tune to finance several important expedi-tions, including his most famous one to the

South Pacific in 1768 with Captain JamesCook on the Endeavour. This voyagelasted for three years and resulted in thediscovery of the unique flora and fauna ofAustralia. On his return with his enormouscollection of specimens, he received an in-vitation to meet King George III, who hadexpressed a great interest in the expedition.His continued friendship with the king in-creased his influence in scientific circlesand he became president of the Royal Soci-ety for life in 1778. Following his appoint-ment to the post of honorary director ofKew Gardens, he helped to establish amajor collection of living plant species andto make Kew famous as a center of excel-lence for plant cultivation. He is also fa-mous for introducing the tea plant to India(from China) and breadfruit to the WestIndies (from Tahiti). His extensive libraryand herbarium are housed in the BritishNatural History Museum.

bark The outermost tissue of the stemand roots in woody plants, on the outsideof the phellogen (cork cambium). If thesame phellogen functions from year toyear, as in beech and oak, then the bark issmooth and consists only of tissue externalto the phloem, mainly cork. If new corkcambia arise every few years, as in mostspecies of woody plants, then the bark in-cludes dead phloem and cortex as well ascork, and is termed rhytidome. As bark in-creases in thickness, the outer layers maysplit in distinctive patterns. The textureand patterning of the bark is often charac-teristic of the species. The bark is punctu-ated by lenticels, small patches of loosecells, which allow air to penetrate the livingtissues below.

basal body See kinetosome.

base 1. (Chemistry) A compound thatreacts with an acid to produce water plus asalt. In solution, it forms ions that can reactwith hydrogen ions.2. (Biochemistry) A nitrogenous molecule,either a PYRIMIDINE or a PURINE, that com-bines with a pentose sugar and phosphoricacid to form a nucleotide, the fundamental

25

base

unit of nucleic acids. The most abundantbases are cytosine, thymine, and uracil(pyrimidines) and adenine and guanine(purines).

base pairing The bonding relationshipbetween purine and pyrimidine bases in thenucleotides of DNA and RNA. The pairingof these bases, sustained by hydrogenbonding, is highly specific: adenine (A) onone strand of the nucleic acid pairs onlywith thymine (T) in the complementarystrand of DNA, or with uracil (U) in RNA;guanine (G) pairs only with cytosine (C).The large numbers of hydrogen bonds thatform between the base pairs along a DNAor RNA molecule stabilize the doublehelix. The specificity of the pairing ensuresthat during REPLICATION and TRANSCRIP-TION accurate copies are made of the ge-netic material. The number of base pairs isa measure of the length of the DNA orRNA molecule, which is often expressed inunits of 100 bases (kilobases), meaning100 base pairs.

base ratio The ratio of the molar quan-tities of two bases in DNA, or of one basepair with the other base pair. The baseratio of 1:1 for the bases adenine andthymine and the bases cytosine and gua-nine was the first clue to the specificity ofBASE PAIRING in the nucleic acids. The basepair ratio, AT:GC, is constant within aspecies but differs between species.

basic stain See staining.

basidioma (pl. basidiomata) The fruit-ing body of the Basidiomycota, exceptingthe rusts and smuts, formerly called the ba-sidiocarp. It may be of a fleshy, corky, orspongy nature. The basidioma is composedmainly of sterile tightly packed hyphae; inmost species there is a distinct fertile layer,the hymenium, on the surface of the basid-ioma that bears the basidial hyphae which,after meiosis, give rise to the basidiospores(see basidium).

basidiomycete See Basidiomycota.

Basidiomycota (basidiomycetes) A phy-lum of the Fungi that includes the mush-rooms and toadstools. Basidiomycotes arecharacterized by their spore-bearing struc-tures (see basidium), each of which usu-ally bears four sexually produced spores(basidiospores) on its outer surface. Inmost basidiomycotes the basidia are borneon a special fertile layer, the hymenium, onthe surface of a fruiting body, the BASIDI-OMA.

basidiospore See Basidiomycota; basid-ium.

basidium (pl. basidia) The cell of the BA-SIDIOMYCOTA that produces the sexualspores (basidiospores). In most basidiomy-cotes the basidia develop in the BASIDIOMA

from hyphal tips in the hymenium and maybe club-shaped or cylindrical.

basifixed Stamens in which the antherlobes are attached at their base to the fila-ment, and are not capable of independentmovement. Compare dorsifixed; versatile.

basipetal Developing from the apexdownward so that the youngest structuresare furthest from the apex. Basipetal differ-entiation is seen in the formation of proto-and metaxylem in the stem. The term mayalso be applied to the movement of sub-stances toward the base, for example themovement of auxin in shoot tissues. Com-pare acropetal. See also centrifugal; cen-tripetal.

B chromosome (accessory chromosome)An extra chromosome found in addition tothe normal number of chromosomes char-acteristic of a species (the A chromosomes).B chromosomes are known from morethan 20 families of plants and are alsofound in some fungi.

beet sugar See sucrose.

Benedict’s test A test for the presence ofa dissolved reducing sugar. Benedict’s solu-tion is an alkaline solution of copper(II)sulfate, which is reduced to insoluble cop-

base pairing

26

per(I) oxide by reducing sugars, giving ared precipitate.

Bentham, George (1800–84) Britishbotanist. Having developed an interest inplant taxonomy from the age of 17, Ben-tham abandoned (1833) his career in lawto devote himself entirely to his botanicalcollection and library. In 1854 he pre-sented his collection to the Royal BotanicGardens at Kew, where he worked for therest of his life. His famous Handbook ofBritish Flora was published in 1858.Between 1862 and 1883 he collaboratedwith Joseph HOOKER to publish GeneraPlantarum, a world flora describing 7569genera and 97 000 species, which is con-sidered his greatest achievement.

benthic Of, relating to, or living in thezone at the bottom of a sea, lake, or otherbody of water. See benthos. Comparepelagic.

benthos The organisms that live on thebottom sediments of freshwater and ma-rine ecosystems. They include plants andalgae (phytobenthos) that grow below lowtide mark.

berry A succulent fruit, usually contain-ing more than one seed, that does not burstopen when ripe. The ovary wall remainsfleshy after fertilization of the ovule exceptfor the development of a thin outer skin. Aberry with a hard rind, such as a cucumber,is called a pepo. A berry with a leatheryrind in which the seeds are separated bysegments, as in the citrus fruits, is called ahesperidium. The epicarp and mesocarpmay be brightly colored thus attractinganimals, which disperse the fruit, as in thetomato. Compare drupe. See also pericarp.

beta-oxidation See oxidation.

beta-pleated sheet (b -pleated sheet) Atype of protein secondary structure inwhich polypeptide chains run close to eachother and are held together by hydrogenbonds at right angles to the main chain be-tween the NH and C=O groups on one

chain and the C=O and NH groups on theother chain. The structure is folded in reg-ular ‘pleats’. Fibers having this type ofstructure are usually composed of aminoacids with short side chains. The chainsmay run in the same direction (parallel) oropposite directions (antiparallel). See con-formation; secondary structure.

bicollateral bundle The arrangementof tissues in the vascular bundles in whichthe phloem is situated on both sides of thexylem. This is thought to aid transport ofnutrients in plants with long trailing stems,such as the marrow. Compare concentricbundle; collateral bundle.

biennial A plant that completes its lifecycle within two years. In the first year itproduces foliage only and photosynthe-sizes. The food is stored during the winterin a swollen underground root or stem. Inthe second year, the stored food is used toproduce flowers, fruits, and seeds. Manyimportant crops, such as carrot andparsnip, are biennials. Some biennials canbe induced to act as annuals and flower inthe first year by appropriate cold or hor-mone treatments. Compare annual;ephemeral; perennial.

bilateral symmetry The arrangementof parts in an organism in such a way thatthe structure can only be divided into sim-ilar halves (mirror images) along oneplane. Bilateral symmetry is characteristicof most free-moving animals, where oneend constantly leads during movement. Inplants, bilateral symmetry is seen particu-larly in flowers (e.g. snapdragon), the con-dition being termed zygomorphy. See alsoradial symmetry.

binomial distribution (normal distribu-tion; Gaussian distribution) The ideal-ized distribution of a population ofstatistical values centered around a mean,when departures from the mean are due therandom occurrence of a large number ofsmall independent effects. It represents theprobability of each value occurring and is

27

binomial nomenclature

represented on a graph by a bell-shapedcurve or histogram.

binomial nomenclature A system ofclassification introduced by Linnaeus, theSwedish botanist, in which each species isgiven two names. The first is the genericname, written with a capital letter, whichdesignates the genus to which the speciesbelongs. The second is the specific name,indicating the species. The generic and spe-cific names are in Latin and are printed initalic type. For example, the sunflower be-longs to the species Helianthus annuus.Helianthus is the generic name and annuusis the specific name. See International Codeof Botanical Nomenclature.

bioassay An experimental technique formeasuring quantitatively the strength of abiologically active chemical by its effect ona living organism. For example, the vita-min activity of certain substances can bemeasured using bacterial cultures. The in-crease in bacterial numbers is comparedagainst that achieved with known stan-dards for vitamins. Plant growth hormonescan be estimated by their effect in causingcurvature of oat coleoptiles.

Biochemical Oxygen Demand (BOD)The standard measurement for determin-ing the level of organic pollution in a sam-ple of water. It is the amount of oxygenused by microorganisms feeding on the or-ganic material over a given period of time.Sewage effluent must be diluted to complywith the statutory BOD before it can bedisposed of into clean rivers.

biochronology The measurement of geo-logical time using biological events. Partic-ular strata of rocks can be approximatelydated according to the presence or absenceof certain fossils. The first and last appear-ances of particular fossil species representspecific moments in geological time.

biodegradable Describing organic com-pounds that can be decomposed by bacte-ria and other microorganisms, such as theconstituents of sewage, as compared with

nonbiodegradable compounds, such asmost plastics. See also decomposer; pollu-tion.

biodiversity The number and variety oforganisms in a given locality, community,or ecosystem. High biodiversity is typicalof complex and highly productive ecosys-tems, such as tropical rainforests, where asmall area can contain many differentspecies of animals, plants, and other or-ganisms. Biodiversity is often used as an in-dicator of the health of such ecosystems.See also Convention on Biological Diver-sity.

bioenergetics The study of energytransfer in living organisms.

biogenesis The theory that living thingsoriginate only from other living things asopposed to nonliving matter. The theorybecame accepted as a result of the work ofRedi and Pasteur, who showed that dirt,for example, does not itself produce bacte-ria or maggots, but that bacteria and mag-gots come only from spores or eggs alreadyexisting in the dirt. This theory satisfacto-rily explains the occurrence of existing or-ganisms, but not the origins of the firstorganisms.

biogenic Describing structures pro-duced as a result of the activity of living or-ganisms, in particular rocks (e.g. chalk,limestone, diatomaceous earth) and fossils.

biogeochemical cycle The movementof chemical elements from the physical en-vironment into organisms and back to theenvironment in a more or less circularpathway. If the elements concerned are es-sential to life, this cycle is called a nutrientcycle, e.g. the NITROGEN CYCLE. Some ofthese cycles are regulated mainly by physi-cal factors. For example, the main con-straint on the phosphorus cycle is the slowrelease of phosphates from rocks by weath-ering. Others, such as the nitrogen cycle,involve biological feedback mechanisms,as nitrates are incorporated into proteinsand released by decomposition. In the CAR-

bioassay

28

BON CYCLE, photosynthetic organisms arethought to be important in balancing thecycle by carbon fixation; carbon com-pounds are slowly released as dead organ-isms decompose.

biogeographical barrier A barrier tothe migration of plants and animals thatprevents different populations mixing.Such barriers may be physical, such asmountain ranges, large rivers, lakes, orseas; or they may be climatic.

biogeographical region A subdivisionof the Earth’s surface based on the particu-lar species of plants and animals present.There are several different schemes of bio-geographical regions, but the most widelyrecognized are: Boreal (northern Eurasiaand North America), Paleotropical (Africaand southeast Asia as far as New Guinea),Neotropical (South America, except the farsouth), Australasian, Cape (South Africa),and Antarctic (New Zealand and southernSouth America). These biogeographical re-gions are subdivided into biogeographicalprovinces.

biogeography The study of the distrib-ution of plants and animals, both past andpresent, and its interpretation in terms ofecology, in particular the distribution ofworld vegetation types and the interactionsbetween humans and their environment.

bioinformatics The creation and main-tenance of databases of biological informa-tion. With the rapid expansion ofbiological knowledge and research, sophis-ticated computerized information-retrievalsystems are necessary to speed up access tobiological information.

biological clock The internal mecha-nism of an organism that regulates CIRCA-DIAN RHYTHMS and various other periodiccycles.

biological conservation The preserva-tion of the world’s natural biological diver-sity and natural habitats and themaintenance, as far as is possible, of the

abiotic factors that influence these. Thismay involve human management of a habi-tat. Modern biological conservation mayalso include sustainable use of biologicalresources, which sometimes conflicts withthe conservation of maximum diversity.See World Conservation Union.

biological control The use of naturalpredators or parasites, instead of chemi-cals, to control pests. The most famoussuccessful example was the introduction ofthe gray moth, Cactoblastis cactorum, intoAustralia to control the prickly pear, Op-untia inermis, which was overrunning vasttracts of land. The moth’s caterpillars eatthe shoots of the plant.

biology The study of living organisms,including their structure, function, evolu-tion, interrelationships, behavior, and dis-tribution.

biomass The weight or volume of livingorganisms of one particular species perunit area (species biomass), or of all thespecies in the community (community bio-mass). See also pyramid of biomass.

biome A major regional community ofplants and animals with similar life formsand environmental conditions. It is thelargest geographical biotic unit, and isnamed after the dominant type of vegeta-tion, such as tropical rain forest, grassland,or coral reef.

biometry The application of statistics tothe analysis of biological phenomena.

biorhythm A periodic physiological orbehavioral change that is controlled by abiological clock. Circadian rhythms are anexample.

biosphere The part of the earth and itsatmosphere that is inhabited by living or-ganisms. The Earth is made up of threeconcentric zones: the lithosphere (solidearth), the hydrosphere (water layer), andthe atmosphere. The biosphere includesparts of all three of these zones. It is some-

29

biosynthesis

times considered to represent a singleecosystem, the ecosphere.

biosynthesis Chemical reactions inwhich a living cell builds up its necessarymolecules from other molecules present.See anabolism.

biosystematics The area of systematicsin which experimental taxonomic tech-niques are applied to investigate the rela-tionships between taxa. Such techniquesinclude serological methods, biochemicalanalysis, breeding experiments, compara-tive morphology, cytological examination,and RNA analysis, in addition to the moreestablished procedures of comparativeanatomy. Evidence from ecological studiesmay also be brought to bear. See also mol-ecular systematics.

biota The plants, animals, and other liv-ing organisms that live together in a par-ticular place.

biotechnology The application of tech-nology to biological processes for indus-trial, agricultural, and medical purposes.For example, bacteria such as Penicilliumand Streptomycin are used to produce anti-biotics and fermenting yeasts produce alco-hol in beer and wine manufacture. Recentdevelopments in genetic engineering haveenabled the large-scale production of hor-mones, blood serum proteins, and othermedically important products. Geneticmodification of farm crops, and even live-stock, offers the prospect of improved pro-tection against pests, or products withnovel characteristics, such as new flavorsor extended storage properties. See alsogenetic engineering.

biotic environment The biological fac-tors acting on an organism, which arisefrom the activities of other living organ-isms, as distinct from physical factors.

biotin A water-soluble vitamin generallyfound, together with VITAMINS in the Bgroup, in the vitamin B complex. It iswidely distributed in natural foods, egg

yolk, kidney, liver, and yeast being goodsources. Biotin is required as a coenzymefor carboxylation reactions in cellular me-tabolism.

biotrophic Obtaining nutrients fromthe living tissues of another organism.Compare autotrophism.

biotype 1. A naturally occurring groupof individuals all with the same geneticcomposition, i.e. a clone of a pure line.Compare ecotype.2. A physiological race or form within aspecies that is morphologically identicalwith it, but differs in genetic, physiological,biochemical, or pathogenic characteristics.

bipinnate Describing a pinnate leafwhose leaflets are pinnately subdivided,e.g. sensitive plant (Mimosa pudica).

bisexual See hermaphrodite.

Biuret test A test for the presence ofproteins, in which sodium hydroxide isadded to the test solution, followed bydrops of 1% copper(II) sulfate solution. Aviolet color indicates the presence of pro-teins or peptides (excluding dipeptides).Amino acids have no effect.

bivalent Describing any pair of homolo-gous chromosomes when they pair upduring MEIOSIS. Pairing of homologouschromosomes (synapsis) commences at oneor several points on the chromosome and isclearly seen during PACHYTENE of meiosis I.

black earth See chernozem.

bladder 1. A modified leaf, found on thestems of members of the bladderwort fam-ily, that develops into a distended structurefor trapping small invertebrates. The blad-der is thin-walled and closed by a one-wayvalve.2. An air-filled sac found in large numbersin the thalli of certain seaweeds (e.g. blad-derwrack).

blending inheritance Inheritance in

biosystematics

30

which the characters of the parents appearto blend into an intermediate form in theoffspring. It is seen in characters controlledby a single gene whose alleles are codomi-nant, as in pink flowers of Antirrhinum,and also in characters such as yield, whichare determined by many genes. Until the re-discovery of Mendel’s work, it wasthought that all characteristics becomeblended in the offspring. See Mendel’slaws.

blepharoplast See kinetosome.

blight A plant disease in which seriousleaf damage appears suddenly. It is usuallydue to fungi or oomycetes, such as thepotato blight (Phytophthora infestans), oroccasionally to bacterial infections, as infireblight of pears, which is caused by Er-winia amylovora.

bloom A sudden growth of microorgan-isms in a freshwater or marine ecosystem,often known as an algal bloom. This mayoccur naturally in spring, when growth ofmicroorganisms is for a time unchecked byherbivores. It may also be brought on bynutrient enrichment of the water, often dueto fertilizers being washed off agriculturalland, or to seawater entering the marineenvironment.

blue-green bacteria See Cyanobacte-ria.

BOD See Biochemical Oxygen Demand.

bog A plant community found in wetacid areas where rainfall is high, the at-mosphere is continually moist, and decom-position rates are slow, favoring PEAT

formation. Bogs are common in uplandsand western areas of north temperate andsome southern temperate regions. Typicalbog plants of north temperate regions in-clude the bog mosses (Sphagnum), cottongrasses (Eriophorum), rushes (Juncus),sedges (Carex), and certain heaths (e.g.Calluna vulgaris, Erica tetralix).

Boletales (boletes) An order of basid-

iomycote fungi having fleshy mushroom-shaped fruiting bodies in which the hyme-nium (spore-bearing surface) lines theinner surface of vertical tubes (pores) onthe underside of the cap (e.g. Boletus).Compare Agaricales.

bolting Premature flowering and seedproduction. Biennial plants will sometimesbolt in their first year; this seriously re-duces their yield.

borax carmine A dye used to stain nu-clei for light microscopy.

Boreal floral region (Holarctic floralprovince) A floral region that consists ofnorthern Eurasia and North America. Seefloral province.

boron A MICRONUTRIENT found in verylow concentrations in plant tissues but es-sential for plant growth. It is involved inphenol metabolism and membrane func-tion. It is implicated in the uptake of cal-cium, the development of apical meristems,and pollen germination.

botanical 1. Relating to the study ofplants.2. A cultivated plant of an unaltered wildspecies.3. An insecticide derived from a plant, e.g.pyrethrum (from Chrysanthemum flow-ers).

botany The scientific study of plants.

bracket fungi A member of the basid-iomycote order Aphyllophorales, whosefruiting bodies (basidiomata) typically pro-ject from the trunks of trees like shelves ordomed brackets. These fruiting bodies areusually tough, sometimes woody. Thespores are produced on small teeth orlamellae rather than gills.

bract A modified leaf that developsbelow a flower or an inflorescence on itsaxis. It may be reduced or highly colored,as in the scarlet bracts of Poinsettia. In thedaisy family the numerous bracts which

31

bracteole

subtend the inflorescence are known as aninvolucre. In the grasses bracts are borne inpairs below the spikelets and are calledglumes. Each individual floret of thespikelet is also subtended by two bracts,the lower being called the lemma and theupper the palea.

bracteole A small leaflike organ on thepedicel of a flower in an inflorescence thatis itself subtended by a bract, as in the blue-bell. Compare bract.

bract scale The structure found in largenumbers in the female cone of gym-nosperms, each bearing an ovuliferousscale in its axil.

branch A lateral stem arising from an-other stem of the same form.

Brassica A genus of plants of the cab-bage family (Brassicaceae) that includesmany vegetables and fodder plants.

Brassicaceae (Cruciferae) A family ofannual, biennial, or perennial plants,mainly herbs, that includes many commer-cially important crops, especially the BRAS-SICA species; salad vegetables, such as cress(Sinapis), rocket (Eruca), and watercress(Rorippa); and ornamental plants, e.g.Alyssum, Aubretia, wallflower (Erysi-mum), stock (Matthiola), and honesty (Lu-naria annua).

breeding system The way in which aspecies breeds, including the method ofmate selection, genetic determination ofsex and incompatibility, method of mateselection or pollination, and other factorsthat place limits on the genetic diversity ofthe offspring. See self-incompatibility; sexdetermination; sexual reproduction.

breeding true (pure breeding) The pro-duction of offspring whose genetic makeupfor particular alleles is identical to that oftheir parents. This requires both parents tobe homozygous for same alleles in ques-tion.

Brown, Robert (1773–1858) Britishbotanist. Brown studied medicine andserved as a military medical officer. Duringa visit to London he was introduced to SirJoseph Banks, who encouraged him toapply for the post of naturalist on an expe-dition (1801–06) to survey the coast ofAustralia in the Investigator. Having re-turned with some 4000 plant specimens, hespent the next few years on their classifica-tion. His painstaking observations led himto discover many important features ofplants. He was the first to observe andname the nucleus of cells and to recognizeits importance. He also observed that gym-nosperm ovules were not enclosed in anovary wall, leading to the establishment ofone of the basic differences between an-giosperms and gymnosperms. In 1827,upon observing a suspension of pollengrains in water, he discovered BROWNIAN

MOTION.

brown algae See Phaeophyta.

brown earth The type of soil foundunder deciduous forests in temperate cli-mates. It is rich in humus and slightly acid,due to leaching of lime. When cleared, itprovides good fertile agricultural soil.

Brownian motion The random motionof microscopic particles due to their con-tinuous bombardment by the much smallerand invisible molecules in the surroundingliquid or gas. Particles in Brownian motioncan often be seen in colloids under specialconditions of illumination.

Bryophyta A phylum of simple, mainlyterrestrial, plants called mosses, commonlyfound in moist habitats. They show a het-eromorphic alternation of generations, thegametophyte being the dominant genera-tion. When mature the gametophyte showsdifferentiation into stem and leaves butthere are no roots or vascular tissues, butmosses do possess elongated conductingcells. The sporophyte, which is wholly de-pendent on the gametophyte, is simply aspore capsule borne on a stalk. The sporesgerminate to produce a slender, filamen-

bract scale

32

tous haploid protonema, which producesbuds that develop into leafy moss plants.Formerly, the Bryophyta also included theliverworts and hornworts, which in theFive Kingdoms classification are nowplaced in separate phyla (HEPATOPHYTA andANTHOCEROPHYTA). The collective namefor mosses, liverworts, and hornworts isbryophytes.

bud 1. A compacted undeveloped shootconsisting of a shortened stem and im-mature leaves or floral parts. The youngleaves are folded about the growing tip,and the outermost leaves may be scaly andreduced to protect the growing point. Abud has the potential to develop into a newshoot or flower. Terminal buds are formedat the stem or branch tip. Axillary or lat-eral buds develop in the leaf axils. Buds candevelop adventitiously on other parts of aplant and are sometimes a means of asex-ual reproduction.2. The swelling produced by a unicellularorganism that is reproducing asexually byBUDDING.

budding 1. The production of buds onplants.2. A type of grafting in which the graftedpart is a bud.3. A type of asexual reproduction in whicha new individual is produced as an out-growth (bud) of the parent organism. It oc-curs in some unicellular fungi, such as theyeasts.

buffer A solution that resists any changein acidity or alkalinity. Buffers are impor-tant in living organisms because they guardagainst sudden changes in pH. They in-volve a chemical equilibrium between aweak acid and its salt or a weak base andits salt. In biochemistry, the main buffersystems are the phosphate (H2PO4

–/HPO4

2–) and the carbonate (H2CO3/HCO3

–) systems.

bulb A modified shoot that acts as anorgan of perennation and often vegetative

reproduction. The stem is reduced to adisklike structure, bearing concentric lay-ers of fleshy leaf bases that comprise thefood store. Each leaf has a bud at its basethat is able to develop into a subsidiarybulb. The whole bulb is protected by scaleleaves and adventitious roots arise fromthe base of the stem. In spring or summerone or more buds grow and produce leavesand flowers, exhausting the food supply.The new leaves photosynthesize and foodis stored in their bases thus giving rise to anew bulb. If more than one bud develops,eventually more than one bulb develops.See perennating organ; asexual reproduc-tion.

bulbil A small bulblike organ of asexualreproduction that may form in a leaf axil,as in some lilies, or may replace flowers in an inflorescence, as in the wild leek (Al-lium amploprasum). It may detach itselffrom the parent plant and become inde-pendent.

bundle sheath The ring of parenchyma-tous or sclerenchymatous tissue, usuallyone cell thick, that surrounds the vascularbundle in an angiosperm leaf. The indi-vidual cells are closely packed with noapparent intercellular spaces, and con-duct water and solutes from the vas-cular bundle to the surrounding tissues.Chloroplasts may be present in the bun-dle sheath, and are thought to be con-nected with starch storage in the tropicalgrasses.

bunt (stinking smut) A seed-borne dis-ease of wheat (Avena) and other grasses,caused by the basidiomycote Tilletia.

bur A kind of pseudocarp (false fruit) inwhich the fruit is encased in a barbed in-volucre. The barbs catch in the fur of pass-ing animals and the fruits are thusdispersed. Examples include burdock (Arc-tium) and cocklebur (Xanthium).

butenedioic acid See succinic acid.

33

buttress root

Cactaceae (cacti) A large family of suc-culent xerophytic dicotyledonous plants,found mainly in warm dry parts of theAmericas; some species, notably theprickly pears (Opuntia), have been intro-duced to the Mediterranean and Australia.

Caenozoic See Cenozoic.

Cainozoic See Cenozoic.

calcareous soil (basic soil; alkalinesoil) A soil with a pH greater than 6.0.Calcareous soils occur where lime accumu-lates in the surface layers, usually as a re-sult of a calcareous bedrock such aslimestone, or a calcareous sand depositnear coasts. They are most common in re-gions of light rainfall and are usually richin calcium ions and often other elements.See rendzina.

calcicole Describing plants that thriveon neutral to alkaline soils rich in calciumcarbonate, i.e. chalk or limestone soils,such as marls. Compare calcifuge.

calcifuge Describing plants that growon soils containing very little calcium car-bonate, such as sandy or peaty soils andcannot grow on basic substrates such asmost chalk and limestone soils. Comparecalcicole.

calcium Symbol: Ca. An essential min-eral for animal and plant growth. One ofits most important functions in plants is asan intracellular messenger with steep gra-dients of calcium concentration formingacross membranes with a system of cal-cium pumps. It is an important constituentof the middle lamella between plant cellwalls as calcium pectates. The calcium ions

(Ca2+) cause cross-linking of adjacentchains of pectic acid, thus strengtheningthe cell wall.

callose An insoluble polysaccharide thatis laid down around the perforations insieve plates, at wound or infection sites,and by growing pollen tubes. As sieve tubesage, the callose layers become thicker,eventually blocking the SIEVE ELEMENT ei-ther seasonally or permanently. At woundand infection sites callose provides a phys-ical barrier. When pollen germinates on astigma the growing pollen tubes depositcallose as small plugs, but if the plant isself-incompatible, tubes from its ownpollen will deposit more callose, whichthen blocks the tube, preventing fertiliza-tion. See self-incompatibility.

callus 1. A mass of undifferentiatedparenchyma. If a plant is injured, the sur-rounding uninjured parenchyma cells forma suberin-impregnated callus, sometimes,sealing off the wound.2. In tissue cultures, cells are first de-differentiated to form a callus that can thenbe induced to form adventitious shoots androots. See also graft.

calmodulin A Ca2+-binding proteinfound in high levels in eukaryote cells. Itmediates many calcium-regulated pro-cesses in cells. When bound to calciumions, calmodulin changes shape and bindsto other proteins, such as kinases and phos-phatases, that are involved in signalingpathways, activating or inactivating them.See cascade.

Calvin, Melvin (1911–97) Americanbiochemist noted for his investigations ofthe DARK REACTIONS of photosynthesis

34

C

(using radioactive carbon). He wasawarded the 1951 Nobel Prize for Chem-istry.

Calvin cycle See photosynthesis.

calyptra 1. The root cap: a conical-shaped structure that is formed by the ac-tivity of the meristem at the root apex andforms a protective cap around the root tip.It is constantly replaced by newly formedcells as the older tissue is sloughed off withgrowth of the root through the soil.2. A layer of cells derived from the venterof the archegonium that covers the devel-oping sporophyte. In mosses and liver-worts it ruptures as the seta elongates,being taken up as a hood over the capsulein mosses, and forming the sheath of tissueat the base of the seta in liverworts. Thepresence of the calyptra is necessary for theproper development of the capsule inmosses and the embryo in ferns.

calyptrogen A layer of meristematiccells covering the root apical meristem insome plants (e.g. grasses) that gives rise toand maintains the root cap.

calyx (pl. calices) The outermost part ofa flower, enclosing the other floral partsduring the bud stage. It consists of sepals,often green but sometimes brightly col-ored. The symbol K denotes the calyx inthe floral formula. If the sepals are joinedat their lateral margins a calyx tube isformed, the mouth of which may be ex-tended into lobes or teeth.

CAM See crassulacean acid metabolism.

cambium The ring of dividing cells(meristem) responsible for lateral growthin plants. The primary cambium is found inthe stem and root between the phloem andxylem cells, and by division gives rise to thesecondary phloem and xylem in woody di-cotyledonous plants. There are two zonesof cambium: the VASCULAR CAMBIUM, whichproduces secondary xylem and phloem,and the PHELLEM (cork cambium), whichproduces secondary cortex and cork or

bark. See secondary growth; wood. Seealso annual ring; meristem.

Cambrian The earliest period of the Pa-leozoic era, about 590–510 million yearsago. It is characterized by the appearanceof algae and a proliferation of marine in-vertebrate animal forms, but no trueplants. In Britain, Cambrian rocks arefound in Wales and North-West Scotland.See also geological time scale.

Camerarius, Rudolph Jacob (1665–1721) German botanist. Camerarius wasprofessor of medicine and director of thebotanic gardens at Tubingen. In his exper-iments on sexuality in dioecious plants(with male and female flowers on separateplants) and monoecious plants (with maleand female flowers on the same plant), hedemonstrated that if the female flowerswere isolated from the male flowers or themale flowers were removed, the seeds pro-duced by the female flowers were alwayssterile. His results were published in Desexu plantarum (On the Sex of Plants) in1694. He also described the process of pol-lination and showed that the stamens werethe male organs and that the stigma andstyle were the female organs of a plant.

campylotropous (orthocampylotropous)See ovule.

Canada balsam A yellowish resin withsimilar optical properties to glass, used as amounting medium for microscope slides.

Candolle, Augustin Pyrame de (1778–1841) Swiss botanist. In 1806 the Frenchgovernment invited Candolle to carry out abotanical and agricultural survey of Francethat took him six years to complete. His Théorie élémentaire de la botanique(Elementary Theory of Botany; 1813) in-troduced the term ‘taxonomy’ for classifi-cation. Candolle based his classification onresemblances between the plan of symme-try of the sex organs in various plants, eventhough these were often disguised by loss,fusion, or degeneration. His idea was thateven when these parts appeared to be dif-

35

Candolle, Augustin Pyrame de

ferent, they were in fact homologous andhad a common ancestry. This classificationreplaced that of LINNAEUS, eventually beingimproved upon by BENTHAM and HOOKER.He is also noted for having recognized theinfluence of the soil on the distribution ofvegetation.

cane sugar See sucrose.

canker A localized plant disease inwhich there is considerable NECROSIS of thecortex tissue, especially in woody plants,which becomes surrounded by layers ofcallus tissue. An example is apple canker,caused by the fungus Nectria galligena.

canopy In a woodland, forest, or shrubcommunity, the uppermost layer of vegeta-tion, formed from the branches of thetallest plants. Compare ground layer.

capillarity (capillary action) The rise orfall of liquids in narrow tubes as a result ofthe surface tension of the liquid, whichcauses the water to adhere to solid sur-faces, such as soil particles or the walls ofxylem vessels. When water in the soil or inthe xylem is put under tension, due to theevaporation of water from a particular site(which creates a water potential gradient inthe direction of the evaporation), capillar-ity enables water to overcome the cohesiveforces within the water and move againstgravity in response to the water potentialgradient. It is a key feature in the move-ment of water through the xylem andthrough the finer pores of the soil.

capitulum (pl. capitula) See inflores-cence.

capsid The protein coat of a VIRUS,which surrounds its nucleic acid. Capsidproteins are involved in recognition and in-fection of the host and can be used to iden-tify the virus by serological means oramino acid sequencing.

capsule 1. A dry dehiscent fruit that isformed from several fused carpels. The nu-merous seeds may be released through

pores as in snapdragon (Antirrhinum), orby complete splitting of the capsule (e.g.Iris).2. The structure within which spores of thesporophyte generation of mosses and liver-worts are formed. It is borne at the end ofa stalk (the seta), and ruptures to releasethe spores.3. The mucilaginous covering often foundaround the cell membrane in bacteria.

carbohydrates A class of compoundsoccurring widely in nature and having thegeneral formula type Cx(H2O)y. (Note thatalthough the name suggests a hydrate ofcarbon these compounds are in no way hy-drates and have no similarities to classes ofhydrates.) Carbohydrates are generally di-vided into two main classes: SUGARS andPOLYSACCHARIDES. Simple sugars are mono-saccharides – single sugar units, usuallywith five (pentoses) or six (hexoses) carbonatoms. Ribose and glucose are examples.Oligosaccharides are polymers of two toten monosaccharide units. Polysaccharidesare long linear or branched chains ofmonosaccharide units, which have a struc-tural or storage role in the plant, since theytend to be insoluble and relatively unreac-tive.

Carbohydrates are both stores of en-ergy and structural elements in living sys-tems. Living cells are able to build uppolysaccharides from simple units (an-abolism) or break the larger units down tomore simple units for releasing energy (ca-tabolism).

carbon Symbol: C. An essential elementin plant and animal nutrition that occurs inall organic compounds and thus forms thebasis of all living matter. It enters plants ascarbon dioxide and is assimilated into car-bohydrates, proteins, and fats, forming thebackbones of such molecules. The elementcarbon is particularly suited to such a roleas it can form stable covalent bonds withother carbon atoms, and with hydrogen,oxygen, nitrogen, and sulfur atoms. It isalso capable of forming double and triplebonds as well as single bonds and is thus aparticularly versatile building block. Car-

cane sugar

36

bon, like hydrogen and nitrogen, is farmore abundant in living materials than inthe Earth’s crust, indicating that it must beparticularly suitable to fulfill the require-ments of living processes. See also carboncycle.

carbon cycle The circulation of carbonbetween living organisms and the environ-ment. The carbon dioxide in the atmos-phere is taken up by autotrophic organisms(mainly green plants) and incorporatedinto carbohydrates. The carbohydrates soproduced are the food source of the het-erotrophs (mainly animals). All organismsreturn carbon dioxide to the air as a prod-uct of respiration and of decay. The burn-ing of fossil fuels also releases CO2. Inwater, carbon, combined as carbonatesand bicarbonates, is the source for photo-synthesis.

carbon-14 dating See radiometric dat-ing.

Carboniferous The second most recentperiod of the Paleozoic era, some 355–280million years ago. It is characterized by theevolution on swampy land of amphibians,a few primitive early reptiles, and trees.Aquatic life included sharks and coela-canths. The period is named after the ex-tensive coal deposits that formed from theremains of vast forests of swamp plants.These were dominated by tree-sized lyco-phytes (related to present-day clubmosses)and sphenophytes (horsetails), ferns, andsome early seed plants (mainly pteri-dosperms or ‘seed ferns’) some of whichwhen rapidly buried and compressed overmany millions of years formed the coalmeasures. See also geological time scale.

carboxylase An enzyme that catalyzesthe incorporation of carbon dioxide into asubstrate molecule. Carboxylases are foundin yeasts, bacteria, plants, and animals.The most important carboxylase in plantsis ribulose bisphosphate carboxylase (ru-bisco), which catalyzes the carboxylationof ribulose bisphosphate, the ‘carbon-fixing’ first step in the Calvin cycle.

carboxylic acid An organic compoundof general formula RCOOH, where R is anorganic group and –COOH is the carboxy-late group. Many carboxylic acids are ofbiochemical importance. Those of particu-lar significance are: 1. The lower car-boxylic acids (such as citric, succinic,fumaric, and malic acids), which partici-pate in the respiration in the KREBS CYCLE,and in CRASSULACEAN ACID METABOLISM.Some plants accumulate high concentra-tions of certain carboxylic acids: citrusfruits contain a lot of citric acid, andrhubarb contains oxalic acid.2. The higher acids, which are bound inLIPIDS. These are also called fatty acids, al-though the term ‘fatty acid’ is often used todescribe any carboxylic acid of moderate-to-long chain length. The fatty acids con-tain long hydrocarbon chains, which maybe saturated (no double bonds) or unsatu-rated (C=C double bonds). Plant fattyacids are often unsaturated: oleic acid is thecommonest example.

carcerulus (pl. carceruli) A type of cap-sular fruit that breaks up at maturity intoone-seeded segments or nutlets. Thecarcerulus is typical of the Lamiaceae fam-ily, e.g. deadnettle (Lamium).

carnivorous plant (insectivorous plant)A plant adapted to supplementing photo-synthesis by obtaining nutrients, especiallynitrates, from insects and other small ani-mals. The plants have various means oftrapping and killing the insects, which arethen digested by enzymes secreted by theplant. Examples include butterworts (Pin-guicula), with slippery inrolling leaves;sundews (Drosera), with sticky glandularhairs that close over the trapped insects;Venus’ fly trap (Dionaea muscipula), withhinged leaves that snap shut over the vic-tim, triggered by sensitive hairs; andpitcher plants, with leaves modified toform pitchers into which small animals falland drown.

carotene A carotenoid pigment, exam-ples being lycopene, and a- and b-carotene.The latter compounds are important in an-

37

carotene

imal diets as a precursor of vitamin A. Red,yellow, or orange carotene pigments occurin all photosynthetic cells, where they func-tion as accessory pigments by screeningother photosynthetic pigments from photo-oxidation. Some carotenes give brightcolors to animal-dispersed fruits, e.g. lyco-pene in tomatoes.

carotenoids A group of yellow, orange,or red pigments comprising the carotenesand xanthophylls. They are found in allphotosynthetic organisms, where theyfunction mainly as accessory pigments inphotosynthesis. They contribute, with an-thocyanins, to the autumn colors of leavessince the green pigment chlorophyll, whichnormally masks the carotenoids, breaksdown first. They are also found in someflowers and fruits, e.g. tomato. Carot-enoids have three absorption peaks in theblue–violet region of the spectrum.

carpel The female reproductive organ ofa flowering plant. It consists of an ovary,containing one or more ovules, a stalk, orstyle (absent in some plant groups), and aterminal receptive surface, the stigma.Each flower may have one or more carpelsborne singly giving an apocarpous gynoe-cium or fused together giving a syncarpousgynoecium. The carpel is homologous withthe megasporophylls of certain spore-bear-ing vascular plants (pteridophytes) and theovuliferous scales of gymnosperms. It hasevolved by fusion of the two edges of themegasporophyll. This development can bemost clearly seen in simple carpels such asthose of the Fabaceae.

carrier 1. An organism that carries a re-cessive, often harmful, gene masked in thephenotype by a normal dominant gene.2. An individual infected with pathogenicmicroorganisms without showing symp-toms of disease. Such carriers can transmitthe infection to others.

caruncle A fleshy outgrowth from aseed, similar to, but smaller than, an aril.Caruncles are often waxy or oily. Theymay arise from the placenta, micropyle, or

funicle and are seen in castor oil seeds(Ricinus) at the micropyle and in violetseeds (Viola) at the hilum. The oily carun-cles of violet seeds attract ants, which helpto disperse them.

caryopsis (grain) (pl. caryopses) A dryindehiscent fruit, typical of the grasses. It issimilar to an ACHENE except that the ovarywall is fused with the seed coat.

cascade A sequence of biochemical reac-tions, each triggered by the previous one.Cascades are important in signaling path-ways, where extracellular signals (such ashormones or ions) bind with receptors onthe cell membrane, triggering changes inmembrane proteins that activate proteinson the cytoplasmic side of the membrane,setting off a chain of reactions.

Casparian strip An impervious band ofthickening on the radial and transversewalls of the endodermis. It consists of de-posits of suberin and cutin and ensures thatall the water and solutes entering the stelepass through the cytoplasm of the endo-dermal cells, thus allowing for selective fil-tering of solutes and control of flow rate.

catabolism Metabolic reactions in-volved in the breakdown of complex mol-ecules to simpler compounds as, forexample, in respiration. The main functionof catabolic reactions is to provide energy,which is stored as ATP, for use in the syn-thesis of new structures, cell-to-cell sig-nalling, or the movement of motile cells.See also metabolism. Compare anabolism.

catalase An enzyme present in bothplant and animal tissues that catalyzes thebreakdown of hydrogen peroxide, a toxiccompound produced during metabolism,into oxygen and water. It also catalyzes theoxidation of substrates by hydrogen perox-ide. Catalase is found in the cytoplasm insmall organelles called peroxisomes.

catalyst A substance that increases therate of a chemical reaction without beingused up in the reaction. Enzymes are highly

carotenoids

38

efficient and specific biochemical catalysts.They lower the activation energy needed toinitiate reaction by combining temporarilywith the reactants.

cation A positively charged ion, formedby removal of electrons from atoms ormolecules. In electrolysis, cations are at-tracted to the negatively charged electrode(the cathode). Compare anion.

catkin See inflorescence.

C2 cycle See photorespiration.

cdl See critical day length.

cell The basic unit of structure of all liv-ing organisms, excluding viruses. Cellswere discovered by Robert HOOKE in 1665,but SCHLEIDEN and SCHWANN in 1839 werethe first to put forward a clear cell theory,stating that all living things were cellular.Prokaryotic cells (typical diameter 1 m m)are significantly smaller than eukaryoticcells (typical diameter 20 m m). The largestplant cells are the fibers of false nettles(Boehmeria nivea), up to 550 mm in lengthand stinging nettles (Urtica dioica) up to 75 mm long; the smallest are the epider-mal cells of roses (0.04 mm in diameter).Among algae, the internode cells ofstoneworts (Chara spp.) reach lengths upto 80 mm, while the individual indepen-dent cells of Chlamydomonas are only0.092 mm in diameter. All cells contain, oronce contained genetic material in the formof DNA, which controls the cell’s growthand some of its activities; in eukaryotes thisis enclosed in the membrane-bounded nu-cleus. All contain cytoplasm, containingvarious organelles and are surrounded by aplasma membrane. This controls entry andexit of substances. Plant cells and mostprokaryotic cells are surrounded by rigidcell walls. In multicellular organisms cellsbecome specialized for different functions;this is called differentiation. Within thecell, further division of labor occurs be-tween the organelles.

cell cycle The ordered sequence of

phases through which a cell passes leadingup to and including cell division (mitosis).It is divided into four phases G1, S, and G2(collectively representing interphase), andM-phase, during which mitosis takes place.Synthesis of messenger RNA, transferRNA, ribosomes, and protein and generalgrowth of the cell occurs in G1, and repli-cation of DNA and synthesis of histonesoccurs during the S phase, as chromosomesdivide to form chromatids. The materialsrequired for spindle formation are formedin G2, when there is a further burst ofgrowth before prophase begins. The timetaken to complete the cell cycle varies indifferent tissues.

cell division See meiosis; mitosis; ami-tosis.

cell fractionation The separation of thedifferent constituents of the cell into ho-mogenous fractions. This is achieved bybreaking up the cells in a mincer or grinderand then centrifuging the resultant liquid.The various components settle out at dif-ferent rates in a centrifuge and are thus sep-arated by appropriately altering the speedand/or time of centrifugation. The compo-nents are thus separated according to theirsize and weight.

cell lineage The theory stating that cellsarise only from pre-existing cells. The celllineage of a structure traces the successivestages that the cells pass through from thetime of their formation in the zygote totheir appearance in the mature functionalstructure.

cell membrane See plasma membrane.

cellobiose See cellulase.

cell plate A structure that appears inlate anaphase in dividing plant cells and isinvolved in formation of a new cell wall atthe telophase stage of mitosis. It is formedby fusion of vesicles from the Golgi appa-ratus, resulting in a flattened membrane-bounded sac spreading across andeffectively dividing the cell. Cell-wall poly-

39

cell plate

saccharides and pectic substances con-tained in the vesicles contribute to growthof the new wall and middle lamella insidethe sac. The process begins at the phrag-moplast, a barrel-shaped region at the for-mer site of the spindle equator, wheremicrotubules remain and the vesicles clus-ter, possibly oriented by the microtubules.Endoplasmic reticulum and ribosomes arealso present. The cell-plate membraneseventually form the two new plasma mem-branes of the daughter cells. See cytoki-nesis.

cell theory The theory that all organ-isms are composed of CELLS and cell prod-ucts and that growth and developmentresults from the division and differentia-tion of cells.

cellular slime molds See Acrasiomy-cota.

cellulase An enzyme that hydrolyzes1,4-glycosidic linkages in cellulose, yield-ing cellobiose (a disaccharide) and glucose.It is important in the degradation of plantcell walls in living plants (e.g. in leaf ab-scission).

cellulose A polysaccharide forming theframework of the CELL WALLS of all plants,many algae and other protoctists, andsome fungi. Cellulose molecules are un-branched chains that together form a rigidstructure of high tensile strength. Bundlesof molecules form microfibrils, which maybe aligned in the primary cell wall eithertransversely or longitudinally. Celluloseforms an important source of carbohydratein the diets of herbivores, and is a majorconstituent of dietary fiber in human diets.The individual units in cellulose are b-1,4linked D-glucose molecules.

cell wall A rigid wall surrounding thecells of plants, fungi, bacteria, and algae.Plant cell walls are made of cellulose fibersin a cementing matrix of other polysaccha-rides. The walls of some algae differ, e.g.the silica boxes enclosing diatoms, and thecalcareous layer on the cell walls of cal-

careous algae. Fungi differ, with their wallsusually containing chitin. The walls ofsome fungal spores contain the pigmentmelanin, which protects against the dam-aging effects of ultraviolet radiation. Bacte-rial walls are more complex, containingpeptidoglycans – complex polymers ofamino acids and polysaccharides. Cellwalls are freely permeable to gases, water,and solutes. They have a mechanical func-tion, allowing the cell to become turgid byosmosis, but preventing bursting. This con-tributes to the support of herbaceousplants. Plant cell walls can be strengthenedfor extra support by addition of lignin (asin xylem and sclerenchyma) or extra cellu-lose (as in collenchyma). They are an im-portant route for movement of water andmineral salts. Other modifications includethe uneven thickening of guard cells, thesieve plates in phloem, the suberization ofendodermal cells, the waterproof coveringsof epidermal and cork cells.

The cellulose in cell walls takes theform of microfibrils. At cell division inplants the primary wall is laid down on themiddle lamella of the cell plate as a loosemesh of cellulose microfibrils. This givesan elastic structure that allows cell expan-sion during growth. Later the secondarywall grows and acquires greater rigidityand tensile strength. New cellulose fibersare laid down in layers, parallel withineach layer, but orientated differently in dif-ferent layers. The Golgi apparatus providespolysaccharide-filled vesicles that depositwall material by exocytosis, guided by mi-crotubules.

Cenozoic (Caenozoic; Cainozoic) Thepresent geological era, beginning some 65million years ago, and divided into two pe-riods, the Tertiary and the Quaternary. It ischaracterized by the rise of modern organ-isms, especially mammals and floweringplants. During this era, the world’s tallestmountain ranges such as the Rockies,Andes, European Alps and Himalayaswere formed, with dramatic effects on cli-mate and plant distribution. See also geo-logical time scale.

cell theory

40

centrarch Denoting a protostele inwhich protoxylem is at the center of theaxis. Compare endarch; exarch; mesarch.

centric bundle See concentric bundle.

centrifugal Developing from the centeroutwards so the youngest structures are atthe outer edge. Centrifugal xylem differen-tiates from the center outwards, and a cen-trifugal inflorescence (e.g. the dichasialcyme) is one in which the progression offlower opening is from the center to theperiphery. Compare centripetal. See alsoacropetal; basipetal.

centriole A cell organelle consisting oftwo short tubular structures orientated atright angles to each other. Each tubularstructure is about 500 nm long and 150 nmin diameter. The centriole lies outside thenucleus of animal cells and many fungaland protoctist cells, but is absent in cells ofmost higher plants. Prior to cell division itreplicates, and the sister centrioles move toopposite ends of the cell to lie within thespindle-organizing structure, the centro-some. At the start of cell division, fibrilsappear to radiate from the centrioles,forming starlike ASTERS. These form thepoles of the developing spindle. However,the centriole is not essential for spindle for-mation, although an analogous structure,the basal body, is responsible for organiz-ing the microtubules of UNDULIPODIA.

centripetal Developing from the outsideinwards so the youngest structures are atthe center. Centripetal xylem differentiatesfrom the outside toward the center, and acentripetal inflorescence (e.g. the capitu-lum) is one in which the outer flowers openbefore those in the center. Compare cen-trifugal. See also acropetal; basipetal.

centromere The region of the chromo-some that contains the kinetochore, thestructure that becomes attached to the nu-clear spindle during mitosis and meiosis. Itcontains DNA of no known significance.Following the replication of chromosomes,resultant chromatids remain attached at

the centromere. The centromere is a spe-cific genetic locus and remains relativelyuncoiled during prophase, appearing as aprimary constriction. It does not stain withbasic dyes.

centrosome A structure found in all eu-karyotic cells, except fungi, that forms theSPINDLE during cell division. It lies close tothe nucleus in nondividing cells. Micro-tubules extend across the cell from the cen-trosome, which controls the distributionand assembly of the microtubules thatmake up the cytoskeleton, thus controllingthe cell’s shape and, in some cells, move-ment. The centrosome probably plays animportant role in the reorganization of thecell that occurs during differentiation. Atthe start of cell division the centrosome di-vides, and the sister centrosomes move toopposite ends of the cell, organizing thespindle. In animal and protoctist cells thecentrosome contains two short barrel-shaped structures, the CENTRIOLES, butthese are not directly involved in spindleformation. In fungi, the function of thecentrosome is served instead by the spindlepole body.

chalaza (pl. chalazae or chalazas) Theregion of an angiosperm ovule where thenucellus and integuments merge. Whenovule orientation is orthotropous the cha-laza corresponds to the point where the fu-nicle is attached but in anatropous andcampylotropous ovules the chalaza is somedistance from the funicle.

chalazogamy A method of fertilizationin angiosperms in which the pollen tube en-ters the ovule by the chalaza instead ofthrough the micropyle, penetrating the pla-centa en route. Chalazogamy is seen in cer-tain trees and shrubs, e.g. beech. Compareporogamy.

chamaephyte A perennial plant that isable to produce new growth from restingbuds near (within 0.25 m of) the soil sur-face. Chamaephytes are usually smallbushes (e.g. heather (Calluna vulgaris)).See also Raunkiaer’s plant classification.

41

chaparral

chaparral Sclerophyllous scrub vegeta-tion of coastal and inland mountain re-gions of southwestern North America,maintained by burning and grazing and, insome areas, possibly from past land clear-ance. It consists of dense thickets of broad-leaved evergreen bushes and shrubs andtwisted dwarf trees up to 2.5 meters (8 ft)tall. The dominant plants are sages (Lami-aceae) and evergreen oaks and, in drierareas, chamise (Adenostoma fascicula-tum), manzanita (Arctostaphylos pun-gens), and species of Ceanothus. Seemaquis. Compare garrigue.

Charophyceae A class of the Chloro-phyta (green algae) that includes thestoneworts, formerly placed in a separatephylum Charophyta. They resemble trueplants in many ways and are thought to bethe most closely related algal group toplants.

Charophyta See Charophyceae.

chasmogamy The production of flow-ers that open their petals so that cross pol-lination is possible. Compare cleistogamy.See also anemophily; entomophily.

chelation The process of forming a sta-ble chemical complex (chelate) with agiven substance, usually a metal ion, thatinvolves the formation of at least twochemical bonds. Chelates are usually or-ganic ring compounds, the ion beingchelated to form the closure of the ring.Chelating agents can be used to removetoxic metal ions from soil or water. Someplants achieve tolerance to toxic ions bychelating them, usually by combinationwith an organic compound, such as citrateor malate; the chelate is then deposited inthe vacuole. Chlorophyll is also a chelate,the porphyrin part of the molecule beingbound to iron.

chemical fossils Particularly resistantorganic chemicals present in geologicalstrata that are thought to indicate the exis-tence of life in the period when the rockswere formed. Chemical fossils (e.g. alkanes

and porphyrins) are often the only evi-dence for life in rocks of PRECAMBRIAN age.

chemiosmotic theory The theory pro-posed by Peter Mitchell in 1961 to explainhow electron transport and ATP synthesisare coupled during photosynthesis and res-piration. It requires a membrane that per-mits the movement of protons from insideto outside, but not vice versa. An energy-requiring ‘proton pump’ (the ELECTRON-TRANSPORT CHAIN) causes movement ofprotons (H+ ions) from the internal matrixof the organelle to the outside, creating apH gradient across the membrane. As theH+ ions diffuse back across the membrane.This gradient of potential energy drives re-actions at specific sites on the membrane,where the enzyme ATPase uses the energyto drive ATP synthesis, so the energy canbe transferred to reaction sites elsewhere.

chemoautotrophism (chemosynthesis)See autotrophism; chemotrophism.

chemoheterotrophism See chemotro-phism; heterotrophism.

chemoreceptor An organ or structurethat responds to a particular chemicalstimulant.

chemosynthesis See autotrophism; che-motrophism.

chemotaxis (chemotactic movement) ATAXIS in response to a chemical concentra-tion gradient. The spermatozoids of primi-tive plants are often positively chemotactic,swimming toward the female organs in re-sponse to a chemical secreted by the latter.For example, the archegonium (femaleorgan) of the moss Funaria secretes su-crose. Motile organisms may also be nega-tively chemotactic, swimming away fromconcentrations of toxic chemicals.

chemotaxonomy (biochemical systemat-ics; biochemical taxonomy) A system ofclassification of plants based on the natureof their metabolic products (metabolites).Techniques involved include chromatogra-

Charophyceae

42

phy and electrophoresis, and the sequenc-ing of nucleic acids and proteins.

chemotrophism A type of nutrition inwhich the source of energy for the synthe-sis of organic requirements is chemical.Most chemotrophic organisms are het-erotrophic (i.e. chemoheterotrophic) andtheir energy source is always an organiccompound; animals, fungi, and most bac-teria are chemoheterotrophs. If auto-trophic (i.e. chemoautotrophic or chem-osynthetic) the energy is obtained byoxidation of an inorganic compound; forexample, by oxidation of ammonia to ni-trite or a nitrite to nitrate (by nitrifyingbacteria), or oxidation of hydrogen sulfideto sulfur (by colorless sulfur bacteria).Only certain specialized bacteria are chemo-autotrophic. Compare phototrophism.

chemotropism (chemotropic movement)A TROPISM in which the stimulus is chemi-cal. The hyphae of certain fungi (e.g.Mucor) are positively chemotropic, grow-ing toward a particular source of food.Pollen-tube growth down the style is bothpositively and negatively chemotropic: thetube grows toward chemicals secreted bythe pistil, and away from atmospheric oxy-gen.

chernozem (black earth) A freely drain-ing CALCAREOUS SOIL found in regions withhigh summer rainfall, characterized byhumus distributed fairly evenly throughoutthe profile. The upper horizon of the soil isvery thick and black with humus; it mergesinto a brown lime-rich horizon below.Chernozems are rich in plant nutrients andhave a good crumb structure, allowingdrainage and aeration. They are associatedwith grasslands in temperate climates suchas the prairies of North America, the pam-pas of South America, and the Russiansteppes, where they support highly produc-tive agricultural systems.

chiasma (pl. chiasmata) A connectionbetween homologous chromosomes seenduring the prophase stage of meiosis. Chi-asmata represent a mutual exchange of

material between homologous, nonsisterchromatids (CROSSING OVER) and provideone mechanism by which RECOMBINATION

occurs, through the splitting of linkagegroups. Chiasmata also help to hold ho-mologs together until the start ofanaphase, thus helping to ensure the cor-rect distribution of chromosomes into thedaughter nuclei.

chimera An individual or part of an in-dividual in which the tissues are a mixtureof two genetically different tissues. It mayarise naturally due to mutation in a cell of a developing embryo, producing a lineof cells with the mutant gene, and hence different characteristics compared tosurrounding cells. It may also be inducedexperimentally. In plants, chimaeras pro-duced from two different species areknown as graft hybrids. For example, abud may develop at the junction betweenthe scion and stock with a mixture of tis-sues from both.

chi-squared test A statistical test usedto compare observed frequencies of exper-imental sampling results with expected(hypothetical) frequencies, usually calcu-lated from the results assuming no effect ofthe experimental variable. It is suitable forobservations that fall into discrete classes.The value:

(observed – expected)2/expectedis calculated for each class, then the valuesare added together. The sum is checkedagainst a table of c2 values at various de-grees of freedom to determine the degree ofsignificance of the difference between ob-served and expected values is significant.

chitin A nitrogen-containing polysac-charide found in the cell walls of mostfungi and in some animals. Chitin is a poly-mer of N-acetylglucosamine. It consists ofmany glucose units, in each of which one ofthe hydroxyl groups has been replaced byan acetylamine group (CH3CONH).

chlamydospore A thick-walled restingspore produced by certain fungi (e.g.

43

chlorinated hydrocarbon

smuts) and funguslike protoctists (e.g.Mucor).

Chlorella See Chlorococcales.

chlorenchyma A form of parenchymain which the cells contain many chloro-plasts and relatively large intercellularspaces, as in the mesophyll of the leaf.

chlorinated hydrocarbon A hydrocar-bon in which one or more hydrogen atomsare replaced by chlorine atoms. Some chlo-rinated hydrocarbons are used as insecti-cides, e.g. aldrin, dieldrin, and DDT.

chlorine Symbol: Cl. An element foundin trace amounts in plants. The chlorideion (Cl-) can pass easily through cell mem-branes, and is involved in the maintenanceof osmotic and ionic balance in the plant.

Chlorococcales An order of Chloro-phyta (green algae) that contains single-celled and colonial algae that lackundulipodia, e.g. Chlorella, Volvox, foundin freshwater plankton and most terrestrial

habitats. Only the gametes and zoosporesare motile. They reproduce asexually bymeans of zoospores or nonmotile sporesand sexually by fusion of similar-lookinggametes (isogamy).

chlorophylls A group of photosyntheticpigments. They absorb blue-violet and redlight and hence reflect green light, impart-ing the green color to green plants. Themolecule consists of a hydrophilic (water-loving) head, containing magnesium at thecenter of a porphyrin ring, and a long hy-drophobic (water-hating) hydrocarbon tail(the phytol chain), which anchors the mol-ecule in the lipid of the membrane. Differ-ent chlorophylls have different chemicalgroups attached to the head. Chlorophyllsare located in the chloroplasts, and are in-volved in the light reactions of photosyn-thesis, absorbing light energy and initiatingelectron transport. There are four kinds ofchlorophyll, a, b, c, and d, which differ intheir molecular structure. Chlorophyll a isuniversal in all photosynthesizing eukary-otes. It converts the absorbed light energyto chemical energy. Chlorophylls b, c, and

chlorine

44

Chlorophyll: the structural formula of chlorophyll a

d are accessory pigments. Chlorophyll boccurs in plants and in the Chlorophyta;the other chlorophylls occur in certaingroups of algae. See absorption spectrum;photosynthetic pigments. See also bacteri-ochlorophyll.

Chlorophyta (green algae) A phylum ofprotoctists comprising mainly freshwateralgae with some marine and terrestrialforms. They contain the pigments chloro-phyll a and b together with carotenes andxanthophylls. The Chlorophyta store foodas starch and fat and have cell walls con-taining cellulose and hemicellulose. Someof the commoner orders include: Volvo-cales, unicellular and colonial plants, e.g.Chlamydomonas, Volvox; Chlorococcales,including unicellular and colonial plants,e.g. Chlorella, Pediastrum; Ulotrichales,filamentous, and thallose plants, or coeno-cytic tubes, e.g. Ulothrix, Ulva, Entero-morpha; and Oedogoniales, e.g. thefilamentous Oedogonium.

chloroplast A photosynthetic PLASTID

containing chlorophyll and other photo-synthetic pigments. It is found in all photo-synthetic cells of plants and protoctists butnot in photosynthetic prokaryotes. It has amembrane system containing the pigmentsand on which the light reactions of PHOTO-SYNTHESIS occur. The surrounding gel-likeground substance, or stroma, is where thedark reactions occur. The typical higher-plant chloroplast is lens-shaped and about5 m m in length. Various other forms existin the algae, e.g. spiral in Spirogyra, stellatein Zygnema, and cup-shaped in Chlamy-domonas. The number per cell varies, e.g.one in Chlorella and Chlamydomonas andin most hornworts (Antherocyta), two inZygnema, and about one hundred in pal-isade mesophyll cells of leaves.

Chloroplast membranes form elon-gated flattened fluid-filled sacs calledthylakoids. The sheetlike layers of the thy-lakoids are called lamellae. In all plants ex-cept algae, the thylakoids overlap atintervals to form stacks, like piles of coins,called grana. In this way the efficiency ofthe light reactions seems to be improved.

On the thylakoid membranes are sphericalstructures partially embedded in the mem-brane. These are the photosynthetic units(formerly called quantasomes), which con-tain the pigments and associated lipids andproteins of photosystems I and II. Photo-system I units are about 17.5 nm in diame-ter, and photosystem II units about 11 nm.Other, smaller particles on the thylakoidmembranes are ATPases. The stroma maycontain storage products of photosynthe-sis, e.g. starch grains. Apart from enzymesof the dark reactions, the stroma also con-tains typical prokaryotic protein-synthesiz-ing machinery including circular DNA (seechloroplast DNA) and smaller ribosomes.There is now strong evidence that chloro-plasts and other cell organelles, such asmitochondria, represent prokaryotic or-ganisms that invaded heterotrophic eu-karyotic cells early in evolution and arenow part of an indispensable symbioticunion (see endosymbiont theory). In C4plants there are two types of chloroplast.

chloroplast DNA (cp-DNA) The DNAfound in a chloroplast. It is in circularform, rather like that of prokaryotes. Itcodes for only some of the proteins neededby the chloroplast.

chlorosis The loss of chlorophyll fromplants resulting in yellow (chlorotic)leaves. It may be the result of the normalprocess of senescence, lack of key mineralsfor chlorophyll synthesis (particularly ironand magnesium), or disease.

chlor-zinc-iodide See Schultze’s solu-tion.

chromatid One of a pair of replicatedchromosomes found during the prophaseand metaphase stages of mitosis and meio-sis. During mitosis, sister chromatids re-main joined by their centromere untilanaphase. In meiosis it is not untilanaphase II that the centromere divides,the chromatids being termed daughterchromosomes after separation.

chromatin The loose network of

45

chromatin loop

threads seen in nondividing nuclei that rep-resents the chromosomal material, consist-ing of DNA, protein (mainly histone), andsmall amounts of RNA. It is classified aseuchromatin or heterochromatin on thebasis of its staining properties, the latterstaining much more intensely with basicstains because it is more coiled and com-pact. Euchromatin is thought to be activelyinvolved in transcription and thereforeprotein synthesis, while heterochromatin isinactive. Euchromatin stains more inten-sively than heterochromatin during nucleardivision. See also chromosome.

chromatin loop A region of uncoiledDNA, usually about 200 nm long, that ex-tends out in a loop from the chromosome.This uncoiled condition is thought to facil-itate TRANSCRIPTION and probably indicatesan area where genes are active.

chromatography A method of analyz-ing materials involving the separation byselective absorption of the various com-pounds as identifiable bands. For instance,a mixture of substances in solution ispassed slowly down a long column packedwith alumina. The different compoundsmove at different rates and separate intobands. The medium, together with thebands of separated material, are collec-tively called a chromatogram.

In general chromatography involves atest material being carried by a movingphase (liquid or gas) through a stationaryphase (solid or liquid). Different sub-stances move at different rates (dependingon their absorption–desorption) and aretherefore spatially separable, the leastreadily absorbed being carried the farthest.Colorless materials can be used if somemeans of detecting them is used (electronicdetection, radioactive labeling, or ninhy-drin developer). See also paper chromatog-raphy; thin-layer chromatography; gaschromatography; gel filtration.

chromatophore A name generally ap-plied to a pigment-bearing structure, usu-ally a membrane lamella or vesicle.

chromomere A small beadlike deeplystaining structure seen in chromosomesduring prophase of mitosis and meiosis. Itis thought to represent an area where thechromosome is relatively tightly coiled.Homologous chromosomes have chromo-meres in identical places along their length.

chromoplast (chromatophore) A col-ored plastid, i.e. one containing pigment.They include chloroplasts, which containthe green pigment chlorophyll and aretherefore photosynthetic, and nonphoto-synthetic chromoplasts. The term is some-times confined to the latter, which are bestknown in flower petals, fruits (e.g.tomato), and carrot roots. They are yellow,orange, or red owing to the presence ofcarotenoid pigments.

chromosome 1. One of a group ofthreadlike structures of different lengthsand shapes in nuclei of eukaryotic cells.They consist of DNA with RNA and pro-tein (mostly histones) and carry the GENES.During nuclear division the chromosomesare tightly coiled and are easily visiblethrough the light microscope. After divi-sion, they uncoil and may be difficult tosee, the long narrow threads of uncoiledDNA being only about 2 nm in diameter.Along the length of the uncoiled chromo-somes are beadlike structures called nu-cleosomes, highly organized aggregationsof DNA and histones.

The number of chromosomes per nu-cleus is characteristic of the species exceptin aneuploid plants. Normally one set(haploid) or two sets (diploid) of chromo-somes are present in the nucleus. However,many or perhaps a majority of plants arepolyploid in origin, i.e. they have multiplesets of chromosomes, though most act asdiploids in their reproduction. In somespecies there are diploid and polyploid va-rieties that look almost identical, thoughsometimes they may be referred to as sepa-rate species. In bryophytes the main plantbody is haploid, as is the case in manyalgae (see alternation of generations). Inearly prophase of mitosis and laterprophase of meiosis, the chromosomes

chromatography

46

split lengthwise into two identical chro-matids held together by the CENTROMERE.The length of the arms of the chromosomeis determined by the position of thecentromere. The centromere contains thekinetochore, a structure involved in the at-tachment of the chromosomes to the spin-dle during nuclear division. In diploid cells,there is a pair of sex chromosomes; the re-mainder are termed autosomes. Each chro-mosome contains one DNA molecule,which is folded and coiled. Histones play amajor architectural role, holding the coiled(helical) DNA in supercoils or beads, about10 nm in diameter and consisting of about170 base pairs. These in turn are coiled toform a hollow fiber, 30 nm in diameter,which itself forms a hollow coil measuringabout 240 nm in diameter. It is this that isvisible as the typical looped threadlikechromatin material of the nondividingchromosome. See chromomere; chromo-some map.2. See bacterial chromosome.

chromosome map (genetic map) A dia-gram showing the order of genes along achromosome. Such maps have traditionallybeen constructed from information gainedby linkage studies to give a linkage map.The techniques employed differ accordingto the type of organism being studied.Many plants and animals can be crossedexperimentally to study inheritance pat-terns of particular genes. However, the ad-vent of new molecular techniques hasdramatically changed the nature of chro-mosome mapping in all organisms. Thegrowing science of PROTEOMICS seeks to an-alyze the genome by starting with the pro-tein products of genes and working back tothe genome.

Mapping a genome involves severalsteps. The first is to assign each gene to a particular chromosome. This can beachieved by, for example, somatic-cellhybridization or using a gene probe. Thenext step is to determine the relative positions of the genes on a particular chro-mosome. This involves comparing RESTRIC-TION FRAGMENT LENGTH POLYMORPHISMS

between individuals and constructing a

linkage map of all restriction sites, i.e. sitesthat are cleaved by restriction enzymes.These restriction sites can then be used asmarkers for closely neighboring genes. Thelast step is to construct a physical map ofthe base sequence of the chromosomalDNA. One approach uses cloned DNAsegments obtained from a gene library ofthe chromosome. These clones can be fit-ted together to form a series of overlappingsegments (contig) that corresponds to aparticular region of the chromosome. Thebase sequence of the contig is then deter-mined, and hence the base sequence of thechromosomal DNA. See gene library; re-striction map.

chromosome mutation A change inthe number or arrangement of genes in achromosome. If chromosome segmentsbreak away during nuclear division theymay rejoin the chromosome the wrongway round, giving an inversion. Alterna-tively, they may rejoin a different part ofthe same chromosome, or another chromo-some, giving a translocation. If the segmentbecomes lost, this is termed a deficiency ordeletion; it is often fatal. A part of a chro-mosome may be duplicated and occur ei-ther twice on the same chromosome or ontwo different nonhomologous chromo-somes: this is a duplication. Chromosomemutations can occur naturally but their fre-quency is increased by the effect of x-raysand chemical mutagens. Such aberrationsmay be visible by microscopy, especiallywith the use of certain stains. See also genemutation.

chromosome number The number ofchromosomes possessed by a particularspecies. It may be expressed as either thehaploid (n) or diploid (2n) number. Formany species, chromosome number is auseful taxonomic character. Exceptions in-clude species exhibiting APOMIXIS. Chro-mosome number also gives an indication ofpolyploidy, where numbers vary by exactmultiples between related species.

Chrysomonada A phylum of golden-brown algae, formerly called the Chryso-

47

cilium

phyta, common in fresh temperate waters,with a few species in marine waters.Chrysomonads are mainly motile unicellu-lar algae, but there are some colonial formsand a few complex filamentous forms.

ciliate 1. Describing a structure, such themargin of a leaf, fringed with fine hairs.2. A unicellular protoctist of the phylumCiliophora, whose members are usuallycovered in short undulipodia (cilia) andhave two different kinds of nuclei, mi-cronuclei and macronuclei.

cilium (pl. cilia) A whiplike extension ofcertain eukaryotic cells that beats rapidly,thereby causing locomotion or movementof fluid over the cell. Cilia are short UN-DULIPODIA, 2–10 m m long.

The beat of each cilium comprises an ef-fective downward stroke followed by agradual straightening (limp recovery). Ciliabeat in such a way that each is slightly outof phase with its neighbor (metachronalrhythm), thus producing a constant ratherthan a jerky movement or flow of fluid.The basal bodies of cilia are connected bythreadlike strands (neuronemes), which co-ordinate the beating of neighboring cilia.

circadian rhythm (diurnal rhythm) Adaily rhythm of various metabolic activi-ties in animals and plants. Such rhythmspersist even when the organism is not ex-posed to 24-hour cycles of light and dark,and are thought to be controlled by anendogenous biological clock. Circadianrhythms are found in the most primitiveand the most advanced of organisms. ThusEuglena shows a diurnal rhythm in thespeed at which it moves to a light source.Stomata open and close in a daily cycle,which persists for several days in totaldarkness. Experiments have failed to revealthe type and location of the control mech-anisms involved. See also biorhythm.

circinate See vernation.

circumnutation The spontaneous spi-ral growth of the shoot tips of certainplants, particularly climbers. The directionof rotation is often constant in a species;

for example, Convolvulus always rotatesin an anticlockwise direction. For climbersnutation increases the likelihood of a solidsupport being found. It is also marked inmany leaf tendrils and to a lesser extent inroots, flower stalks, and the sporangio-phores of some fungi. Gravity has beenshown to be involved in the response: if aplant is turned upside down, the tip and thelast few spirals unwind and straighten outbefore winding up in the characteristic wayin the new direction.

cisterna (pl. cisternae) A flattened mem-brane-bounded sac of the endoplasmicreticulum or the Golgi apparatus, being thebasic structural unit of these organelles.

cis–trans effect (complementation test)The phenomenon resulting from recombi-nation within a gene (cistron), in which amutation is only expressed in the pheno-type if the mutant pseudoalleles are on dif-ferent homologous chromosomes.

cis–trans test A test that determineswhether two mutations that have the sameeffect occur in the same gene or in differentgenes. The mutations may be in either thecis position (i.e. on the same chromosome)or the trans position (one on each ho-molog). If the mutations are in differentgenes then a normal phentotype resultswhether the mutations are in the cis ortrans position, since they are masked bycorresponding dominant genes on theother homolog. However, if the mutationsare in the same gene then a normal pheno-type will result only if the mutations are inthe cis position. In the trans position themutant phenotype is expressed, since bothalleles of the locus are mutants.

cistron A unit of function, i.e. a segmentof DNA that determines a single polypep-tide chain of a protein molecule or a mes-senger RNA molecule. Its boundaries maybe defined by the cis–trans test.

CITES The Convention on Interna-tional Trade in Endangered Species of WildFauna and Flora: an agreement signed be-tween some 160 countries, prohibiting

circadian rhythm

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commercial trade in endangered speciesand products derived from them. As morespecies have become endangered, the list ofprotected species has lengthened. CITESsets out degrees of protection for differentspecies, allowing threatened but not en-dangered species to be exported and im-ported under license.

citric acid A six-carbon carboxylic acid,occurring in the juice of citrus fruits, par-ticularly lemons, and present in manyother fruits. Citric acid is biologically im-portant because it participates in the KREBS

CYCLE, where it is formed by the conden-sation of acetyl CoA and oxaloacetate, a rate-limiting step of the cycle. The sys-tematic name is 2-hydroxypropane-1,2,3-tricarboxylic acid

citric acid cycle See Krebs cycle.

cladistics A method of classification inwhich the relationships between organismsare based on selected shared characteris-tics. These are generally assumed to havebeen derived from a common ancestor, inthe evolutionary process of cladogenesis,although the ‘transformed cladists’ believethat shared characteristics alone provide a logical basis for classification withoutpostulating evolutionary relationships. Thepatterns of these shared characteristics aredemonstrated in a branching diagramcalled a cladogram. The branching pointsof the cladogram may be regarded either asan ancestral species (as in an evolutionarytree) or solely as representing shared char-acteristics.

cladode A modified internode of thestem that functions as a leaf, being flat-tened and highly photosynthetic. It may bea xerophytic adaptation and is seen inbutcher’s broom (Ruscus aculeatus) andprickly pear (Opuntia). The true leavesmay be reduced to spines or scales. Unlikea leaf, a cladode has buds on its surface.

cladogram See cladistics.

Cladophorales An order of Chloro-

phyta (green algae), generally calledsiphonaceous algae. They are multicellularsaclike forms or branched filaments withbranching rhizoidlike holdfasts, e.g. Cla-dophora.

class A taxonomic rank that is subordi-nate to a phylum (or sometimes a divisionin plant taxonomy) and superior to anorder. The Latin names of plant classes endin –ae, e.g. Dicotyledonae. Classes may bedivided into subclasses, with the Latin end-ing –idae, e.g. Rosidae.

classification The grouping and arrange-ment of organisms into a hierarchicalorder. The aim is usually to aid in identifi-cation of organisms or represent their phy-logenetic relationships, or, ideally, both.An important aspect of classifications istheir predictive value. For example if acharacteristic is found in one member of agroup of plants, then it is also likely to befound in the other members of that groupeven though the characteristic in questionwas not used in the initial construction ofthe classification.

Artificial classification systems are de-signed to aid identification, and may notreflect true relationships. e.g. LINNAEUS’ssystem, which was based on the number ofstamens and carpels in the flower. Naturalclassification systems are based on morecharacteristics, and thus thus are better forpredicting relationships. Phenetic classifi-cation systems are based on many charac-ters, and may also be weighted in favor ofparticular characters. Phylogenetic classifi-cation systems attempt to reflect the evolu-tionary relationships of taxa. The validityof these systems is now being tested bymRNA studies. Compare cladistics. Seetaxon; phylum; class; order; family; genus;species.

clay Extremely fine-textured SOIL madeup of small mineral particles, less than0.002 mm in diameter, formed mainlyfrom aluminum and magnesium silicates.Clay soils become very sticky and diffi-cult to work when wet and can easily be-come waterlogged. Nutrient availability to

49

cleavage polyembryony

plants can be a problem as the nutrientsmay become chemically bound to the sur-faces of the particles.

clearing 1. In the preparation of perma-nent microscope slides, the stage betweendehydration and embedding, the purposeof which is to remove the dehydratingagent and replace it by a substance that ismiscible with the embedding substance.Clearing also renders the tissues transpar-ent. Clearing agents include benzene andxylene.2. In the preparation of cell material forstaining, the dissolving away of cell con-tents so that the distribution of tissues maybe better observed, using reagents such assodium hypochlorite.3. A treeless, usually grassy area in a wood-land or forest.

cleavage polyembryony The splittingof an embryo into several identical parts,each of which may develop into a matureembryo. It is a form of natural cloning.Cleavage polyembryony occurs in somegymnosperms, e.g. in Pinus, but usuallyonly one embryo develops to maturity. Seeclone.

cleistogamy The production of closedflowers. It is a method of ensuring self-pollination and occurs toward the end ofthe flowering season in certain plants, e.g.wood sorrel (Oxalis acetosella) particu-larly when little or no seed has been set bythe cross-pollinating flowers. Comparechasmogamy.

cleistothecium (pl. cleistothecia) See as-coma.

climacteric The rise in respiration rate,found in the fruits of some species, associ-ated with fruit ripening and senescence. Itmay be induced artificially by treatmentwith ethylene (ethene).

climatic climax A plant communitythat is in equilibrium with a stable climate.It represents the CLIMAX of a succession.

climatic factors Aspects of climate,such as temperature, rainfall, humidity, airmovement, and light, that affect living or-ganisms. Compare biotic environment;edaphic factors.

climax The final community in a succes-sion of natural plant communities in onearea under a particular set of conditions.Mixed woodland with oak is the naturalclimax vegetation in much of lowlandBritain, but on chalk it is often beechwood.A climax is self-perpetuating, at least for atime, although there are constant smallchanges, e.g. when a tree dies pioneeringplants may invade or the tree may be re-placed by a different species. Succession toa climax can be held at any stage by humanintervention, such as grazing on chalkdownland. Such an equilibrium is called asubclimax, plagioclimax, disclimax, or bi-otic climax. See clisere.

climbing ferns Ferns of the familySchizaeaceae, found in most parts of theworld in warm and tropical regions, in-cluding North America. There are fourgenera: Anemia, Lygodium, Mohria, andSchizaea. Many climbing ferns have longcreeping rhizomes.

cline A graded series of characters, usu-ally morphological, exhibited by a speciesor other related group of organisms, usu-ally along a line of environmental or geo-graphical transition. The populations ateach end of the cline may be substantiallydifferent from one another.

clinostat See klinostat.

clisere A succession of climax communi-ties in a given area, each giving way to thenext as a result of climatic changes. Seealso climax; sere.

clone A group of organisms or cells thatare genetically identical. In nature, clonesare derived from a single parental organ-ism or cell by mitotic cell division, asexualreproduction, or parthenogenesis. In ge-netic engineering, multiple identical copies

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50

of a gene are produced in cloning vectors,such as plasmids and phages. See genecloning. See also apomixis.

clubmoss See Lycophyta.

clubroot A serious disease of the cab-bage family (Brassicaceae) caused by Plas-modiophora brassicae (phylum Plasmo-diophora), in which the roots becomeswollen and misshapen, with characteristicgall-like swellings. Above-ground symp-toms include wilting in bright sunlight anda reddish, bluish, or yellowish tinge to theleaves.

CoA See coenzyme A.

coadaptation The evolution and main-tenance of certain genetic characteristicsthat increase the effectiveness of a relation-ship between two species. Flower/pollina-tor relationships are an example, in whichadaptations of flowers for pollination byanimals is to the mutual advantage of bothspecies (coevolution). See mutualism.

cobalt See micronutrient.

coccus (pl. cocci) A spherical-shapedbacterium. Cocci may be found singly, inpairs (e.g. Diplococcus), or chains (e.g.Streptococcus), or in regularly or irregu-larly packed clusters.

co-dominance See incomplete domi-nance.

codon A group of three nucleotide bases(i.e. a nucleotide triplet) in a messengerRNA (mRNA) molecule that codes for aspecific amino acid or signals the beginningor end of the message (start and stopcodons). Since four different bases arefound in nucleic acids there are 64 (4 ´ 4 ´4) possible triplet combinations. Thearrangement of codons along the mRNAmolecule constitutes the genetic code.When synthesis of a given protein is neces-sary the segment of DNA with the appro-priate base sequences is transcribed intomRNA. When the mRNA migrates to the

ribosomes, its string of codons is pairedwith the anticodons of transfer RNA mol-ecules, each of which is carrying one of theamino acids necessary to make up the pro-tein. Most amino acids are coded for bymore than one codon. The genetic code istherefore said to be degenerate. The termcodon is sometimes also used for thetriplets on DNA, which are complemen-tary to those on mRNA, but contain thebase uracil instead of thymine. See transferRNA.

coenocyte An area of cytoplasm con-taining many nuclei and enclosed by a cellwall, typically found in many fungi, certaingreen algae (e.g. Cladophora), and somemembers of the Xanthophyta. Such a cell isdescribed as coenocytic.

coenzyme See cofactor.

coenzyme A (CoA) A coenzyme that isimportant in the synthesis and reactions offatty acids. In the KREBS CYCLE it combineswith pyruvic acid, leading to loss of carbondioxide.

coenzyme Q (ubiquinone) A coenzymethat is an essential component of the respi-ratory electron-transport chain.

cofactor (coenzyme) A nonprotein sub-stance that helps an enzyme to carry out itsactivity. Cofactors may be cations or or-ganic molecules, known as coenzymes.Many enzymes require metal cofactors,e.g. ferredoxin requires ferrous and ferricions. The metal ion may be the catalyticcenter of the cofactor, or it may serve tobind the enzyme and substrate together tohold the enzyme in a catalytically activeconformation. Unlike enzymes cofactorsare, in general, stable to heat. When a cat-alytically active enzyme forms a complexwith a cofactor a holoenzyme is produced.An enzyme without its cofactor is termedan apoenzyme. See also prosthetic group.

cohesion theory A theory to explainhow sap rises up the xylem. It suggests thatthe lowering of water potential in the leaf

51

coleoptile

due to an increase in evaporation causeswater to flow toward the leaf down a waterpotential gradient. Strong cohesive forcesbetween the water molecules in the xylemprevent the water column breaking.

colchicine An alkaloid drug, obtainedfrom the autumn crocus Colchicum au-tumnale, which is used experimentally toprevent spindle formation in mitosis ormeiosis by preventing tubulin polymerizinginto microtubules. It has the effect of halt-ing cell division at metaphase, the stage atwhich the chromosomes have duplicated togive four homologs for each chromosome.See anther culture.

cold hardening (cold acclimation)Processes taking place in plants before theonset of winter that prepare them to with-stand low temperatures. For example,plants may accumulate solutes, such asoligosaccharides and the amino acid pro-line, to make their tissues frost-resistant.

coleoptile A sheathlike structure thatprotects the developing plumule in grasses.Some think that, together with the SCUTEL-LUM, it represents the COTYLEDON, whileothers believe it is the first plumular leaf.The coleoptile contains very little chloro-phyll and is usually light-sensitive.

coleorhiza (pl. coleorhizae) The protec-tive sheath surrounding the radicle ingrasses.

coliform bacteria Gram-negative rod-shaped bacteria able to obtain energy aero-bically or by fermenting sugars to produceacid or acid and gas. Most are found in thevertebrate gut (e.g. Escherichia coli), butsome are present in soil, water, or as plantpathogens. Many are pathogenic to hu-mans (e.g. Salmonella).

collateral bundle A kind of vascularbundle in which the phloem is external tothe xylem and on the same radius. Com-pare bicollateral bundle; concentric bun-dle.

collenchyma A specialized type ofparenchyma, usually located just beneaththe epidermis, that functions as supportingtissue. The cell walls are irregularly thick-ened with cellulose and pectin, the thicken-ing giving distinct patterns to the cells incross-section. The thickening is usuallygreatest in the corners of the cells or adja-cent to the intercellular spaces. Col-lenchyma is the first strengthening tissue tobe formed in young plants and is able toexpand as the young tissues continue de-velopment. Compare sclerenchyma.

colloid A substance that is composed ofparticles dispersed through another sub-stance. Such particles range from 10–7 to10–3 cm; they are too small to see with alight microscope, yet cannot pass througha selectively permeable membrane. Pro-teins in the cytoplasm are an example.

colony A body form in which many cellswith similar form and function aregrouped together, as in many algae, e.g.Volvox. Such a colony is often surroundedby mucilage.

columella 1. The structure present insporangia of many zygomycete fungi (e.g.Mucor) produced by formation of a dome-shaped septum cutting off the sporangiumfrom the sporangiophore.2. The central column of sterile tissue in thesporangium of liverworts and mosses.

commensalism An association betweentwo organisms in which one, the commen-sal, benefits and the other remains unaf-fected either way, e.g. an epiphyte on thebranch of a tree. Compare amensalism;mutualism; parasitism; symbiosis.

community A general term coveringany naturally occurring group of differentorganisms living together in a certain envi-ronment, and interacting with each other.See association; consociation.

community biomass See biomass.

companion cell An elongated thin-

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52

walled cell cut off longitudinally from thesame meristematic cell as the SIEVE EL-EMENT with which it is closely associated. Ithas a nucleus, dense cytoplasm, and manymitochondria, and is thought to provide,via PLASMODESMATA, the needs of the lessmetabolically active, enucleated sieve el-ement and perhaps plays a role in regulat-ing the flow of metabolites through thesieve tube.

compass plant A plant with its leafedges permanently aligned due north andsouth. Such plants thus avoid receiving thestrong midday rays of the sun directly onthe leaf blades, but are positioned to usefully the weaker rays of the morning andevening sun from the east and west. Thebest-known example is the compass plantof the prairies (Silphium laciniatum).

compatibility The coexistence of differ-ent bacterial plasmids in the same host cell.Plasmids that cannot exist in the same hostcell belong to the same incompatibilitygroup.

compensation depth See photic zone.

compensation point The concentra-tion of carbon dioxide at which, in a closedsystem, the rate of photosynthesis is ex-actly balanced by the combined rates ofrespiration and photorespiration, so thatnet exchange of oxygen and carbon diox-ide is zero.

The point at which photosynthesis doesnot increase with increased light intensityis termed the light saturation point. Thispoint occurs at much higher light intensi-ties in C4 plants than C3 plants.

competence A characteristic of embry-onic cells and meristematic cells wherebythey have the potential to differentiate intoany of several different types of cell. Onceplant cells have differentiated, most losecompetence, except under special condi-tions. See totipotency.

competition The utilization of the sameresources by one or more organisms of the

same or different species living together ina community, when the resources are notsufficient to fill the needs of all the organ-isms. Plants, particularly the dominantplants of a community, compete mainly asseedlings are establishing, but once this hasbeen achieved, the adult plant often out-competes any potential competitor unlessaffected by herbivores or pathogens.

competitive inhibition A form of en-zyme inhibition in which the inhibitorcompetes with the substrate to bind withthe active site of the enzyme. A competitiveinhibitor is usually structurally similar tothe substrate, e.g. malonate is a competi-tive inhibitor of the enzyme succinate de-hydrogenase. It may bind to the site butwill not react to produce products.

complementary DNA (cDNA) A formof DNA synthesized by genetic engineeringtechniques from a messenger RNA tem-plate using a reverse transcriptase enzyme.It is used in cloning to obtain gene se-quences from mRNA isolated from the tis-sue to be cloned. It differs from the originalDNA sequence in that it lacks intron and promoter sequences. Labeled single-stranded cDNA is used as a gene probe toidentify common gene sequences in differ-ent tissues and species. See gene cloning.

complementary genes Genes that canonly be expressed in the presence of othergenes; for example if one gene controls theformation of a pigment precursor and an-other gene controls the transformation ofthat precursor into the pigment, then bothgenes must be present for the color to de-velop in the phenotype. Such interactionsbetween genes lead to apparent deviationsfrom the 9:3:3:1 dihybrid ratio in the F2.

Compositae See Asteraceae.

composite fruit See multiple fruit.

compound fruit See aggregate fruit.

concentric bundle (centric bundle) Avascular bundle in which the xylem and

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confocal microscopy

phloem are arranged in rings, one com-pletely surrounding the other. Compare bi-collateral bundle; collateral bundle.

conceptacle A flask-shaped reproduc-tive cavity that develops on the swollen tips(receptacles) of the thalli of certain brownalgae, (e.g. bladder wrack). Female concep-tacles are lined with unbranched sterilehairs (paraphyses) and the oogonia de-velop on short stalks projecting from thechamber wall. Male conceptacles containbranched paraphyses that bear the an-theridia. Both female and male concepta-cles open to the exterior via a pore, theostiole.

condensation reaction A chemical re-action in which two molecules are joinedtogether with the elimination of a simplermolecule or group. Many condensation re-actions involve the elimination of water,e.g. the formation of peptide bonds be-tween amino acids in the formation of pep-tides. Compare hydrolysis.

conduplicate See ptyxis.

cone See strobilus.

confocal microscopy A form of lightmicroscopy involving the use of two mi-croscopes, one on each side of the speci-men. The first microscope focuses anilluminated aperture onto the specimen,while the other also focuses on the speci-men, but receives the light transmittedthrough it. This produces an image of a sin-gle plane in the specimen. It can be re-peated at different depths of focus to buildup a three-dimensional picture.

conformation The spatial arrangementof atoms in a molecule. In affecting themolecule’s shape and the distribution ofchemical groups and electrostatic chargeson its surface, the conformation deter-mines the potential energy and reactivity of the molecule that results from the at-tractive and repulsive forces between thedifferent parts of the structure. The confor-mation of PROTEINS is critical to their effec-

tiveness as enzymes. Their conformation isaffected by the sequence of amino acidsand the type, location, and nature of thebonds between different parts of the mol-ecule, or between the polypeptide chainand various cofactors and prostheticgroups.

conidiophore See conidium.

conidiospore See conidium.

conidium (conidiospore) (pl. conidia)An asexual spore of certain fungi, espe-cially of the Ascomycota and FungiAnamorphici e.g. Pythium and Albugo.Conidia are cut off externally in chains at the apex of a specialized hypha, theconidiophore.

Coniferophyta (conifers) The largestphylum of gymnosperms, comprisingabout 560 species of evergreen trees andshrubs, with many important species, e.g.Pinus (pine), Picea (spruce), Taxus (yew),and Abies (fir). They dominate the vast bo-real forests of the northern hemisphere.Most are evergreen, but Larix (larches)and Taxodium (swamp cypresses) are de-ciduous.

conjugated protein A protein that onhydrolysis yields not only amino acids butalso other organic and inorganic sub-stances. They are simple proteins com-bined with nonprotein groups (prostheticgroups). See also glycoprotein; lipoprotein.

conjugation 1. A type of sexual repro-duction found in the conjugating greenalgae (phylum Gamophyta), and certainfungi, in which a conjugation tube formsbetween cells of two individuals and thegamete formed in one cell (the male ga-mete) moves through the tube and fuseswith the gamete of the other cell (the fe-male gamete) to form a zygote. Spirogyraand Rhizopus are examples.2. In prokaryotes, a means of transferringgenetic information from one bacterium toanother across a bridgelike structure. InEscherichia and related genera, two indi-

conformation

54

viduals join by a conjugation bridge andpart of the genetic material of one, thedonor (or male) cell, is transferred to therecipient (or female) cell.

connective The parenchymatous tissuethat joins the two lobes of the anther andcontains the vascular strand. See stamen.

consociation A climax of natural vege-tation dominated by one particular species,such as oakwood, dominated by the oaktree, or Calluna heathland dominated bythe heather, Calluna vulgaris. Many conso-ciations together may form an association,for example oakwood, beechwood, andashwood consociations together make up adeciduous forest association. See also asso-ciation.

conspecific Describing individuals thatbelong to the same species.

consumer An organism that feeds uponanother organism, e.g. all animals and par-asitic and insectivorous plants. Compareproducer. See also trophic level.

continental drift The theory that pre-sent-day continents have arisen by thebreaking up and drifting apart of a previ-ously existing ancient land mass (Pangaea).There is much evidence to support thetheory, and it serves to explain the distrib-ution of contemporary and fossil plantsand animals. Continental drift is now be-lieved to reflect the movement over geolog-ical time of underlying plates in the earth’scrust – the theory of plate tectonics.

continuous variation (quantitative vari-

55

Convention on Biological Diversity

Conjugation: the stages occurring in Spirogyra

ation; multifactorial inheritance) A formof variation in which the characters showcontinuous variation within a certain rangeof values. Examples are grain yield inwheat and height in humans. Such traitsare typically controlled by many differentgenes (polygenes). The identification andmanipulation of genetic loci determiningquantitative traits (quantitative trait loci(QTLs)) is important in plant and animalbreeding.

contractile root A specialized root de-veloped by certain bulb- and corm-formingplants that serve to pull the bulb or cormdown to the appropriate depth in the soil,e.g. Crocus.

Convention on Biological DiversityAn international convention, signed at theUnited Nations Rio Summit in 1992, thataims to conserve biodiversity, the sustain-able use of natural resources, and the equi-table sharing of the benefits arising fromexploitation of genetic resources. It coversall aspects of biodiversity, from geneticresources to ecosystems, and aims to con-serve resources while promoting sustain-able development and to encourage thesharing of costs and benefits between de-veloping and developed countries. TheUnited Nations Environment Program(UNEP) responded by commissioning theGlobal Biodiversity Assessment, to collectand review data, theories, and opinion re-lating to current issues worldwide, whichprovided valuable information on therapid rate at which environmental changeis taking place and natural ecosystems arebeing modified.

convergent evolution (convergence)The development of similar (analogous)structures in unrelated organisms as a re-sult of living in similar ecological condi-tions. An example is the evolution ofsimilar succulent forms in the Cactaceaeand the Euphorbiaceae, in which the leavesare reduced to pale spines. See also analo-gous.

copper Symbol Cu. A metal trace el-

ement (micronutrient), essential for plantgrowth. It is found in the enzyme cy-tochrome c oxidase, which is part of therespiratory electron-transport chain, andin the protein plastocyanin, which is in-volved in photosynthetic electron trans-port. Copper deficiency results in chlorosis,suggesting that copper may be involved in the synthesis of chlorophyll. See also micronutrient.

coppice (copse) A woodland managedfor wood production by cutting trees backto ground level at regular intervals (usually10–15 years) and allowing adventitiousshoots to grow up from the base. Theyoung shoots, which are often verystraight, are used for fencing, charcoalburning, and firewood. Occasional treesmay be left to mature: these are termedstandards.

coralloid roots A type of root regularlyproduced by cycads, which contains sym-biotic nitrogen-fixing cyanobacteria inroot nodules, giving the roots a knobblycoral-like appearance.

cordate Heart-shaped, usually referringjust to the base of leaves, e.g. the leaves ofthe common blue violet (Viola papil-ionacea).

cork See phellem.

cork cambium See phellogen.

corm An organ of perennation and veg-etative reproduction, consisting of a shorterect fleshy swollen underground stem,usually broader than high and coveredwith membranous scales. It stores foodmaterial in the stem and bears buds in theaxils of the scalelike leaf remains of theprevious years’ growth. One or more newcorms form from these buds, and lie abovethe shrivelled remains of the previousyear’s corm. The corm’s position in the soilis often maintained with the aid of con-tractile roots. Examples of corms are cro-cus and gladiolus. Compare bulb.

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corolla A collective term for the petalsof a flower. The corolla is denoted in thefloral formula by the symbol C. Where themargins of the petals are partly or com-pletely fused, a corolla tube is formed.

corona 1. In flowering plants, any typeof outgrowth from the petals or sepals,such as the trumpet of the daffodil flower(Narcissus).2. A group of cells at the tip of the oogo-nium in certain green algae (Chlorophyta),such as the stonewort Chara and the mer-maid’s cup (Acetabularia).

corpus The central region of the meri-stem below the tunica where cell divisionsare in all directions giving both increasedwidth and length to the apex. The tissues ofthe stele and cortex are derived from thecorpus. See tunica–corpus theory.

cortex (pl. cortices or cortexes) A pri-mary tissue in roots and stems of vascularplants derived from the corpus meristem,that extends inwards from the epidermis to the phloem. It usually consists ofPARENCHYMA cells but other tissues (e.g.collenchyma and sclerenchyma) may bepresent.

corymb See inflorescence.

cothallus See thallus.

cotyledon (seed leaf) The first leaf of theembryo of seed plants, which is usuallysimpler in structure than later-formedleaves. Cotyledons play an important partin the early stages of seedling development.For example they act as storage organs insome seeds, such as peas and beans whenmature. They form the first photosyntheticorgan in seeds showing epigeal germina-tion, e.g. sunflower (Helianthus) but inothers they remain underground. Mono-cotyledons and dicotyledons are so termedbecause they normally contain one andtwo cotyledons respectively although thereare exceptions.

covalent bond A chemical bond in

which one or more pairs of electrons areshared between two atoms. It is often rep-resented by a single line between the sym-bols of the two atoms that are bondedtogether. Compare hydrogen bond; ionicbond.

cp-DNA See chloroplast DNA.

C3 plant A plant in which the first prod-uct of photosynthesis is a 3-carbon acid,glycerate 3-phosphate (phosphoglycericacid). Most plants are C3 plants. They arecharacterized by high carbon dioxide com-pensation points owing to PHOTORESPIRA-TION. Compare C4 plant.

C4 plant A plant in which the first prod-uct of photosynthesis is a 4-carbon dicar-boxylic acid, oxaloacetic acid. C4 plantshave evolved from C3 plants by a modifica-tion in carbon dioxide fixation, leading tomore efficient carbon dioxide uptake. Themodified pathway is called the Hatch–Slack pathway or the C4 dicarboxylic acidpathway. In the leaves, the mesophyll cellssurrounding the vascular bundles (bundlesheath cells) contain the carbon dioxide-fixing enzyme phosphoenolpyruvate car-boxylase (PEP carboxylase) in theircytoplasm. This has a higher affinity forcarbon dioxide than ribulose bisphosphatecarboxylase (RUBP carboxylase). The prod-uct of carbon dioxide fixation is oxalo-acetate, which is rapidly reduced to the C4acid malate or transaminated to the C4 acidaspartate. These C4 acids are transportedto the bundle-sheath cells encircling theleaf veins, where they are decarboxylated,releasing CO2, which is then refixed as inC3 plants.

C4 plants are mainly tropical or sub-tropical, including many tropical grasses(e.g. maize, sugar cane, and sorghum).They are more efficient than C3 plants,producing more glucose per unit leaf area.See also bundle sheath; crassulacean acidmetabolism.

crassulacean acid metabolism (CAM)A form of photosynthesis first discoveredin the family Crassulacaeae and since

57

Cretaceous

found in many other succulent plants, suchas cacti. It allows plants of arid environ-ments to keep their stomata closed to con-serve water during the day, opening themat night to exchange gases. Carbon dioxidetaken in through the stomata at night isconverted into an organic acid for storage.Instead of combining with ribulose bispho-sphate, it combines with the 3-carbonphosphoenolpyruvate (PEP) to form ox-aloacetic acid, which is then converted tomalic acid for storage in the vacuole. Thefollowing day, when the stomata areclosed, the malic acid is broken down andthe carbon dioxide is released to take partin the light reactions of photosynthesis.When stressed by drought, plants withcrassulacean metabolism (CAM plants)can last for days or even weeks withoutopening their stomata, conserving waterand living on stored resources. CompareC3 plant; C4 plant.

cremocarp A dry fruit splitting into twoone-seeded portions. The portions aretermed mericarps.

Cretaceous The most recent period ofthe Mesozoic era, 145–66 million yearsago. The dominant vegetation on land inthe early Cretaceous was forests of cycads,conifers, gingkgoes and ferns. Most mod-ern-type ferns, gymnosperms, and an-giosperms arose during the Cretaceous,and by the end of the period the floweringplants had replaced the gymnosperms asthe dominant terrestrial vegetation, form-ing vast broad-leaved forests with magno-lias, figs, poplars, sycamores, willows, andherbaceous plants and a great variety of in-sect pollinators. The Cretaceous is namedafter the large amounts of chalk (fossilizedplankton) found in rocks of the period. Seealso geological time scale.

crista (pl. cristae) The structure formedby folding of the inner mitochondrialmembrane, giving a large surface area forelectron transport.

critical day length (cdl) The amount oflight per day that is the maximum a short-

day plant may receive and still flower, andconversely, the minimum a long-day plantneeds to flower. Cocklebur, a short-dayplant, will not flower if given more than15fi hours light per day; i.e. its cdl is 15fihours. Henbane, a long-day plant, willonly flower if given more than 11 hourslight per day; i.e. the cdl is 11 hours. In re-ality, however, it is the length of the darkperiod that it critical, short-day plants re-quiring more than a critical length of darkperiod before they will flower, and viceversa. See photoperiodism.

cross 1. The act of cross-fertilization.2. The organism resulting from cross-fertilization.

crossing over The exchange of materialbetween homologous chromatids by theformation of chiasmata. This results in therecombination of linked genes. Such reas-sortment of genes is the main source of ge-netic variation during sexual reproduction.Unequal crossing over, in which one chro-matid ends up with one copy of a chromo-some segment while the other has threecopies, results from crossing over afterpairing between homologous chromo-somes that are not properly aligned. Seealso chiasma; independent assortment;tetrad.

crossover frequency See chromosomemap.

cross-pollination The transfer of pollenfrom the anthers of one individual to thestigma of another individual, usually of thesame species, with subsequent germinationof the pollen and growth of the pollen tube.This transfer is achieved by intermediaryagents such as birds (ornithophily), bats,insects (entomophily), wind (anemophily),or water (hydrophily). See incompatibility.Compare self-pollination.

crown gall A disease of plants caused bythe soil-borne bacterium Agrobacteriumtumefasciens. It causes galls both andabove and below ground on a wide rangeof plants, especially fruit trees.

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Cruciferae See Brassicaceae.

crumb structure The texture of a soil interms of the size of the soil particles andhow they interact, often determined simplyby rubbing the soil with fingers. Some soils,such as sandy soils, have a loose crumbstructure with good drainage, whereas inclay soils the particles are small and sticktogether to give a dense crumb structureand poor drainage

cryptogam In early classifications, anyplant that reproduces by spores or gametesrather than by seeds. Cryptogams werethus named because early botanists consid-ered their method of reproduction to behidden (cryptic). They included the algae,fungi, mosses, liverworts, hornworts,ferns, clubmosses, and horsetails, the vas-cular plant group often being termed vas-cular cryptogams. Compare phanerogam.

cryptophyte (geophyte) A plant inwhich the resting buds are below the soilsurface or in water. See also Raunkiaer’splant classification.

cultivar Any agricultural or horticul-tural ‘variety’. The term is derived from thewords cultivated variety.

culture A population of microorganismsor dissociated cells of a tissue grown on orwithin a solid or liquid medium for exper-imental purposes. This is done by inocula-tion and incubation of the nutrientmedium. See also tissue culture.

culture medium A mixture of nutrientsused, in liquid form or solidified with agar,to cultivate microorganisms, such as bacte-ria or fungi, or to support tissue cultures.

cushion plant A plant that forms a tightlow hummock as an adaptation to cold anddry or windy situations. Cushion plantsusually have small thick leaves to minimizewater loss, or hairy leaves to reduce tran-spiration. Cushion plants are common inarctic–alpine habitats.

cuticle A waterproof, protective layer ofwax secreted by the epidermis that coversthe external surface of the plant, with theexception of the stomata and lenticels.Some seeds also have a cuticle. Its thicknessvaries with the species and environment.Xerophytes tend to have thick cuticles, anddry conditions often induce cuticular thick-ening. Its main function appears to be re-striction of water loss from the plant. Onaverage, only about 5% of the water lostfrom a plant is via the cuticle (cuticulartranspiration).

cutin A group of substances chemicallyrelated to fatty acids forming a continuouslayer called the cuticle on the epidermis ofplants, interrupted only by stomata orlenticels. Being fatty in nature, cutin iswater-repellent, therefore helping to re-duce transpiration. It is also protective, forexample preventing invasion by parasites.See suberin.

cutinization The impregnation of aplant cell wall with cutin.

cutting A part of a plant that is removedfrom the parent and encouraged to growinto another plant. Cuttings are a means ofasexual propagation and may vary in sizefrom buds, leaves or root segments to largeshoots.

Cyanobacteria A phylum of Eubacteriacontaining the blue-green bacteria (for-merly called blue-green algae) and thegreen bacteria (chloroxybacteria). Bothgroups convert carbon dioxide into or-ganic compounds using photosynthesis,generally using water as a hydrogen donorto yield oxygen, like green plants. Cyano-bacteria are an ancient group, and theirfossils (stromatolites) have been dated atup to 3500 million years old. They arespherical (coccoid) or form long micro-scopic filaments of individual cells.

Cycadophyta (cycads) A phylum ofcone-bearing gymnosperms (about 75 liv-ing species) with palmlike compoundleaves and special CORALLOID ROOTS at or

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cypsela

near the ground surface, which containsymbiotic nitrogen-fixing cyanobacteria.

cyclic AMP (cAMP; adenosine-3¢ ,5¢ -monophosphate) A form of adenosinemonophosphate (see AMP) formed fromATP in a reaction catalyzed by the enzymeadenyl cyclase. It has many functions,acting as an enzyme activator, geneticregulator, chemical attractant, secondarymessenger, and as a mediator in the activ-ity of many hormones, including epineph-rine, norepinephrine, vasopressin, ACTH,and the prostaglandins.

cyclic photophosphorylation In PHO-TOSYNTHESIS, the flow of electrons fromPhotosystem I, after excitation by ab-sorbed light energy, along the ferredoxin/cytochrome b/f/plastocyanin electron-transport chain and back to Photosystem Ito be reenergized. Cyclic photophosphory-lation drives ATP synthesis, but does notproduce NADPH.

cyclins Regulatory proteins whose con-centration varies with certain stages of theCELL CYCLE. Cyclins help control progressthrough the various stages of the cell cycleby activating protein kinases (cyclin-dependent protein kinases).

cyclosis (cytoplasmic streaming) Thestreaming of cytoplasm in a circular mo-tion around the cell observed in someplants, particularly young sieve tube el-ements.

cycyanocobalamin (vitamin B12) Oneof the water-soluble B-group of VITAMINS.

cymose inflorescence (cyme; definite in-florescence) See inflorescence.

cypsela An achenelike fruit that devel-ops from an inferior ovary and containssome tissues not derived form the carpel. Itis common in the Asteraceae, e.g. dande-lion (Taraxacum), whose fruits bear a ringof hairs (the pappus) derived from thecalyx.

cyst A thick-walled resting spore.

cysteine A sulfur-containing AMINO ACID

synthesized from methionine and serine. Itis involved in the synthesis of biotin. It alsoacts as a store of sulfur for biosynthesis,and can be broken down to pyruvate.

cystine A compound formed by the join-ing of two cysteine AMINO ACIDS through a–S–S– linkage (a cystine link). Bonds of thistype are important in the conformation ofproteins, acting as cross-links between dif-ferent parts of a polypeptide or betweendifferent polypeptide chains, thus deter-mining the pattern of folding. It is also re-quired to the synthesis of the coenzymethiamin pyrophosphate, which is involvedin many decarboxylation reactions.

cystolith A deposit of calcium carbon-ate arising internally on a stalk from thecell walls of large modified epidermal cellsin some flowering plants, e.g. stinging net-tle (Urtica dioica).

cytidine (cytosine nucleoside) A NUCLE-OSIDE formed when cytosine is linked to D-ribose via a b-glycosidic bond.

cytochrome oxidase (cytochrome aa3)The enzyme that catalyzes the final step inthe respiratory electron transport chain, inwhich water is formed from hydrogen ionsand oxygen. See respiration.

cytochromes Conjugated proteins con-taining heme, that act as intermediates inthe electron-transport chains of photosyn-thesis and respiration. The iron ion in theporphyrin prosthetic group of cytochromeschanges its oxidation state from ferric(Fe3+) to ferrous (Fe2+) when the cy-tochrome is reduced. There are four mainclasses, designated a, b, c, and d, mostbound to membranes.

cytogamy The fusion of gametes duringfertilization. Compare karyogamy.

cytokinesis The division of the cyto-plasm after nuclear division (mitosis or

cyst

60

meiosis). In the cells of plants and mostalgae it involves formation of a new plantcell wall by means of a structure called thecell plate. In primitive protoctists, as in an-imals, the cell membrane ingrows underthe control of a ring of actin microfila-ments, and a furrow constricts the cellequator until it pinches the cell in two. Seecell plate.

cytokinin One of a class of plant hor-mones concerned with the stimulation of cell division, nucleic acid metabolism,and root–shoot interactions. Cytokininsare often purine derivatives: e.g. kinetin (6-furfuryl aminopurine), an artificial cy-tokinin commonly used in experiments topromote cell division in tissue cultures; andzeatin, found in maize cobs. Natural cy-tokinins are all derivatives of adenine.

Cytokinins are produced in roots,where they stimulate cell division. They arealso transported from roots to shoots in thetranspiration stream, where they are essen-tial for healthy leaf growth. Subsequentmovement from the leaves to youngerleaves, buds, and other parts may occur in

the phloem and be important in sequentialleaf senescence up the stem.

cytology The study of cells; cell biology.

cytoplasm The living contents of a cell,excluding the nucleus and large vacuoles,in which many metabolic activities occur.It is contained within the plasma mem-brane and comprises a colorless substance(hyaloplasm) containing organelles andvarious inclusions (e.g. crystals and insolu-ble food reserves). The cytoplasm is about90% water. It is a true solution of ions (e.g.potassium, sodium, and chloride), smallmolecules (e.g. sugars, amino acids, andATP), and a colloidal solution of large mol-ecules (e.g. proteins, lipids, and nucleicacids). It can be gel-like, usually in its outerregions, or sol-like. A complex cytoskel-eton of F-actin filaments and microtubulesis often present, and appears to control thestructure of the cytoplasm, cell move-ments, and movements of the cytoplasmand conformation of the membranes. Seecytoskeleton; organelle; protoplasm.

cytoplasmic inheritance (extrachromo-

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cytotaxonomy

2,4-D (2,4-dichlorophenoxyacetic acid)A synthetic AUXIN used as a potent selec-tive weedkiller. Monocotyledenous specieswith narrow erect leaves (e.g. cereals andgrasses) are generally resistant, while di-cotyledonous plants are often very suscep-tible. The compound is thus particularlyeffective in controlling weeds in cerealcrops and lawns.

dark-ground illumination A light mi-croscopy technique used to study transpar-ent material, such as living cells ormicrobes. The specimen is lighted from theside, so that only diffracted light from thespecimen passes through into the objective.The specimen thus appears bright against adark background. The difference in refrac-tive index between organelles and the sur-rounding cytoplasm causes the boundariesbetween organelles and cytoplasm to re-flect more light.

dark reactions (light-independent reac-tions) A group of reactions that follow theLIGHT REACTION in PHOTOSYNTHESIS andform glucose and other reduced productsfrom carbon dioxide. They are not depen-dent on light, although they can take placein the light. The dark reactions use energy(as ATP) and reducing power (as NADPH)generated by the light reaction of photo-synthesis to reduce carbon dioxide. In eu-karyotes, the dark reactions take place inthe stroma of the chloroplast by one of twopathways, according to whether the plantis a C3 PLANT or a C4 PLANT. The reducedcarbon dioxide is incorporated into carbo-hydrates in the Calvin cycle. See also cras-sulacean acid metabolism.

Darwin, Charles Robert (1809–82)British naturalist noted for his theory of

evolution by NATURAL SELECTION. Darwinfirst became interested in natural history atCambridge University. He was inspired bythe geologist Adam Sedgwick and encour-aged by the professor of botany at Cam-bridge, John Henslow, to apply for theposition of naturalist on a survey of the Pa-cific and South America on HMS Beagle in1831. On this voyage, he observed manydifferent animal species and was struck bythe way in which they gradually changedfrom one location to another, in particular,the finches of the Galápagos Islands. Herecorded 14 species of finches on these is-lands, each species dominating a particularpart of the islands in different conditions.He speculated that it was probably un-likely that they could all have arisen spon-taneously as separate species and thoughtthat it was more probable that they hadevolved from a mainland species of finch,possibly from Ecuador. It took 20 years forDarwin to collect enough evidence to pub-lish his seminal work On the Origin ofSpecies by Means of Natural Selection(1859), having been stimulated to publishby an unexpected letter in 1858 from theBritish naturalist Alfred Russel WALLACE,outlining theories similar to Darwin’s own.Darwin and Wallace read a joint paper ontheir theories to the Linnaean Society inLondon in the summer of 1858. The publi-cation of Darwin’s book led to a storm ofcontroversy among scientists and church-men as his theory challenged the biblicalaccount of the Creation. Darwin continuedto publish more works, including his TheDescent of Man (1871), in which he ap-plied his theory of evolution to humans,implying that they had descended fromapelike creatures

Darwinism Darwin’s explanation of

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D

the mechanism of evolutionary change,namely, that in any varied population oforganisms only the best adapted to that en-vironment will tend to survive and repro-duce. Individuals that are less well adaptedwill tend to perish without reproducing.Hence the unfavorable characteristics, pos-sessed by the less well-adapted individuals,will tend to disappear from a species, andthe favorable characteristics will becomemore common. Over time the characteris-tics of a species will therefore change, even-tually resulting in the formation of newspecies. Darwin called this process of selec-tive birth and death NATURAL SELECTION.See also neo-Darwinism.

dating techniques Methods used to de-termine the age of rocks, fossils, or archae-ological remains. There are two mainmethods. Relative dating assesses the ageof a specimen in comparison to other spec-imens. Absolute dating involves assessingthe actual age of a specimen by using somereliable measure of time. See also den-drochronology; radiometric dating.

day-neutral plant A plant that requiresno particular photoperiod to flower. Seephotoperiodism.

deamination A type of chemical reac-tion in which an amino group (NH2) is re-moved from a compound. Amino acids canbe broken down by oxidative deamination.Transamination of many amino acidsyields glutamate, which can be deaminatedto a-ketoglutarate, which can then enterthe Krebs cycle to be further broken downwith the release of energy. See transamina-tion.

De Bary, Heinrich Anton (1831–88)German botanist. De Bary abandoned a ca-reer in medicine in Frankfurt to devote therest of his life to his botanical studies, be-coming professor of botany at a number ofGerman universities. His work on fungirepresents a major landmark in the under-standing of and fight against plant dis-eases. His observations of fungi in theirnatural habitat and his methods of cultur-

ing them enabled him to work out their de-velopment and life cycles, thus layingdown the foundations of modern mycol-ogy. In his Researches on Fungal Blights(1853) outlining certain rust and smut dis-eases, he explained that fungi are the causeand not the effect of diseases. He alsoshowed that a lichen was an association be-tween an alga and a fungus and coined theterm ‘symbiosis’ for dissimilar plants livingtogether to their mutual benefit.

decarboxylase An enzyme that cat-alyzes the decarboxylation of carboxylicacids, including the conversion of aminoacids to amines.

deciduous Denoting plants that season-ally shed all their leaves, for example be-fore the winter, or dry season. It is anadaptation to prevent excessive water lossby transpiration when water is scarce.Compare evergreen.

decomposer An organism that feedsupon dead organisms, breaking themdown into simpler substances. Decom-posers recycle nutrients, making themavailable to producer organisms. Bacteriaand fungi are important decomposers inmost ecosystems. See also food chain;trophic level.

decussate Describing leaves in oppositepairs and with alternating pairs at right an-gles to each other up the stem.

dedifferentiation The reversion of adifferentiated cell to the meristematic (un-differentiated) state, which is capable ofdifferentiating again. It occurs naturallyjust before the production of secondarymeristems or in response to wounding orstimulation by growth factors. See callus;totipotency.

deficiency disease A disease caused bydeficiency of a particular essential nutri-ent (such as a MICRONUTRIENT or trace el-ement), usually with a characteristic set ofsymptoms. For example, iron deficiency re-sults in CHLOROSIS between the veins, par-

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deficiency disease

ticularly in young leaves that are still syn-thesizing chlorophyll. Nitrogen deficiencycauses stunting of growth and chlorosis ofolder leaves, as chlorophyll pigments arebroken down so their components can bereused in the young growing parts of theplant.

definite growth See determinate growth.

definite inflorescence See cymose in-florescence.

definitive nucleus See polar nuclei.

deforestation (disafforestation) The per-manent removal of forests, especially bymeans of logging for commercial timberand clearing for agriculture and human set-tlement. A major cause of loss of naturalhabitats and biodiversity worldwide, it canlead to soil erosion, flooding downstream,and increasing drought in neighboringareas.

degenerate code A part of the geneticcode in which some amino acids are codedfor by more than one triplet of nucleotidebases (codons). For some amino acids, onlythe first two bases appear to be important,and it matters little what the third base is.For example, leucine is coded for by CUU,CUC, CUA, and CUG, and by UUA andUUG.

dehiscent Describing a fruit or fruitingbody that opens at maturity to release theseeds or spores. Dehiscence may be violentto aid seed dispersal. Compare indehiscent.

dehydration A process followed whentissues are prepared for permanent micro-scope slides. Water is removed by immers-ing the tissue in increasing strengths ofethyl alcohol. The alcohol concentrationmust be increased gradually as otherwisethe cells would dehydrate too quickly andshrink. Dehydration is necessary becausewater does not mix with the chemicals usedin clearing and mounting sections.

dehydrogenase An enzyme that cat-

alyzes the removal of certain hydrogenatoms from specific substances in biologi-cal systems. Dehydrogenases are usuallycalled after the name of their substrate, e.g.lactate dehydrogenase. Some dehydroge-nases are highly specific, with respect toboth their substrate and coenzyme, whilstothers catalyze the oxidation of a widerange of substrates. Many require the pres-ence of a coenzyme, which is ofteninvolved as a hydrogen acceptor. Dehydro-genases catalyze the transfer of two hydro-gen atoms from substrates to NAD andNADP. See also alcohol dehydrogenase.

deletion See chromosome mutation.

deme A subpopulation of a species: adiscrete interbreeding group of organisms,with recognizable cytological or geneticcharacters, that is spatially distinct fromother such groups, although it may be ad-jacent to them. Compare subspecies.

denaturation A process that causes un-folding of the peptide chain of proteins orof the double helix of DNA. These changesmay be brought about by a variety of phys-ical factors: change in pH, temperature, vi-olent shaking, and radiation. The primarystructure remains intact. Denatured pro-teins and nucleic acids show changes inphysical and biological properties; dena-tured proteins, for example, are often in-soluble in solvents in which they wereoriginally soluble.

dendrochronology A method of ar-chaeological dating by the ANNUAL RINGS oftrees, used when the lifespans of living andfossil trees in an area overlap. Exact datesfor sites can be calculated and the methodis more accurate than radiometric datingtechniques. Bristlecone pines, which canlive for up to 5000 years, have been used insuch work.

denitrification The chemical reductionof nitrate by soil bacteria. Denitrifying bac-teria such as Pseudomonas, Micrococcus,and Clostridium use nitrate as the terminalelectron acceptor in anaerobic respiration.

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64

The process is important in terms of soilfertility since the products of denitrifica-tion (e.g. nitrites and ammonia) cannot beused by plants as a nitrogen source. Com-pare nitrification. See nitrogen cycle.

dentate Toothed: describing leaf mar-gins that have a series of outward-pointingnotches.

denticulate Describing a leaf marginthat is finely toothed.

deoxyribonuclease See DNase.

deoxyribonucleic acid See DNA.

deoxyribose A pentose (five-carbon)sugar that is a component of the nu-cleotides that make up DNA.

deoxy sugar A SUGAR in which oxygenhas been lost by replacement of a hydroxylgroup (OH) with hydrogen (H). The mostimportant example is DEOXYRIBOSE.

dermatogen (protoderm) See histogentheory.

desert An area in which the rate of evap-oration exceeds the rate of precipitationfor most of the time. A desert may form inany climate where average annual precipi-tation is less than 250 mm and intermit-tent. A ‘true desert’ has no plant life at all.The term is also used for semideserts inwhich plant and animal life is sparsely dis-tributed, and adapted to long periods ofdrought. EPHEMERAL plants are commonand there may also be succulent or xero-phytic perennials. Cold deserts includeparts of the TUNDRA and may also form inhot regions with annual rainfall of less than400 mm.

desiccation The drying out of an organ-ism due to evaporation.

desmids A group of green algae of thephylum GAMOPHYTA that are basically sin-gle-celled but in some species form fila-mentous or irregular colonies.

determinate growth (definite growth) Aform of growth that has a finite limit, i.e.once a maximum size is reached, no furthergrowth occurs. It occurs, for example, inannual and biennial plants and plant or-gans such as internodes. The term is some-times applied to cymose inflorescences, inwhich growth ceases with the productionof a terminal flower bud. Compare indefi-nite growth.

detritus Fragments of dead material,such as leaf litter and products of thebreakdown of organic material by decom-posers.

Deuteromycota See Fungi Anamorphici.

Devonian The geological period knownas the ‘Age of Fish’, some 405–355 millionyears ago, between the Silurian and theCarboniferous periods of the Paleozoic era.It was also the time when the first major in-vasion of the land by plants occurred. Thevery first land plants date from the late Sil-urian, but during the Devonian many vas-cular land plants appeared, such as therhyniophytes (all now extinct), clubmosses(Lycophyta), horsetails (Sphenophyta),and eusporangiate ferns (early members ofthe Filicinophyta. The gymnosperms prob-ably arose in the Late Devonian. See alsogeological time scale.

De Vries, Hugo (1848–1935) Dutchplant physiologist and geneticist. An experton Netherlands flora, De Vries showedthat turgor pressure of the fluid in plantcells was partially responsible for growthand coined the term ‘plasmolysis’ for theeffect on cells of water loss during wilting.His work in this area formed a basis fortheories of osmosis.

In the 1880s he developed an interest inheredity, proposing that there were struc-tures in the nucleus (which he called ‘pan-genes’) that determine characteristics in theoffspring. He carried out plant breedingexperiments, and the results of his crossesshowed segregation of characters in theprogeny in a ratio of 3:1. He came across

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De Vries, Hugo

MENDEL’s work in 1900 and immediatelypublished his results.

In The Mutation Theory (1901–03), DeVries postulated that new species couldarise from a single dramatic mutation. Hehad observed different types of eveningprimrose within a single colony and be-lieved them to be the result of mutations. Ithas subsequently been discovered that theplants De Vries believed to be mutantswere triploids or tetraploids, but his essen-tial premise continues to be important,showing that variation is essential for evo-lution within a species.

dextrin Any of a class of intermediatesproduced by the hydrolysis of starch. Fur-ther hydrolysis eventually produces themonosaccharide glucose.

dextrose (grape sugar) An outdatedname for GLUCOSE.

diakinesis The last stage of thePROPHASE in the first division of MEIOSIS.Chiasmata are seen during this stage, andby the end of diakinesis the nucleoli andnuclear membrane have disappeared. Dur-ing diakinesis the sister chromatids of ho-mologous pairs of chromosomes completetheir separation, and the chromosomes coiltightly, shortening and thickening.

dialysis The separation of large mol-ecules from smaller ones by DIFFUSION

through a selectively permeable membranethat allows through only molecules up to acertain size. For example, proteins can beseparated from amino acids or glucose inthis way. Compare osmosis.

Diatoms A phylum of the Protoctistawhose members are unicellular algae foundin freshwater, the sea, and soil. Much ofplankton is composed of diatoms and theyare thus important in FOOD CHAINS. Di-atoms are diploid unicells with silica cellwalls (frustules) composed of two overlap-ping valves ornamented with perforations,which are arranged differently in eachspecies. The frustules of dead diatoms arehighly resistant to decay, and form deep

sediments on the floors of oceans and lakescalled diatomaceous earth or kieselguhr,which is used in insulating materials, fire-proof cements, filters, and in the manufac-ture of explosives.

diatropism (diageotropism) See tropism;gravitropism.

dicaryon See dikaryon.

dichasial cyme (dichasium) See inflo-rescence.

dichogamy The condition in which theanthers and stigmas mature at differenttimes thus helping prevent self-pollination.Compare homogamy. See protandry; pro-togyny.

dichotomy Forked branching producedby division of the growing point into twoequal parts, seen for example in Fucus.

Dicotyledonae A class of floweringplants (phylum Angiophyta) characterizedby having two cotyledons in the seed. Theyinclude herbs, shrubs, and trees and sec-ondary growth is normal. Examples are thesunflower and the oak. The flower partsare arranged in fours or fives, or multiplesthereof, and the leaf veins are branched.The vascular bundles of the stem arearranged in a ring within a single endoder-mis and pericycle, giving a eustele. Com-pare Monocotyledonae.

dictyosome A stack of membrane-bounded sacs (cisternae) that, togetherwith associated vesicles (Golgi vesicles),forms the GOLGI APPARATUS. The term isusually only applied to plant cells, wheremany such stacks are found. In contrast,the Golgi apparatus of most animal cells isa continuous network of membranes. Theterm dictyosome is little used today.

dictyostele A modified solenostele thatis broken up by large leaf gaps so crowdedtogether that they overlap. The tube of astelar tissue is thus broken up into a mesh,each small segment of remaining vascular

dextrin

66

tissue being called a meristele. It is found incertain fern stems (e.g. Dryopteris).

differentially permeable membraneSee osmosis.

differentiation A process of changeduring which cells with generalized formbecome morphologically and functionallyspecialized to produce the different celltypes that make up the various tissues andorgans of the organism. See also toti-potency.

diffuse-porous Describing wood inwhich vessels of approximately equal di-ameter tend to be evenly distributed sothere is no obvious growth ring. Diffuse-porous wood is seen for example in yellowbirch (Betula lutea). Compare ring-porous.

diffusion The movement of moleculesalong a concentration gradient from areasof high concentration to areas of lowerconcentration as a result of random move-ment. Gaseous exchange in plants takeplace by diffusion, and some of the move-ment of ions through plant fluids is alsodue to diffusion. See dialysis; transpiration.Compare active transport; osmosis.

diffusion pressure deficit (DPD) Thenet force that causes water to enter a plantcell. It is the difference between OSMOTIC

PRESSURE (P) and TURGOR PRESSURE:DPD = P – TP.

See water potential.

dihybrid (dihybrid cross) An organismthat is heterozygous at two loci, formed bycrossing homozygous parents with differ-ent alleles at two given loci: for example,Mendel’s cross between yellow round(YYRR) and green wrinkled (yyrr) gardenpeas to give a yellow round dihybrid(YyRr). When a dihybrid is selfed, a char-acteristic dihybrid ratio of 9:3:3:1 is ob-tained in the offspring. Nine plants exhibitboth dominant characters, six plants showone dominant and one recessive character,and one plant exhibits both recessives.Compare monohybrid.

dikaryon (dicaryon) A fungal hypha ormycelium whose cells each contain two dif-ferent nuclei, arising from the fusion of twocompatible cells, each with one nucleus.The nuclei do not fuse immediately, in-stead dividing independently but simulta-neously. Dikaryosis is common inascomycetes and basidiomycetes, and re-sults from the fusion of two monokaryoticmycelia of different mating types.

dimorphism The existence of two dis-tinct forms. Examples include male and fe-male plants in dioecious species, aerial andsubmerged leaves, alternation of gameto-phyte and sporophyte stages of a life cycle,and mesophyll and bundle-sheath chloro-plasts.

dinitrogenase The enzyme that cat-alyzes NITROGEN FIXATION.

dinoflagellate See Dinomastigota.

Dinomastigota (dinoflagellates) A phy-lum of the Protoctista containing mainlysingle-celled marine or freshwater algaethat swim in a twirling manner by means oftwo UNDULIPODIA that lie at right angles toeach other in two grooves within the or-ganism’s rigid body wall (test). The bodywall is often stiffened by cellulose platesunder the cell membrane. Many dinoflagel-lates possess stinging organelles (tricho-cysts) that they discharge to catch prey;and some produce potent toxins that arecapable of killing fish. Planktonic dinofla-gellates can multiply rapidly to form redtides. The phosphorescent seas of the trop-ics are usually due to bioluminescent di-noflagellates. Roughly half of all knowndinoflagellates are capable of photosynthe-sis. Many form symbioses with marine in-vertebrates such as corals and clams.

dinucleotide A compound of two nu-cleotides linked by their phosphate groups.Important examples are the coenzymesNAD and FAD.

dioecious Denoting a plant species inwhich male and female reproductive or-

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diplobiontic

gans are borne on separate individuals.Compare hermaphrodite; monoecious.

diplobiontic Describing life cycles show-ing a typical alternation of generationswith haploid and diploid somatic bodies.Ferns and mosses are diplobiontic organ-isms. Compare haplobiontic.

diploid A cell or organism containingtwice the haploid number of chromosomes(i.e. 2n). In plants exhibiting an alternationof generations the sporophyte is diploid,and the gametophyte haploid. In lowerplants such as mosses and liverworts thepersistent vegetative plant is haploid, thesporophyte being a relatively short-livedphase. Higher plants are normally alwaysdiploid. Exceptions are those species inwhich polyploidy occurs.

diplont A diploid organism that repre-sents the vegetative stage in life cycles inwhich haploidy is restricted to the gametes.Such diplontic life cycles are found in someprotoctists, such as the diatoms and certainPhaeophyta (brown algae), including thewracks. Compare haplont; alternation ofgenerations.

diplotene In MEIOSIS, the stage in late PRO-PHASE I when the pairs of chromatids beginto separate from the tetrad formed by theassociation of homologous chromosomes.Chiasmata can often be seen at this stage.

disaccharide A SUGAR with moleculescomposed of two monosaccharide units.Sucrose and maltose are examples. Theseare linked by a –O– linkage (glycosidiclink). See also glycosidic bond.

discontinuous variation (qualitativevariation; qualitative inheritance) A formof variation in which a character has twoor more distinct forms in a population. Ex-amples are Mendel’s pea characters. It gen-erally occurs when there are two or moreallelic forms of a major gene in a popula-tion. See polymorphism.

disjunct Denoting a species or genus

whose distribution is not continuouswithin its range. Examples are species thatare found in the Alps, arctic regions, andnorth European mountains and magnoliasthat occur in eastern North America, Cen-tral America, and southeast Asia, but notin the intervening regions. Such disjunc-tions may arise where climate change re-duces the range of a species, restrictingthem to certain isolated areas (refugia)where the local climate is more suitable forthem, i.e. since the last Ice Age for the cold-loving arctic–alpine species now confinedto isolated mountain tops. The other majorcause of disjunction is CONTINENTAL DRIFT.

disk floret See inflorescence.

distal Denoting the part of an organ,limb, etc., that is furthest from the origin orpoint of attachment. Compare proximal.

distely Having two steles, e.g. the stemof Selaginella kraussiana. The steles arejoined only at branches. See also polystely.

disulfide bridge In PROTEINS, a covalentbond formed between the sulfhydryl (–SH)groups of different cysteine molecules.Such bridges are important in stabilizingthe tertiary structure of peptides and pro-teins.

diurnal 1. Describing an event or activityoccurring during the day or a species orprocess that is active only by day and notby night.2. Occurring at daily intervals. See circa-dian rhythm.

divergent evolution The evolution ofdifferent forms from a single basic struc-ture in response to different selection pres-sures. For example, the basic angiospermstem is vertical and elongated, but somecacti have swollen water-storing stems,while other xerophytic plants that havelost their leaves during the course of evolu-tion have evolved flattened leaflike stemsfor photosynthesis, e.g. butcher’s broom(Ruscus aculeatus).

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division The equivalent of a phylum inthe Five Kingdoms classification. The termdivision was retained for the plant king-dom until recently. Division names of

plants (and hence phylum names too) end

in -phyta (e.g. Bryophyta, Filicinophyta),

while those of fungi end in –mycota (e.g.

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DNA

DNA

Ascomycota, Basidiomycota). Divisionsmay be divided into subdivisions.

DNA (deoxyribonucleic acid) A nucleicacid, mainly found in the CHROMOSOMES,that contains the hereditary information oforganisms. The molecule is made up of twohelical polynucleotide chains coiled around

each other to give a double helix. Phos-phate molecules alternate with deoxyri-bose sugar molecules along both chains,linked by phosphodiester bonds. Eachsugar molecule is also joined to one of fournitrogenous bases – adenine, guanine, cy-tosine, or thymine. The two chains arejoined to each other by hydrogen bondingbetween bases. The sequence of basesalong the chain makes up a code – the ge-netic code – that determines the precise se-quence of amino acids in proteins (seemessenger RNA; protein synthesis; tran-scription).

In DNA, the two purine bases (adenineand guanine) always bond with the pyrim-idine bases (thymine and cytosine), and thepairing is quite specific: adenine withthymine and guanine with cytosine. DNAis the hereditary material of all organismswith the exception of RNA viruses. To-gether with RNA and histones it makes upthe chromosomes of eukaryotic cells. SomeDNA viruses contain single-stranded DNArather than the double helix. See also junkDNA; replication; selfish DNA; RNA.

DNA hybridization See nucleic acidhybridization.

DNA ligase An enzyme, used in DNAREPLICATION and the in vitro synthesis ofDNA, that joins together segments ofnewly synthesized polynucleotide chainsusing energy from ATP. See replication.

DNA polymerase See polymerase.

DNA probe (gene probe) A nucleic acidconsisting of a single strand of nucleotideswhose base sequence is complementary tothat of a particular DNA fragment beingsought, for example a gene on a chromo-some or a restriction fragment in a DNAdigest. The probe is labeled (e.g. with a ra-dioisotope or a fluorescent compound) sothat when it binds to the target sequence,both it and the target can be identified (by autoradiography or FLUORESCENCE MI-CROSCOPY). DNA probes are used to ana-lyze fragments of DNA or RNA generatedby restriction enzymes and determine their

DNA hybridization

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DNA: the double helix

sequence in the original nucleic acid, aprocess called restriction mapping (see re-striction map). They are also used to detectgene and chromosome mutations. See alsoFISH.

DNase (deoxyribonuclease) Any en-zyme that hydrolyzes the phosphodiesterbonds of DNA. DNases are classified intotwo groups, according to their site of ac-tion in the DNA molecule (see endonucle-ase; exonuclease).

domain 1. A discrete part of the tertiarystructure of a PROTEIN that has a particularfunction, e.g. as the binding site for thesubstrate of an enzyme.2. A taxonomic grouping of organisms thatranks higher than kingdom. There are gen-erally considered to be three domains: AR-CHAEA, BACTERIA, and EUKARYA. The FIVE

KINGDOMS CLASSIFICATION recognizes onlytwo ‘superkingdoms’: Prokarya (which in-cludes the single kingdom Bacteria com-prising both archaea and the bacteria) andEukarya.

domatium (pl. domatia) See ant plant.

dominant 1. An allele that, in a het-erozygote, prevents the expression of an-other (recessive) allele at the same locus.Organisms with one dominant and one re-cessive allele thus appear identical to thosewith two dominant alleles, the difference intheir genotypes becoming apparent only onexamination of their progenies. The domi-nant allele usually controls the normalform of the gene, while mutations are gen-erally RECESSIVE.2. A plant species that is the most impor-tant member of a particular communityand affects the structure of that commu-nity.

dormancy A period of minimal meta-bolic activity of an organism or reproduc-tive body. It is a means of surviving aperiod of adverse environmental condi-tions, e.g. cold or drought. Seeds, spores,cysts, and perennating organs of plants arepotentially dormant structures. See estiva-tion.

dormin (abscisin II) A former name forABSCISIC ACID.

dorsal 1. Designating the lower or abax-ial surface of a lateral organ such as a leaf.2. Designating the upper surface awayfrom the substrate in thallose plants suchas liverworts. Compare ventral.

dorsifixed A stamen in which the fila-ment is fused to the back of the anther.Compare basifixed; versatile.

dorsiventral Describing a structure thathas distinct upper and lower surfaces. Adorsiventrally flattened structure has flatdorsal and ventral surfaces and very nar-row lateral surfaces.

double fertilization The fusion of onepollen nucleus with the egg to form the zy-gote and of the other pollen nucleus with apolar nucleus to form the triploid en-dosperm nucleus. The process is restrictedto certain angiosperms but a primitiveform is known from some gymnosperms.

double helix See DNA.

double recessive An organism contain-ing both recessive alleles of a particulargene and thus expressing the recessive formof the gene in its phenotype. Double reces-sives, being of known genotype, are oftenused in test crosses to establish whether theorganism to which it is crossed is heterozy-gous or homozygous for the same gene. Seebackcross.

DPD See diffusion pressure deficit.

drupe (pyrenocarp) A fleshy indehiscentfruit containing one or more seeds eachsurrounded by a hard stony wall, the endo-carp. Drupes with one seed include plumsand cherries while many-seeded drupes in-clude holly and elder fruits. Blackberriesand raspberries are collections of smalldrupes or drupelets.

duplex Double, or having two distinctparts. The term is particularly used to de-scribe the double helix of the Watson–

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dwarfism

ecocline A CLINE that is due to a specificenvironmental factor, such as a gradient ofheavy metal concentration in the soil.

ecological niche The functional role ofan organism in a community. It includesthe habitat in which the organism lives, theresources it uses, and the periods of time inwhich it is present and/or active there andits interactions with other organisms. Itcan normally be defined only by the pres-ence of the organism and is used more ex-tensively for animals than for plants. If twospecies occupy the same niche, then com-petition may occur until either one has re-placed the other or the two divide the nichebetween them to some extent. A similarniche may be occupied by different speciesin different areas. For example differentspecies of grasses are dominant in grass-lands in different parts of the world.

ecology The study of the relationshipsof organisms to one another and to theirliving (biotic) and nonliving (abiotic) envi-ronment.

ecosystem (ecological system) A unitmade up of all the living and nonlivingcomponents of a particular area that inter-act and exchange materials with eachother. The concept of the ecosystem differsfrom that of the community in that moreemphasis is placed on abiotic factors. Var-ious studies have been made to attempt toitemize the energy flow of an entire eco-system, taking into account factors such asincoming radiation, photosynthetic effi-ciency, etc. The term can be applied on var-ious scales, from small ponds to the wholeplanet.

ecotype A population of individuals

within a species adapted genetically to thecombination of environmental factors intheir local habitat, but still able to repro-duce with other ecotypes belonging to the same species and produce fertile off-spring. Differences between ecotypes maybe physiological or morphological. Com-pare biotype. See also adaptive radiation;speciation.

ectomycorrhiza (ectotrophic mycorrhiza)(pl. ectomycorrhizae) See mycorrhiza.

ectoparasite See parasitism.

ectoplasm The outer gel-like layer ofthe cytoplasm in the cells of plants andsome protoctists, which lies immediatelybeneath the cell membrane and contains adense array of microtubules.

ectotrophic mycorrhiza See mycor-rhiza.

edaphic factors The physical, chemical,and biological characteristics of the soilthat together form an important compo-nent of the habitat because of their influ-ence on plant distribution. The mainedaphic factors are water content, pH, or-ganic matter, and soil texture.

EDTA Ethylenediamine tetra-acetic acid,a compound that acts as a chelating agent,reversibly binding iron, magnesium, andother positive ions. It is used as a competi-tive inhibitor of certain enzymes that havemetal ion cofactors, and as a source ofiron, which it releases slowly into the solu-tion.

effector A molecule that can combinewith a repressor in an OPERON, making the

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E

REPRESSOR inactive and thus enabling MES-SENGER RNA and protein to be produced.

egg apparatus The three haploid nucleithat are situated at the micropylar end ofthe embryo sac in most flowering plants.The central nucleus is the female gameteand those to either side of it are called thesynergids. See embryo sac. See also antipo-dal cells.

egg cell See ovum.

elaioplast (lipidoplast; oleoplast) A col-orless plastid (leucoplast) storing lipids(fats or oils).

elaiosome See oil.

elater One of many elongated spirallythickened cells that are formed within the capsule of certain liverworts (e.g.Marchantia and Pellia). When the elatersare exposed to the air they dry out un-evenly because of the differential thicken-ing. The resulting twisting movements ofthe cells help in dispersing the spores.

electron carrier A molecule that canfunction as both an acceptor and a donorof electrons and/or protons in an electron-transport system. See coenzyme Q; cy-tochromes; electron-transport chain; fer-redoxins.

electron micrograph See micrograph.

electron microscope See microscope.

electron-transport chain (respiratorychain) A chain of membrane-linked oxi-dation–reduction reactions involving pro-teins and enzymes, during which electronsare transferred from an initial electrondonor through a series of intermediates toa final electron acceptor, resulting in theformation of ATP. The main electron-transport systems in plants are involved inrespiration and the light reaction of PHO-TOSYNTHESIS. Heterotrophic bacteria havea similar respiratory electron-transportchain.

In eukaryotic cells, the enzymes andother components of the respiratory elec-tron-transport chain are located in theinner membrane of the MITOCHONDRIA.During aerobic respiration the reducedcoenzyme NADH, produced by the KREBS

CYCLE in the mitochondrial matrix gives uptwo electrons to the first component in thechain, NADH dehydrogenase, and two hy-drogen ions (H+) are discharged from thematrix of the mitochondrion into the inter-membrane space. The electrons are trans-ferred along the chain to a carrier molecule(COENZYME Q), and then in sequence to aseries of cytochromes, finally acting withthe enzyme cytochrome oxidase to reducean oxygen atom, which combines with twoH+ ions to form water. During this electrontransfer, a further two pairs of H+ ions arepumped into the intermembrane space,making a total of six per molecule ofNADH. If FADH2 is the electron donor,only four H+ ions are pumped across.

The function of electron transport inthe mitochondrion is to phosphorylateADP to ATP.

electrophoresis The migration of elec-trically charged particles toward oppo-sitely charged electrodes in solution underan electric field – the positive particles tothe cathode and negative particles to theanode. The rate of migration varies withmolecular size and shape. The techniquecan be used to separate or analyze mixtures(e.g. of proteins or nucleic acids).

elicitors Molecules that initiate cell sig-naling pathways leading to the activationof plant defense genes and the productionof PHYTOALEXINS and other defensivechemicals in response to infection by apathogen.

elongation The enlargement of cellsafter mitotic division that results from theuptake of water by OSMOSIS until the cellsbecome turgid. Elongation also involvesthe enzyme-mediated stretching of the cellwalls. Hormones, especially AUXINS, are in-volved in the promotion of elongation.TROPISMS are due to differential elongation

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elongation

of cells on different sides of root or shoot,resulting in curvature.

elongation factor See translation.

elution The removal of an adsorbedsubstance in a CHROMATOGRAPHY columnor ion-exchange column using a solvent(eluent), giving a solution called the eluate.The chromatography column can selec-tively adsorb one or more componentsfrom the mixture to ensure efficient recov-ery of these components graded elution isused. The eluent is changed in a regularmanner starting with a nonpolar solventand gradually replacing it by a more polarone. This will wash the strongly polar com-ponents from the column.

emasculation The removal of the sta-mens of a plant before the anthers burst to prevent self-pollination or unwantedcrosses to nearby plants.

Embden–Meyerhof–Parnas pathwaySee glycolysis.

embedding The sealing of tissue pre-pared for permanent microscope slides in asolid block of paraffin wax prior to sec-tioning. After CLEARING, tissues are placedin two or three baths of molten paraffinwax. When the tissue is completely infil-trated by the wax it is allowed to harden.As the wax is opaque the block must bemarked to insure correct orientation whensectioning. Tissue prepared for electronmicroscopy may be embedded in tougherepoxy resins such as Araldite and cut witha diamond knife. See also fixation.

embryo The organism that developsfrom the zygote of a vascular plant afterthe proembryo has differentiated into em-bryo and SUSPENSOR but before germina-tion, i.e. while the young plant is stillnutritionally dependent on parental tis-sues. In the Filicinophyta, Sphenophyta,and Lycophyta the embryo sporophyteproduces a footlike structure that remainsembedded in the tissues of the parent ga-metophyte plant, absorbing nutrients like

the suspensor of higher plants. See embryosac.

embryogeny (embryony) 1. The devel-opment of an EMBRYO from the momentthe ZYGOTE divides.2. The development of embryos fromdiploid somatic cells in embryo culture.Such embryos are often termed embryoids,to distinguish them from embryos formedfrom zygotes. They are capable of givingrise to normal plants. Embryoids that de-velop from pollen grains grow into haploidplants.

embryology The study of the develop-ment of embryos, usually from fertiliza-tion.

embryony See embryogeny.

embryo sac A large oval thin-walled sacin the nucellus of flowering plants in whichegg fertilization and subsequent embryodevelopment occurs. It corresponds to thefemale GAMETOPHYTE of lower plants andcontains a number of nuclei derived by di-vision of the MEGASPORE nucleus.

There are commonly eight nuclei in theembryo sac: the egg apparatus at the mi-cropylar end, made up of an egg nucleusand two synergid nuclei; three antipodalcells at the opposite chalazal end that prob-ably aid embryo nourishment; and twopolar nuclei in the center that fuse to formthe primary endosperm nucleus. Otherplants have four nuclei or sixteen nuclei intheir embryo sacs. At fertilization one male

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Embryo sac

nucleus fuses with the egg nucleus to formthe zygote, while the second male nucleusfuses with the primary endosperm nucleusto form a triploid cell, or more rarely a sec-ond diploid or a pentaploid cell that latergives rise to the ENDOSPERM. In the gym-nosperms the megaspore gives rise to a cellthat is termed the embryo sac because of itssimilarity to the angiosperm structure. Seenucellus; micropyle.

Emerson effect The observation byRobert Emerson in 1957 that PHOTOSYN-THESIS proceeds much faster when chloro-plasts are illuminated with light at awavelength of 650 nm as well as at 700 nm(the absorption wavelength of chlorophylla molecules). It contributed to the discov-ery of PHOTOSYSTEMS I AND II.

endangered species See CITES; RedData Book.

endarch Denoting a stele in which meta-xylem develops to the outside of pro-toxylem. Compare centrarch; exarch; mes-arch.

endemic Describing a population orspecies that is restricted geographically.Some endemics have evolved in geographi-cal isolation on islands or mountain tops;others may be relics of once widespreadspecies that have become restricted owingto climate change, geological change, orhuman activity.

endocarp See pericarp.

endocytosis The bulk transport of ma-terials into cells across the plasma mem-brane by processes not involving diffusionor active transport. It is described aspinocytosis (cell drinking) or phagocytosis(cell eating) depending on whether the ma-terial is fluid, containing molecules in solu-tion, or solid respectively. The processinvolves extension and invagination of theplasma membrane to form small vesicles inpinocytosis (pinocytotic vesicles) or vac-uoles in phagocytosis (food vacuoles). Thecontents are often digested by enzymes

from lysosomes. Compare exocytosis. Seelysosome.

endodermis The innermost part of thecortex, consisting of a single layer of cellsthat controls the passage of water andsolutes between the cortex and the stele. Aclearly defined endodermis is seen in allroots and in the stems of the non seed-bear-ing vascular plants and some dicotyledons.See Casparian strip; passage cells.

endogenous Produced or originatingwithin an organism. Compare exogenous.

endomitosis The duplication of chro-mosomes without division of the nucleus,causing polyploidy and leading to an in-crease in nuclear and cytoplasmic volume.Endomitosis occurs regularly in certain tis-sues, such as the phloem cells of some legu-minous plants. Compare amitosis; mitosis.

endomycorrhiza (endotrophic mycor-rhiza) (pl. endomycorrhizae) See mycor-rhiza.

endonuclease An enzyme that catalyzesthe hydrolysis of internal bonds of polynu-cleotides such as DNA and RNA, produc-ing short segments of linked nucleotides(oligonucleotides). See also DNase; restric-tion endonuclease.

endophyte A bacterium, protoctist, orfungus that lives inside a plant or otherphotosynthetic organism, but is not a par-asite. Compare epiphyte.

endoplasm The inner layer of cyto-plasm in plant cells, which contains themain organelles. Compare ectoplasm.

endoplasmic reticulum (ER) A systemof membranes forming tubular channelsand flattened sacs (cisternae), runningthrough the cytoplasm of all eukaryoticcells and continuous with the nuclear enve-lope. Although often extensive, it was onlydiscovered with the advent of electron mi-croscopy. Its surface is often covered withribosomes, forming rough ER. The pro-

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endoplasmic reticulum

teins they make can enter the cisternae fortransport to other parts of the cell or for se-cretion via the GOLGI APPARATUS. ER lack-ing ribosomes is called smooth ER and isinvolved with lipid synthesis, includingphospholipids and steroids.

endopolyploidy See endomitosis.

endoscopic Describing the type of de-velopment of a plant embryo in which theinner cell formed by the first division of thezygote develops into the embryo, while theouter cell develops into the suspensor. It isseen in many ferns and in all seed plants.Compare exoscopic.

endosperm The nutritive tissue that sur-rounds the embryo in angiosperms. Innonendospermic seeds most of the en-dosperm is absorbed by the developing em-bryo and the food stored in the cotyledons.In endospermic seeds the endosperm re-places the nucellus and is often a richsource of growth-regulating substances.Many endospermic seeds (e.g. cereals andoil seeds) are cultivated for their food re-serves. The endosperm is usually triploidand develops from the primary endospermnucleus, the triploid nucleus resulting fromthe fusion of one of the male gametes fromthe pollen tube with the polar nuclei in theembryo sac.

endospore A resting stage produced bycertain bacteria under unfavorable condi-tions. Endospores are formed within thecell, one in each parent cell, and are sur-rounded by a thick coat containing dipicol-inic acid. On germination the wall is lysedand one vegetative cell is produced. En-dospores can remain viable for several cen-turies and are resistant to heat, desiccation,and x-rays.

endosporium (intine) See pollen.

endosymbiont theory The theory thateukaryotic organisms evolved from symbi-otic associations between bacteria. It pro-poses that integration of photosyntheticbacteria, for example purple bacteria and

cyanobacteria, into larger bacterial cellsled to their permanent incorporation asforerunners of the plastids (e.g. chloro-plasts) seen in modern eukaryotes. Simi-larly, other symbiotic aerobic bacteria gaverise to the mitochondria.

endotrophic mycorrhiza (pl. endo-trophic mycorrhizae) See mycorrhiza.

entire Describing the margin of a leaf,sepal, or petal that is smooth and undi-vided.

entomophily Pollination by insects.Various structures and mechanisms haveevolved to attract insects (e.g. showy petalsand nectar) and to insure that they carrypollen away on their bodies.

environment The complete range of ex-ternal conditions under which an organismlives, including physical, chemical, and bi-ological factors, such as temperature, light,the availability of food and water, and theeffects of other organisms.

enzyme A compound that catalyzes bio-chemical reactions. Enzymes are proteins,which act with a given compound (the sub-strate) to produce a complex, which thenforms the products of the reaction. The en-zyme itself is unchanged in the reaction; itspresence allows the reaction to take placeby lowering the activation energy. Thenames of most enzymes end in -ase, addedto the substrate (e.g. lactase) or the reac-tion (e.g. hydrogenase).

Enzymes are extremely efficient cata-lysts for chemical reactions, and specific toparticular reactions. They may have a non-protein part (cofactor), which may be aninorganic ion or an organic constituent(coenzyme). The mechanism of action ofmost enzymes appears to be by active siteson the enzyme molecule. These have veryspecific shapes and charge distributions.The substrate acting with the enzymechanges shape to fit the active site, and thereaction proceeds. Enzymes are very sensi-tive to their environment – e.g. tempera-ture, pH, and the presence of other

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substances. See activation energy. See alsoribozyme.

Eocene The second oldest epoch of theTertiary period, 55–38 million years ago. Itwas a period of widespread temperate andsubtropical forests. The extensive grass-lands of the late Tertiary had not yet devel-oped, but large-hoofed grazing mammals,such as horses and elephants, were evolv-ing rapidly. See also geological time scale.

eosin An acid stain that colors cellulosered and cytoplasm pink. See staining.

ephemeral A plant that has a very shortlife cycle and may complete more than onelife cycle within a year. Such plants areoften referred to as annuals. Examples areshepherd’s purse (Capsella bursa-pastoris)and certain desert plants that grow, flower,and set seed in brief periods of rain. Com-pare annual; biennial; perennial.

epicarp See pericarp.

epicotyl The part of the plumule abovethe cotyledons. Compare hypocotyl.

epidermis The outer protective layer ofcells in plants. In aerial parts of the plantthe outer wall of the epidermis is usuallycovered by a waxy cuticle that preventsdesiccation, protects the underlying cellsfrom mechanical damage, and increasesprotection against fungi, bacteria, etc. Thecells are typically platelike and closelypacked together except where they aremodified for a particular function, as areguard cells. The epidermis arises from thetunica meristem. In damaged stems androots, and in those undergoing secondarygrowth, it is replaced by a secondary layer,the periderm. The specialized epidermalarea of the roots from which the root hairsarise is termed the piliferous layer. Theterm epiblem is sometimes used instead todenote the outermost layer of cells in theroot.

epigeal germination Seed germinationin which the cotyledons form the first pho-tosynthetic organs above the ground, e.g.

sunflower (Helianthus). Compare hy-pogeal germination.

epigyny The type of flower structure inwhich the perianth and androecium are in-serted above the gynoecium, giving an infe-rior ovary, fused with the receptacle. It isseen in the Asteraceae and Rosaceae. Com-pare hypogyny; perigyny.

epilimnion In a stratified lake (a lake inwhich there are distinct layers of differenttemperature densities), the upper layer ofwarm water. See stratification.

epinasty (epinastic movements) A nasticmovement that involves the curving of aplant organ away from the axis (i.e. down-ward) as a result of greater growth on theupper surface, as in the opening of flowers.See nastic movements; leaf mosaic.

epiphylly 1. The growth of a plant onthe leaf of another plant. Such a plant is termed an epiphyll, or is described asepiphyllous. Examples include manymosses.2. The production of adventitious buds ona leaf, e.g. Bryophyllum, where the budsform along the leaf margins, developinginto small plantlets with adventitious rootsthat eventually become detached from theleaf.

epiphyte Any plant growing upon or at-tached to another plant or object merelyfor physical support. Examples of tropicalepiphytes are ferns, orchids, and bromeli-ads. In temperate regions vascular epi-phytes are rare: here most epiphytes arelichens, mosses, liverworts, and algae.

epithelium (pl. epithelia) A tissue con-sisting of a sheet (or sheets) of cells thatcovers a surface. Epithelia may line resincanals in gymnosperms, or, sometimes,gum ducts in dicotyledons, and usuallyhave a secretory role.

equatorial plate An arrangement of thechromosomes in which the centromeres be-come aligned in a single plane at the center

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Equisetales

of the spindle during metaphase of mitosisand meiosis.

Equisetales (horsetails) The single orderof the phylum SPHENOPHYTA, which con-tains the horsetails.

ER See endoplasmic reticulum.

era A unit of the geological time scalethat is made up of several periods, e.g. the Mesozoic Era comprises the Triassic,Jurassic, and Cretaceous periods. Whenused formally, the initial letter is capital-ized.

ergot A fungal disease of many cereals.The hard black sclerotia of Claviceps pur-purea, an ascomycete, replace the grains(see sclerotium). They contain alkaloidsthat can cause severe poisoning and evendeath if taken in with food by humans andother animals.

Escherichia coli A rod-shaped Gram-negative bacterium, some strains of whichare normally present in animal intestines,soil, or water. It is widely used in geneticresearch and genetic engineering as arepository for fragments of DNA incorpo-rated into plasmids in its cytoplasm.

essential amino acid See amino acids.

essential element An element that is in-dispensable for the normal growth, devel-opment, and maintenance of a livingorganism. Some, the major elements, arerequired in relatively large quantities andmay be involved in several different meta-bolic reactions (see carbon; hydrogen;oxygen; nitrogen; sulfur; phosphorus;potassium; magnesium; calcium). Others,such as iron, manganese, molybdenum,boron, zinc, copper, cobalt, iodine, and se-lenium, are required in only small orminute amounts. See micronutrient.

essential oil Any of a number of volatileOILS secreted by aromatic plants that arethe source of characteristic odors or tastes.

ester A compound formed by the con-

denstion reaction of a carboxylic acid withan alcohol:

RCOOH + HOR1 ® RCOOR1 + H2O.

estivation The way in which sepals andpetals are folded in the flower bud beforeexpansion. Compare ptyxis; vernation.

ethanedioic acid See oxalic acid.

ethanoic acid See acetic acid.

ethene See ethylene.

ethylene (ethene) A gaseous hydrocarbon(C2H4), produced in varying amounts bymany plants, that functions as a plant HOR-MONE. Its production may be stimulated byother plant hormones. The amino acid me-thionine is a precursor. Ethylene is in-volved in the control of germination, cellgrowth, fruit ripening, abscission, andsenescence, and it inhibits longitudinalgrowth and promotes radial expansion. Inripening fruit a rapid rise in ethylene pro-duction precedes respiration to reach theclimacteric ripeness: production of ethyl-ene stimulates further production. This iswhy ripe fruit stimulate other fruits toripen quickly and this process can be con-trolled to some extent for fruit storage andtransport.

etiolation The type of growth exhibitedby plants grown in darkness, usually fromseed. They lack chlorophyll and thereforeappear white or yellow. They show less dif-ferentiation and contain reduced amountsof supporting material, such as lignin, con-centrating resources on elongation of in-ternodes. The plant can then reach the lightfaster and photosynthesize and synthesizemore chlorophyll.

etioplast A modified chloroplast formedfrom proplastids in leaves grown in totaldarkness.

Eubacteria A major subkingdom of theBacteria, containing a large and diversegroup of bacteria, principally distinguish-able from the other major subkingdom, theArchaea, by differences in the base se-

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78

quences of RNA subunits of the ribosomes,which are thought to reflect the very earlyevolutionary divergence between the twogroups. Most are unicells that divide by bi-nary fission. The cells can be spherical,rod-shaped, or helical, and some formassemblages of cells, such as branchingfilaments. Most are immotile, but somepossess flagella. They are a ubiquitousgroup, some being found in extreme condi-tions. Cell-wall structure, morphology,and metabolic features are used to distin-guish the different phyla. The Eubacteriacontain both Gram-negative and Gram-positive bacteria, and bacteria with no cellwalls at all.

eucaryote See eukaryote.

euchromatin See chromatin.

Euglenophyta Formerly, a phylum ofaquatic single-celled protoctists that swimusing one or more UNDULIPODIA (flagella),now classified as a class, the Euglenida, ofthe phylum Discomitochondria. It containsboth photosynthesizing and nonphotosyn-thesizing members. For example, membersof the genus Euglena possess a flexiblepellicle surrounding the cell, and a singleundulipodium. Many species containchloroplasts, with pigments similar tothose found in plants, although these or-ganisms may also consume dissolved orparticulate food from their surroundings.

Eukarya In taxonomy, a DOMAIN of liv-ing organisms characterized by having eu-karyotic cells (See eukaryote). In the FiveKingdoms classification, the Eukarya con-stitutes a ‘superkingdom’ containing thekingdoms Animalia, Fungi, Plantae, andProtoctista.

eukaryote (Eukaryotae) All the livingkingdoms except the Bacteria (Archaeaand Eubacteria) are the eukaryotes. Theyare defined by the presence of a much moreelaborate cell than the prokaryotes. Eu-karyotes are characterized by: the geneticmaterial packaged in chromosomes withina membrane-bound nucleus; possession of

mitochondria, and, in photosynthetic eu-karyotes, chloroplasts; a quite differentand much elaborated membrane structure,including internal membranes such as theendoplasmic reticulum and Golgi appara-tus; different-sized ribosomes; and com-plex UNDULIPODIA composed of arrays ofMICROTUBULES. Some of these featuresprobably arose through endosymbiosis ofprokaryotes. See endosymbiont theory.

Euphorbiaceae A large family of di-cotyledonous plants that include thespurges. There are over 8000 species. Theyare mostly trees and shrubs, with but a fewherbaceous species, and are found world-wide, but especially in the tropics. Com-mercially important species include Heveabrasiliensis, the main source of naturalrubber; cassava (Manihot esculenta); andcastor oil (Ricinus communis).

euphotic zone See photic zone.

euploidy The normal state in which anorganism’s chromosome number is anexact multiple of the haploid number char-acteristic of the species. For example, if thehaploid number is 7, the euploid numberwould be 7, 14, 21, 28, etc., and therewould be equal numbers of each differentchromosome. Compare aneuploidy.

eusporangiate Describing the condi-tion, found in clubmosses, horsetails, andcertain ferns in which the sporangia de-velop from a group of initial cells and donot have an elaborate spore dispersalmechanism. Compare leptosporangiate.See also Filicinophyta.

eustele The STELE arrangement found inmost gymnosperms and dicotyledons inwhich the vascular tissue is arranged into aring of discrete bundles separated bymedullary rays, all contained within a sin-gle ring of endodermis and pericycle. It is aform of SIPHONOSTELE.

eutrophic Describing lakes or pondsthat are rich in nutrients and consequentlyare able to support a dense population of

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eutrophic

plankton and littoral vegetation. Eutroph-ication is the process that results when anexcess of nutrients enters a lake, for exam-ple as sewage or from water draining offland treated with fertilizers. The nutrientsstimulate the growth of the algal popula-tion giving a great concentration or‘bloom’ of such plants. When these die theyare decomposed by bacteria, which use upthe oxygen dissolved in the water, so thataquatic animals such as fish are deprived ofoxygen and die from suffocation. Compareoligotrophic.

evergreen Describing plants that retaintheir leaves throughout the year or throughseveral years. Many tropical species ofbroadleaved flowering plants are evergreenand their leaves are thicker and more leath-ery than those of deciduous trees. Waterconservation is not a problem in tropicalevergreen forests, so there is no need toshed leaves in a particular season. In polarand cold temperate regions many of theevergreens are shrubs or trees with needle-like or scalelike leaves, adaptations thatprevent water loss by transpiration. Com-pare deciduous.

evolution The process of genetic changethat occurs in populations of organismsover a period of time. It manifests itself asnew characteristics in a species, and even-tually the formation of new species. SeeDarwinism; natural selection.

exarch Denoting a stele in which themetaxylem develops to the inside of theprotoxylem. Compare centrarch; endarch;mesarch.

excretion The process by which excess,waste, or harmful materials, resulting fromthe chemical reactions that occur withinthe cells of living organisms, are eliminatedfrom the body. In plants most waste prod-ucts accumulate in the vacuoles, whose se-lectively permaeable membranes preventthem interfering in cell reactions. Gaseousexcretion takes place by diffusion throughthe stomata and lenticels. In fungi andsmall protoctists waste products are ex-

creted by diffusion through the cell or bodysurface into the external medium.

exine (exosporium) See pollen.

exocarp (epicarp) See pericarp.

exocytosis The bulk transport of mate-rials out of the cell across the plasma mem-brane. It involves fusion of vesicles orvacuoles with the plasma membrane in areversal of endocytosis. See also lysosome.Compare endocytosis.

exodermis An outermost layer of thick-ened or suberized cortical cells that some-times replaces the epidermal layer in theolder parts of roots if the epidermal cellshave died.

exogamy The fusion of gametes pro-duced by organisms that are not closely re-lated. See outbreeding.

exogenous Produced or originating out-side an organism. Compare endogenous.

exon A segment of a gene that is bothtranscribed and translated and hence car-ries part of the code for the gene product.Most eukaryotic genes consist of exons interrupted by noncoding sequences (seeintron). Both exons and introns are tran-scribed to heterogeneous nuclear RNA(hnRNA), an intermediary form of messen-ger RNA (mRNA); the introns are then re-moved leaving mRNA, which has only theessential sequences and is translated intothe protein. Bacteria do not have introns.

exonuclease An enzyme that catalyzesthe hydrolysis of the terminal linkages ofpolynucleotides such as DNA and RNA,thereby removing terminal nucleotides. Seealso DNase; restriction endonuclease.

exoscopic Describing the type of devel-opment of a plant embryo in which theapex of the sporophyte develops from theouter cell formed by the first division of thezygote, the inner cell giving rise to the foot.Compare endoscopic.

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exosporic The formation of spores out-side the spore-producing organ. In thebasidiomycete fungi, for example, the ba-sidiospores are borne on the tips of thesterigmata, which are outgrowths of thebasidium.

exosporium (exine) See pollen.

explantation The culture of isolated tis-sues of adults or embryos in an artificialmedium for the study of maintenance,growth, and/or differentiation. Such a pieceof transplanted tissue is called an explant,and its placement on the culture medium iscalled inoculation. Explants are also ameans of plant propagation, producingclones and offering a more rapid means ofmultiplication than seed production.

exponential growth A type of growthin which the rate of increase in numbers ata given time is proportional to the numberof individuals present. Thus, when thepopulation is small multiplication is slow,but as the population gets larger, the rateof multiplication also increases. An expo-nential growth curve starts off slowly andincreases faster and faster as time goes by(called the log phase). However, at somepoint factors such as lack of nutrients, ac-cumulated wastes, etc., limit further in-crease, when the curve of number againsttime begins to level off and the curve be-

comes sigmoid (S-shaped). When the pop-ulation remains stable, it has reached thestationary phase. If organisms are intro-duced into a new medium, there is often aperiod during which the population re-mains constant before it begins to increase(called the lag phase). During this phase thecells are increasing in size and synthesizingmaterials necessary for growth under thenew conditions.

extensin See glycoprotein.

extracellular Occurring or situated out-side a cell.

extrachromosomal DNA In eukary-otes, DNA found outside the nucleus of thecell and replicating independently of thechromosomal DNA. It is contained withinorganelles in the cytoplasm, e.g. mitochon-dria, chloroplasts, and plastids, and is re-sponsible for cytoplasmic inheritance. Seecytoplasmic inheritance.

extrorse Denoting anthers in which de-hiscence lines are to the outside of theflower, promoting cross-pollination. Com-pare introrse.

eyespot (stigma) A light-sensitive struc-ture of certain protoctists and invertebrateanimals. The eyespot of unicellular andcolonial algae and their gametes and

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eyespot

F1 The first filial generation; i.e. the firstgeneration that results from a particularcross.

F2 The second filial generation, obtainedby crossing or selfing within the F1 genera-tion. If the characteristic is governed bysimple Mendelian genes, the typical mono-hybrid and dihybrid ratios become appar-ent in the F2 generation.

Fabaceae A large family of dicotyledo-nous flowering plants, commonly knownas legumes. There are about 18,000 spe-cies, which include herbs, trees, shrubs,climbers, and some aquatics. Most specieshave root nodules containing nitrogen-fixing bacteria. The leaves are usually com-pound and pinnate, with stipules. TheFabaceae have a worldwide distributionand many are of economic importance,including peas (Pisum sativum), beans(Phaseolus and Vicia), lentils (Lens culi-naris), and peanuts (Arachis hypogea).Others are used for forage, e.g. clovers(Trifolium) and lucerne (Medicago sativa).There are also ornamental species such aslupins (Lupinus).

facilitated diffusion A passive trans-port of molecules across a cell membranealong a concentration gradient, mediatedby carrier molecules or complexes, usuallyproteins. No energy is expended in thisprocess, but it enables the passage throughthe membrane of molecules that otherwisecould not pass through.

facultative Describing an organism thatcan utilize certain conditions but is not de-pendent on them, i.e. an organism that canadopt an alternative mode of living. For ex-ample, a facultative anaerobe is an organ-

ism that can grow under anaerobic condi-tions but is also able to survive in aerobicconditions. Compare obligate.

FAD (flavin adenine dinucleotide) A de-rivative of riboflavin that is a coenzyme inelectron-transfer reactions. It acts as aprosthetic group to various dehydrogenaseenzymes, the substrate being oxidized asFAD accepts electrons and is reduced toFADH2. See also flavoprotein.

Fagaceae A family of deciduous or ever-green trees and shrubs that includes theoaks (Quercus), beeches (Fagus), southernbeeches (Nothofagus), and chestnuts (Cas-tanea). Members of this family dominatebroad-leaved forests, especially of temper-ate regions.

false fruit See pseudocarp.

family A taxonomic category involvinga collection of similar genera. Families maybe subdivided into subfamilies, tribes, andsubtribes. Plant family names generally endin aceae. Similar families are grouped intoorders.

far-red light Electromagnetic radiationof wavelength approximately 740 nm. Seephytochrome; photoperiodism.

fascicular cambium (pl. cambia) Seeintrafascicular cambium.

fast green See staining.

fat A triglyceride of a long-chain car-boxylic acid (fatty acid) that is solid below20°C. Fats commonly serve as energy stor-age material in some plants, especially inseeds. See also lipid.

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F

fatty acid See carboxylic acid.

feedback inhibition The inhibition ofthe activity of an enzyme (often the first) in a multienzyme reaction sequence by the product of that sequence. When theproduct accumulates beyond an optimalamount it binds to a site (ALLOSTERIC SITE)on the enzyme, changing the shape so thatit can no longer react with its substrate.However, once the product is utilized andits concentration drops again, the enzymeis no longer inhibited and further forma-tion of product results. The mechanism isused to regulate the concentration of cer-tain substances within a cell.

feedback loop (feedback mechanism) Aregulatory mechanism in a system. Thismay involve negative feedback, for exam-ple, an increase in the number of herbi-vores in an ecosystem leads to overgrazingand reduction of the herbivores populationby starvation or increased susceptibility todisease, which allows the plants to growback, and so on – the population of bothspecies fluctuates about a mean. Positivefeedback has the opposite effect, reinforc-ing a change in the system. In certain meta-bolic pathways, for example, the substrateof the first enzyme in the pathway stimu-lates subsequent reactions in the sequence.

fermentation The breakdown of or-ganic substances, particularly carbohy-drates, under anaerobic conditions. It is aform of anaerobic respiration and is seen incertain bacteria and yeasts, and also inplants under waterlogged conditions. Seealcoholic fermentation. See also glycolysis;lactic acid bacteria.

ferns See Filicinophyta.

ferredoxins A group of red-brown pro-teins found in green plants, many bacteria,and certain animal tissues. They containnonheme iron in association with sulfur atthe active site. They are strong reducingagents (very negative redox potentials) andfunction as electron carriers, for examplein photosynthesis and nitrogen fixation.

They have also been isolated from mito-chondria. In photosynthesis ferredoxin isinvolved in electron transfer between pho-tosystem I and the final electron acceptor,NADP.

fertilization (syngamy) The fusion of amale gamete with a female gamete to forma zygote, the essential process of sexual re-production. External fertilization occurswhen gametes are expelled into a waterysubstrate from the parental bodies beforefusion; it is typical of sporebearing plants.Internal fertilization takes place in the seedplants and some clubmosses and ferns,often by means of a pollen tube. Internalfertilization is an adaptation to life in a ter-restrial environment.

Feulgen’s stain A test used to show thepresence of DNA, especially in dividing nu-clei. A sample of tissue is placed in dilutehydrochloric acid at 60oC to hydrolyze theDNA. This exposes the aldehyde groups ofdeoxyribose. The tissue is then soaked inSchiff’s reagent: the development of a ma-genta color indicates the location of DNA.See staining.

fiber A form of SCLERENCHYMA cell thatis often found associated with vascular tis-sue. Fibers are long narrow cells, withthickened walls and finely tapered ends.Their function is more as supporting tissuethan as conducting tissue. Where theyoccur interspersed with the xylem theymay be distinguished from tracheids bytheir narrower lumen.

fiber-tracheid An elongated cell withbordered pits found in wood, intermediatein form between a fiber and a tracheid.

fibrous root system A branching rootsystem in which there is no main root. Infibrous root systems the radicle or pri-mary root either branches or dies and is re-placed by adventitious roots, e.g. grasses(Poaceae). Fibrous root systems are charac-teristic of monocotyledons. Compare tap-root.

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field capacity

field capacity The point at which thesoil contains all the water it can hold bycapillary and chemical attraction. Anymore water added to soil at field capacitywould drain away by gravity. Soil at fieldcapacity has a high water potential. Watercontent at field capacity is usually ex-pressed as a percentage of the weight orvolume of oven-dry soil. This is affected bythe soil texture, clay having a high field ca-pacity and sand a low field capacity.

field layer The herb layer of a plantcommunity. Compare canopy; groundlayer.

filament 1. The stalk of the stamenbearing the anther in angiosperms.2. The vegetative body of the filamentousalgae (e.g. Spirogyra), composed of a lineof similar cells joined by their end walls.3. A strand of protein of diameter 4–15 nmfound inside a cell. Such filaments are usu-ally composed of the contractile proteinsactin and/or myosin, and are thought toplay a role in cell motility, and perhaps alsochange in shape.

Filicinophyta (ferns) The largest phy-lum of spore-bearing vascular plants com-prising the ferns and including about12,000 species. It is divided into two maingroups, the LEPTOSPORANGIATE ferns andthe EUSPORANGIATE ferns. In leptosporan-giate ferns, the sporangia are thin-walledand develop from one cell. In the Filicalesthe leptosporangium has a specializedspore dispersal mechanism involving anannulus of cells with thick walls that ejectss pores explosively as it dries out. Lep-tosporangiate ferns include the typicalferns (Filicales) and the water ferns (Mar-sileales and Salviniales). The Filicales ismuch the largest living group of ferns, andthey normally have large spirally arrangedleaflike fronds (MEGAPHYLLS) derived fromlateral branches of the main axis, some orall of which bear sporangia on their mar-gins or undersurfaces. In eusporangiateferns, the sporangia are thick-walled anddevelop from a number of cells. Eusporan-giate ferns include the adderstongues

(Ophioglossales), Marattiales, and royalferns (Osmundales), which are to some ex-tent intermediate between leptosporan-giate and eusporangiate ferns. The whiskferns (Psilotales) may also be includedhere, but in the Five Kingdoms classifica-tion they have the status of a phylum(Psilophyta). There are also a number offossil genera, leptosporangiate ferns datingback to the Cretaceous era, eusporangiateferns to the Devonian. Ferns do not showsecondary thickening, but in large fernssuch as tree ferns bands of sclerenchymaand the overlapping bases of the frondshelp to support the plant.

fimbriae (sing. fimbria) See pili.

fine structure See ultrastructure.

FISH (fluorescence in situ hybridization)A technique for locating specific sequencesof DNA using fluorescent probes that areviewed under the MICROSCOPE. The probesbind to (hybridize with) specific sequencesof DNA; different colors of probe may beused to distinguish different DNA se-quences. See DNA probe.

fission A type of asexual reproductionin which a parent cell divides into twodaughter cells (binary fission) geneticallyidentical to the parent. Binary fission oc-curs in many unicellular organisms, such asprotoctists, bacteria, and fission yeasts.Fission begins with division of the nucleusby mitosis, followed by cytoplasmic divi-sion, and sometimes sporulation. Comparebudding; fragmentation.

fitness In an evolutionary context, theability of an organism to produce a largenumber of offspring that survive to a re-productive age.

Five Kingdoms classification A classi-fication system that recognizes five king-doms containing at least 96 phyla. Itcomprises two superkingdoms, the Pro-karya (PROKARYOTES) and Eukarya (EU-KARYOTES). The Prokarya contains a singlekingdom, the BACTERIA, which is divided

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into two subkingdoms, the ARCHAEA andthe EUBACTERIA. The Eukarya contains fourkingdoms, the Animalia, Plantae, Fungi,and Protoctista.

fixation 1. The situation in a usuallysmall population when an allele reaches a100% frequency due to the complete lossof all other alleles of that gene. All mem-bers of the population are homozygous forthis particular allele.2. In the preparation of microscope slides,the process by which tissues are rapidlykilled and preserved by chemicals to pre-vent decay or autolysis with minimaldistortion of structure. Fixing agents (fixa-tives) should also render cell organelles andinclusions more visible and harden the tis-sue to prevent shrinkage and distortionduring dehydration, embedding, and sec-tioning. Examples of fixatives areformaldehyde, osmium tetroxide, ethanol,and acetic acid. See also dehydration; em-bedding.3. The incorporation of inorganic materi-als such as carbon and nitrogen into or-ganic compounds by living organisms. Anexample is the incorporation (fixation) ofcarbon from carbon dioxide into new or-ganic compounds during photosynthesis.See also nitrogen fixation.

flaccid Lacking turgor. When a plantloses water to the extent that its cells be-come flaccid, wilting occurs. See plasmoly-sis.

flagellum (pl. flagella) A whiplike ex-tension of prokaryote cells, with a basalbody at its base, whose beat causes loco-motion of the cell. Strictly the term is nowreserved for the bacterial flagellum. Theflagella and cilia of eukaryote cells have aquite different structure and are called UN-DULIPODIA. Bacterial flagella are much sim-pler than undulipodia, being hollowcylinders about 15 nm in diameter, consist-ing of subunits of a protein (flagellin)arranged in helical spirals.

flavin A derivative of riboflavin occur-ring in the flavoproteins; i.e. FAD or FMN.

Flavins are light-sensitive yellow pigmentswith an absorption peak around 370 nm.They are involved in phototropism, and inPHOTOPERIODISM, where they are thoughtto affect phytochrome by transferring light

energy from blue to red wavelengths. Seephytochrome.

flavin adenine dinucleotide See FAD.

flavin mononucleotide See FMN.

flavonoid One of a common group ofplant compounds having the C6–C3–C6chemical skeleton in which C6 is a benzenering. They are an important source of non-photosynthetic pigments in plants.

flavoprotein A conjugated protein inwhich a FLAVIN (FAD or FMN) is the pros-thetic group joined to a protein compo-nent. Flavoproteins are enzymes and manyare dehydrogenases. They act as electron-transport agents and play an importantrole in the electron-transport chains ofrespiration, photosynthesis, and the micro-somal system responsible for the desatu-ration of fatty acids in heterotrophicbacteria.

flocculation The aggregation of soilparticles into crumbs. Compacted struc-tureless clay soils can be flocculated by theaddition of neutral salts, particularly ofcalcium. The addition of lime to saturatedclay soils (liming) is a common agriculturalpractice, improving soil structure by en-

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floral formula

Flavonoid structure

couraging crumb formation and makingheayy soils more workable.

floral formula A series of symbols usedto describe the structure of a flower. Thevarious whorls of structures are abbrevi-ated as follows: K (calyx), C (corolla), P(perianth), A (androecium), and G (gynoe-cium). These letters are each followed by anumber indicating the number of parts inthe whorl. If the number exceeds 12 thenthe symbol ¥ is used to denote an indefinitenumber. Fusion of parts of the whorl is in-dicated by placing the number of parts inbrackets. The position of the gynoecium isshown by a line above or below the letter Gdenoting an inferior or superior ovary re-spectively. The formula is preceded by Å toindicate actinomorphic flowers and ·|· forzygomorphic flowers. The floral formulafor the buttercup would be written:

ÅK5 C5 A¥ G¥

floral province (floral kingdom; floral re-gion) One of the six main regions and afew minor ones into which the world is di-vided on the basis of plant distribution.Each province has one or more centers ofdiversity and each contains unique familiesand genera of plants. The main provincesare the Boreal, which covers all of thenorth temperate; the Neotropical, coveringtropical Central and South America; thePaleotropical, covering tropical Africa andAsia; the Australian; the Cape Province,covering a small area around the Cape ofGood Hope; and the Antarctic, includingNew Zealand and temperate South Amer-ica. Oceanic islands, particularly in the Pa-cific, form minor provinces. There arefloral affinities and connections betweenthe provinces, particularly near theirboundaries.

floret 1. An individual small flower of aclustered inflorescence, e.g. daisy (Bellis).2. In grasses, the lemma, palea, and theflower they enclose.

florigen (flowering hormone) A hypo-thetical plant hormone that has been pos-tulated to account for the transfer of the

photoperiod stimulus from the leaves tothe apex where flowering is induced. Thestimulus can be transferred from one plantto another by grafting, but attempts to iso-late florigen have so far been unsuccessful.See photoperiodism.

flower The characteristic reproductivestructure of an angiosperm. It usually con-sists of an axis or receptacle bearing thesepals, petals, and stamens. The gynoeciumis borne either above the receptacle or en-closed within it. One or more of thesewhorls may be absent. Flower structuresare extremely variable and show numerousadaptations to promote pollination andseed dispersal. See also floral formula; in-florescence.

flowering hormone See florigen.

flowering plants See angiosperms.

fluid-mosaic model See plasma mem-brane.

fluorescence in situ hybridization SeeFISH.

fluorescence microscopy A form ofmicroscopy in which fluorescent probesare added to the material being investi-gated. These probes bind to specific partsof cells or to particular molecules, such asparticular sequences of DNA or protein re-ceptors.

FMN (flavin mononucleotide) A deriva-tive of riboflavin that is a coenzyme in electron-transfer reactions. It acts as aprosthetic group to various dehydroge-nases, such as NADH dehydrogenase,which catalyzes the transfer of electronsfrom NADH to coenzyme Q in the respira-tory electron-transport chain. See alsoflavoprotein.

folic acid (pteroylglutamic acid) One ofthe water-soluble B-group of VITAMINS.Folic acid is important in metabolism invarious coenzyme forms, all of which arespecifically concerned with the transfer

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and utilization of the single carbon (C1)group. It is important in the growth and re-production of cells, participating in thesynthesis of purines and thymine.

follicle A dry dehiscent fruit formedfrom one carpel that splits along one edgeto release its seed, for example Delphiniumfruit.

food chain The chain of organisms ex-isting in any natural community, throughwhich food energy is transferred. Each linkin the food chain obtains energy by eatingthe one preceding it and is in turn eaten bythe organisms in the link following it. Ateach transfer a large proportion (80–90%)of the potential energy is lost as heat, there-fore the number of links in a sequence islimited, usually to 4 or 5. The shorter thefood chain, the greater the available en-ergy, so total energy can be increased bycutting out a step in the food chain, for ex-ample if people consume cereal grain in-stead of consuming animals that eat cerealgrains.

Food chains are of two basic types: thegrazing food chain, which goes from greenplants, to grazing herbivores, and finally tocarnivores; and the detritus food chain,which goes from dead organic matter, to microorganisms, and then to detritus-feeding organisms. The food chains in acommunity are interconnected with oneanother, because most organisms consumemore than one type of food, and the inter-locking pattern is referred to as a food webor food cycle. See trophic level.

food web See food chain.

foot The basal portion of an embryo,sporophyte, or spore-producing body,which is embedded in the tissue of the par-ent.

forest 1. A plant formation dominatedby trees whose crowns touch, forming acontinuous canopy.2. A major plant community in which thedominant plants are trees.

form The lowest taxonomic group,ranking below the variety level, normallyreferring to the recognizably different mor-phology found in several members within aplant species. Subforms may also be recog-nized.

formalin A mixture of about 40%formaldehyde, 8% methyl alcohol, and52% water (the methyl alcohol is presentto prevent polymerization of the formalde-hyde). It is a powerful reducing agent andis used as a disinfectant, germicide, andfungicide and also as a general preservingsolution.

fossil The remains of, or impressions leftin rocks by, long-dead animals and plants.Most fossils consist of hard skeletal mater-ial because soft tissues and organs rot awayvery quickly. Plants seldom contain hardmineral material, so they are preservedwell only under certain conditions. Withrapid burial in a moist anoxic environ-ment, such as the coal swamps of the Car-boniferous period, carbonaceaous films ofplant material like fern fronds may survive.Alternatively, mineral salts from surround-ing rocks gradually replace the hard or-ganic material, to give a cast in a processtermed petrification, as in the various pet-rified forests dotted around the globe. Thehard coats of spores and pollen grains arepreserved better, forming microfossils. Al-ternatively the organic material dissolvesaway leaving an impression or mold in thesurrounding rocks. See also chemical fos-sils; palynology.

fragmentation 1. A form of ASEXUAL

REPRODUCTION in which the plant or algabreaks into fragments, each fragmentgrowing into a new, independent plant.Examples include filamentous algae suchas Spirogyra, and waterweeds like theCanadian pondweed (Elodea). Comparebudding; fission.2. The separation through human activityof once continuous habitats into fragmentsthat may or may not contain viable popu-lations of the organisms present.

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freeze fracturing

frameshift mutation See gene muta-tion.

freeze drying A technique for dehydrat-ing a specimen by the sublimation of iceunder a vacuum. It avoids damage thatwould result from heating.

freeze fracturing A method of prepara-tion of material for electron microscopy,particularly useful for studying mem-branes. Material is frozen rapidly (e.g. byimmersion in liquid nitrogen) thus preserv-ing it in lifelike form. It is then fractured,usually with a sharp knife. The fractureplane tends to follow lines of weakness,such as between the two lipid layers ofmembranes, revealing their internal sur-faces. Carbon or platinum replicas made ofthe surfaces are shadowed for examinationin the electron microscope. In freeze etch-ing the fractured surface is etched, i.e. someice is allowed to sublime away beforeSHADOWING. This exposes more detailedstructure, such as the outer surface of themembrane.

Fries, Elias Magnus (1794–1878)Swedish mycologist. Fries, working atLund University, published Systema Myco-logicum, his three-volume work on theclassification of fungi, in 1921–22. He alsopublished flora covering all the Scandina-vian countries and an important work onEuropean lichens.

frond A term usually applied to largewell-divided leaves as found in ferns,palms, and cycads. The leaflike thalli ofcertain algae and lichens may also betermed fronds. See also megaphyll.

fructification A seed- or spore-bearingstructure. The term is used especially forthe aerial fruiting bodies of fungi, e.g.mushrooms.

fructose A sugar (C6H12O6) found infruit juices, honey, and cane sugar. It is aketohexose, existing in a pyranose formwhen free. In combination (e.g. with glu-cose in the disaccharide sucrose) it exists in

the furanose ring form. Fructose formspolymers called fructosans, which includeINULIN, an important storage polysaccha-ride in the Asteraceae.

fruit The ripened OVARY of a flower thatis usually formed following fertilization ofthe ovule. It may consist of the ripenedovary only or include other parts of theflower. Fruits vary according to themethod of seed dispersal, succulent fruitsnormally being distributed by animalswhile dry fruits may be dispersed by windor water, and dehiscent fruits split to re-lease the seeds with the fruit remaining onthe plant. Fruits are classified according tohow the ovary wall (pericarp) develops, de-pending on whether it becomes fleshy orhard. Fruits are further classified accordingto whether or not the fruit wall opens to re-lease the seeds. See also dehiscent; indehis-cent; multiple fruit; pseudocarp.

Fucales An order of Phaeophyta (brownalgae) in which the haploid generation con-sists only of the gametes. The gametes areproduced in special cavities called concep-tacles, often in the inflated tips of branchesof the thallus, e.g. wracks (Fucus). See al-ternation of generations.

Fuchs, Leonhard (1501–66) Germanbotanist and physician. His beautifully il-lustrated herbal manual De historia stir-pum was published in 1542. It containedabout four hundred German plants andabout a hundred foreign plants, arrangedin alphabetical order. The illustrationswere accompanied by a description of eachplant and its habitat and even informationabout the best time to collect them. Thebook also included a glossary of terms.Fuchs is best known for the genus of orna-mental shrubs Fuchsia, which was namedfor him.

fucoxanthin A brownish xanthophyllpigment found in Diatoms, Chrysomon-ada, Haptomonada, Phaeophyta, andsome Dinomastigota. The light absorbed isused with high efficiency in photosynthe-sis, the energy first being transferred to

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Fruit: a classification of types of fruit

chlorophyll a. It has three absorptionpeaks covering the blue and green parts ofthe spectrum, which penetrate to greaterdepths under water than the red end of thespectrum. See carotenoids.

fumaric acid (trans-butenedioic acid) Anunsaturated dicarboxylic acid, which oc-curs in many plants. The fumarate ion par-ticipates in several important metabolicpathways, e.g. the Krebs cycle, purinepathways, and the urea cycle.

Fungi A kingdom of nonphotosyntheticmainly terrestrial organisms that are quitedistinct from plants and animals, andplaced in their own kingdom, Fungi. Theyare characterized by having cell walls madechiefly of chitin, not the cellulose of plantcell walls, and they all develop directlyfrom spores without an embryo stage.Moreover, undulipodia are never found inany stage of their life cycles. Fungi are gen-erally saprophytic or parasitic, and may beunicellular or composed of filaments(termed hyphae) that together comprise thefungal body or mycelium. Hyphae maygrow loosely or form a compacted mass ofpseudoparenchyma giving well-definedstructures, as in toadstools. Fungi aremajor decomposers in soil, and freshwater. Some form symbiotic associationswith algae or cyanobacteria to formlichens, while others form extremely im-portant mycorrhizae with the roots ofmany plants, including most forest trees.Some parasitic fungi cause diseases in

plants and animals, while others aresources of antibiotics. Yeasts and otherfungi are used in commercial fermentationprocesses such as brewing. There are threemain phyla of Fungi, based on the natureof their fruiting structures: the Zygomy-cota, Ascomycota, and Basidiomycota. Inthe modern Five Kingdoms classification,certain funguslike organisms that were pre-viously considered to be fungi, are nowassigned to phyla in the Kingdom Protoc-tista.

Fungi Anamorphici (Deuteromycota;Fungi Imperfecti; mitosporic fungi) Aphylum of so-called ‘imperfect’ fungi inwhich sexual reproduction is unknown orhas been lost during evolution. Its mem-bers, for example the Penicillium molds,are assigned as a taxonomic convenience,rather than by any true criteria, and someauthorities prefer to allocate the deutero-mycotes to either the Ascomycota (the ma-jority) or the Basidiomycota, on the basisof available evidence. Indeed Penicillium isnow known to have a sexual stage, for-merly regarded as a quite distinct asco-mycete fungus (Talaromyces). Most ofthese fungi reproduce by conidia producedby mitosis (see conidium). There are about15 000 species.

fungicide A chemical that inhibits fun-gal growth. They are used to prevent fun-gal infection as well as to deal with existinginfections.

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galactose A sugar found in lactose andmany polysaccharides. It is an aldohexose,isomeric with glucose. See also sugar.

gall An abnormal outgrowth or swellingon a root, stem, or leaf in response to attack by a parasite or to invasion by asymbiont. ROOT NODULES induced by thenitrogen-fixing bacterium Rhizobium arepart of a symbiotic relationship (see sym-biosis).

gametangium (pl. gametangia) A cell ororgan in which sexual cells (gametes) areproduced. When the gametes are readilydistinguished as male or female, otherterms may be used such as ANTHERIDIUM,ARCHEGONIUM, or OOGONIUM.

gamete A cell capable of fusing with an-other cell to produce a zygote, from whicha new individual organism can develop.Gametes may have similar structure andbehavior (isogametes), as in many simpleorganisms, but are usually dissimilar in ap-pearance and behavior (anisogametes).The typical male gamete is small, motile(by means of UNDULIPODIA), and producedin large numbers. The typical female ga-mete is large because of the food reserves itcontains, and is produced in smaller num-bers than the male gametes. In all exceptthe most primitive organisms, the femalegamete is nonmotile (OOGAMY). Gametesare usually haploid, so fusion of gametesresults in the nucleus of the zygote havingthe diploid number of chromosomes.

gametogenesis The formation of sexcells or gametes.

gametophyte The gamete-producing gen-eration in the life cycle of a plant or multi-

cellular alga. The gametophyte is HAPLOID

and produces sex organs. It derives fromthe germination of haploid spores pro-duced by the SPOROPHYTE generation. It isthe main generation in the life cycle of theBryata, in which the sporophyte is com-pletely or partially dependent upon it. ThePROTHALLUS of Lycophyta (clubmosses),Sphenophyta (horsetails), and Filicino-phyta (ferns), which may be a green struc-ture on the soil surface or a subterraneanmycorrhizal structure, is the gametophyte.In these groups and in the bryophytes,some species produce unisexual gameto-phytes and some produce hermaphroditegametophytes. In the Coniferophyta the fe-male gametophyte is reduced to a nonpho-tosynthetic tissue with up to 2000 nuclei inthe ovule, which produces archegonia (seearchegonium). While in the Angiophytathe female gametophyte is simply the EM-BRYO SAC. The male gametophytes ofconifers and angiosperms are the POLLEN

grains. See also alternation of generations.

Gamophyta A phylum of green algae inwhich sexual reproduction is by conjuga-tion, e.g. Spirogyra and DESMIDS. Amoe-boid gametes are produced, which cross toadjacent filaments through conjugationbridges and fuse to form a zygote, whichusually forms a resistant zygospore. Ongermination, the zygospore undergoesmeiosis to produce haploid cells that giverise to a new thallus. Asexual reproductionis by FRAGMENTATION.

garrigue Scrub woodland, a secondaryformation derived from the original mixedforest, characteristic of limestone areas ofthe Mediterranean region, with low rain-fall and thin, dry soils. It is dominated bylow-growing prickly dwarf shrubs and

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aromatic herbs (most commonly of theLamiaceae), such as lavender (Lavandulavera), sage (Salvia officinalis), and thymes(Thymus), many with drought-resistantfoliage. There are also bulbous plants suchas species of Iris. Compare chaparral;maquis.

gas chromatography A technique widelyused for the separation and analysis ofmixtures. Gas chromatography employs acolumn packed with either a solid station-ary phase (gas–solid chromatography orGSC) or a solid coated with a nonvola-tile liquid (gas–liquid chromatography orGLC). The whole column is placed in athermostatically controlled heating jacket.A volatile sample is introduced into the col-umn using a syringe, and an unreactive car-rier gas, such as nitrogen, passed throughit. The components of the sample will becarried along in this mobile phase. How-ever, some of the components will clingmore readily to the stationary phase thanothers, either because they become at-tached to the solid surface or because theydissolve in the liquid. The time taken fordifferent components to pass through thecolumn is characteristic and can be used toidentify them. The emergent sample ispassed through a detector, which registersthe presence of the different components inthe carrier gas.

gas–liquid chromatography See gaschromatography.

gas–solid chromatography See gaschromatography.

gel filtration (gel-permeation chromatog-raphy) A chromatographic method usinga column packed with porous carbohy-drate-gel beads of standard size and poros-ity. It is a standard technique used forseparating and identifying macromoleculesof various sizes, e.g. proteins or nucleicacids. A solution of the mixture of macro-molecules is added to the top of the columnand allowed to flow through by gravity.The smaller molecules are hindered in theirpassage down the column because they are

better able to penetrate the hydrated poreswithin the particles of the gel. Moleculestoo large to penetrate the pores are ex-cluded, and thus flow more rapidlythrough the column. By analyzing the liq-uid that drips from the bottom of the col-umn (the eluate) at set intervals andcomparing it with a standard (obtained byrunning a known macromolecule throughthe column) information about the sizesand molecular weights of the componentsof the mixture is gathered. The most fre-quently used commercial gel is Sephadex.

gemma (pl. gemmae) 1. A vegetativepropagule produced by mosses, liverworts,and certain Lycophyta (clubmosses). Gem-mae are small dispersal units consisting ofone to several cells, often forming disks,plates, or filaments. They often form ingroups in receptacles called gemma-cupsand eventually become detached from theparent to form new plants.2. See chlamydospore.

gemmation A type of asexual reproduc-tion, seen in mosses and liverworts, involv-ing the production of a group of cells (agemma) that develops into a new individ-ual, before or after separation from theparent.

gene In classical genetics, a unit ofhereditary material located on a chromo-some that, by itself or with other genes, de-termines a characteristic in an organism. Itcorresponds to a segment of the geneticmaterial, usually DNA (although the genesof some viruses consist of RNA). Genesmay exist in a number of forms, termed al-leles. For example, a gene controlling thecharacteristic ‘height’ in peas may havetwo alleles, one for ‘tall’ and another for‘short’. In a normal diploid cell only twoalleles can be present together, one on eachof a pair of homologous chromosomes: thealleles may both be of the same type, orthey may be different. The segregation ofalleles at meiosis and their dominance rela-tionships are responsible for the particulatenature of inheritance. Genes can occasion-

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ally undergo changes, called MUTATIONS, tonew allelic forms.

Although the DNA molecules of thechromosomes account for the great major-ity of genes, genes are also found asPLASMAGENES in certain DNA-containingcytoplasmic bodies (e.g. mitochondria,plastids). A gene can be defined as thesmallest hereditary unit capable either ofrecombination or of mutation. alterna-tively it may be described as a unit of func-tion, a CISTRON, which may be much largerthan a unit of recombination or mutation,and controls the synthesis of a singlepolypeptide or a messenger RNA moleculewhose genetic code is contained within thegene. Research with bacteria has shownthat the smallest unit of recombination ormutation is one base pair, while the size ofa unit of function can be determined by theCIS–TRANS TEST.

gene bank 1. A place where plant ma-terial is stored in a viable condition inorder to preserve plants that are in dangerof becoming extinct in the wild or cultivarsthat are being lost from cultivation. Thematerial also provides a source of genes forbreeding new varieties.2. See gene library.

gene cloning (DNA cloning) A tech-nique of GENETIC ENGINEERING whereby agene sequence is replicated, giving manyidentical copies. The gene sequence is iso-lated by using restriction endonucleases, orby making a complementary DNA from amessenger RNA template using a reversetranscriptase. It is then inserted into the cir-cular chromosome of a cloning vector, i.e.a plasmid or a bacteriophage. The hybrid isused to infect a bacterium, usually Es-cherichia coli, and is replicated within thebacterial cell. A culture of such cells pro-duces many copies of the gene, which cansubsequently be isolated and purified.

genecology The study of ecological ge-netics, or population genetics in relation toenvironment.

gene flow The movement of alleles

within and between populations of thesame species through interbreeding.

gene frequency The proportion of anallele in a population in relation to otheralleles of the same gene.

gene library A collection of clonedDNA fragments derived from the entireGENOME of an organism. The genetic ma-terial of the organism is first broken uprandomly into fragments using restrictionenzymes, for example. Then each fragmentis cloned using a VECTOR (e.g. plasmid orbacteriophage) inside a suitable host, suchas the bacterium E. coli. Particular genes orDNA sequences are identified by a suitableDNA PROBE. See also gene bank; genecloning.

gene mutation An alteration in a singlegene resulting from a change in the num-ber, type, or sequence of nucleotide bases.Unlike CHROMOSOME MUTATIONS, gene mu-tations are not visible under the micro-scope. A point mutation is a change at asingle locus. It may be an addition, dele-tion, inversion, or substitution of one ormore bases. A substitution of one or twobases may still result in the same aminoacid, or at worst to just one amino acidchange, and may have little effect on theresulting protein. However, if the muta-tion results in the loss or gain of one or two bases, it causes the remainder of thegene to be misread during translation, as itchanges the ‘reading frame’; for example,CUC/CAG/GCA becomes UCC/AGG/CA.Such mutations are called frameshift muta-tions, and are more likely to be deleterious.See mutation; chromosome mutation.

gene pool The total number and varietyof genes existing within a breeding popula-tion or species at a given point in time.

gene probe See DNA probe.

generative cell One of the haploid cellsin the POLLEN TUBE of seed plants. In an-giosperms it gives rise directly to the twomale gametes or generative nuclei. In gym-

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nosperms it gives rise to the body cell andstalk cell.

generative nuclei The two gametic nu-clei formed by the division of the genera-tive cell in the pollen tube of angiosperms.One fuses with the egg cell to form a zygotewhile the other either degenerates or, incertain angiosperms, fuses with the polarnuclei to give the primary endosperm nu-cleus.

gene sequencing Determination of theorder of bases making up a gene on a DNAmolecule. Sequencing requires multiple-cloned copies of the gene; long DNA se-quences are cut into more manageablelengths using restriction enzymes. Sincethese cleave DNA at specific points, it ispossible to reconstitute the overall se-quence once the constituent fragmentshave been analyzed individually.

gene splicing 1. The joining of EXONS

after the INTRON sequences have been re-moved, to produce functional messengerRNA that leaves the nucleus to undergoTRANSCRIPTION. In the nucleus this is per-formed by a special assemblage of RNAand proteins called a spliceosome.2. In GENETIC ENGINEERING, the joining ofDNA fragments by the action of the en-zyme DNA ligase.

genetic code The sequence of bases ineither DNA or messenger RNA that con-veys genetic instructions to the cell. Thebasic unit of the code consists of a group ofthree consecutive bases, the base triplet orCODON, which specifies instructions for aparticular amino acid in a polypeptide, oracts as a start or stop signal for translationof the message into polypeptide assembly.For example, the DNA triplet CAA (whichis transcribed as GUU in mRNA) codes forthe amino acid valine. There are 64 differ-ent triplet combinations but only 20 aminoacids; thus many amino acids can be codedfor by two or more triplets. The code issaid to be degenerate, since some aminoacids are coded for by more than onetriplet: in certain cases only the first one or

two bases, are necessary to insure the cod-ing of a specific amino acid. Three triplets,termed ‘nonsense triplets’, do not code forany amino acid and have other functions,e.g. marking the beginning and end of apolypeptide chain.

genetic drift (Sewall Wright effect) Thefluctuation of allele frequencies in a smallpopulation due entirely to chance. If thenumber of matings is small, then the actualnumbers of different types of pairing maydepart significantly from the number ex-pected on a purely random basis. Geneticdrift is one of the factors that can disturbthe HARDY–WEINBERG EQUILIBRIUM.

genetic engineering The direct intro-duction of foreign genes (from other indivi-duals or species or artificially synthesized)into an organism’s genetic material by mi-cromanipulation at the cell level. Geneticengineering techniques bypass crossbreed-ing barriers between species to enable genetransfer between widely differing organ-isms. The commonest method of geneticengineering is RECOMBINANT DNA TECHNOL-OGY. Genes may be take from living tissuesby isolating the RNA produced when thegene is active, and recreating the DNA se-quence of the gene using the enzyme re-verse transcriptase. Gene transfer can beachieved by various methods, many ofwhich employ a replicating infective agent,such as a virus, plasmid, or phage for DNAsegments up to 24 000 base pairs in length,a cosmid for segments up to 45 000 basepairs, or a YAC (yeast artificial chromo-some) for pieces up to 50 000 base pairs, asa VECTOR (see gene cloning). Other meth-ods include microinjection of DNA intocell nuclei and direct uptake of DNAthrough the cell membrane. Recognizingwhether or not transfer has occurred maybe difficult unless the new gene confers anobvious visual or physiological characteris-tic. Consequently the desirable gene maybe linked to a marker gene, e.g. a gene con-ferring resistance to an antibiotic in thegrowth medium. The transferred genemust also be linked to appropriate regula-tory DNA sequences to insure that it works

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in its new environment and is regulatedcorrectly and predictably.

Initial successes in DNA transfer wereachieved with bacteria and yeast. Humangenes coding for medically useful proteinshave been transferred to bacteria. Humaninsulin, growth hormone, and interferonare now among a wide range of therapeu-tic substances produced commerciallyfrom genetically engineered bacteria. Ge-netically engineered vaccines have also

been produced by transfer of antigen-coding genes to bacteria. Genes have beenintroduced to crop plants for variousreasons, for instance to reduce damageduring harvest or to make them resistant to the herbicides used in controlling weeds.Genetically modified tomatoes and soyabeans are now widely available. There is also hope that in future many geneticdiseases will be treatable by manipulatingthe faulty genes responsible. However, ge-

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Genetic code

netic engineering raises many legal and eth-ical issues, and the introduction of geneti-cally modified organisms into theenvironment requires strict controls andmonitoring.

genetic fingerprinting A technique foridentifying individuals by means of theirDNA. The DNA being tested is extractedfrom cells (from blood, semen, tissue frag-ments, etc.) and broken into fragments of600–700 bases each, using restriction en-zymes. The human genome contains manyloci where short base sequences are re-peated in tandem, with great variation be-tween individuals in the number of suchrepeats. These so-called variable numbertandem repeats (VNTR) can be identifiedusing special DNA probes, thus providinga virtually unique set of markers for anygiven individual. This technique is used inveterinary and human medicine to estab-lish the parentage of individuals, and inforensic science to identify individualsfrom traces of body tissue or fluids. Evenminute amounts of DNA can now be am-plified, using the POLYMERASE CHAIN REAC-TION, to provide sufficient material forgenetic fingerprinting.

genetic map A map showing the se-quence of particular genes or segments ofDNA on a chromosome. Restriction en-zymes and DNA probes are used to deter-mine the exact sequence, but it is possibleto get a rough estimate by analyzing thefrequency of recombination between thealleles of linked genes. See chromosomemap; DNA probe; restriction endonucle-ase.

genetics The term coined by the Britishbiologist Bateson (1861–1926) to describethe study of inheritance and variation andthe factors controlling them. Today thesubject has four main subdivisions –Mendelian genetics (classical genetics),population genetics, cytogenetics, and mol-ecular or biochemical genetics.

genome A complete haploid chromo-some set. Haploid organisms have one

genome, diploid organisms have two ho-mologous sets, polyploid organisms havemany sets sometimes from the same ances-tor (autopolyploids) and sometimes fromdifferent ancestors (allopolyploids). Inter-specific hybrids also have two or more dif-ference genomes.

genotype The genetic make-up of an or-ganism. The actual appearance of an indi-vidual (the phenotype) depends on thedominance relationships between alleles inthe genotype and the interaction betweengenotype and environment. Compare phe-notype.

genus (pl. genera) A taxonomic categoryinvolving a collection of similar species.Genera may be subdivided into subgenera,and also, especially in plant taxonomy,into sections, subsections, series, and sub-series. The scientific name of a species al-ways includes the genus name (or itsabbreviation) as the first word of the bino-mial. Similar genera are grouped into fam-ilies. See binomial nomenclature.

geological time scale A system of mea-suring the history of the earth by studyingthe rocks of the earth’s crust. Since newrocks are generally deposited on top of ex-isting material, those lower down are old-est, although this is often disrupted by e.g.volcanic activity, tectonic movements anderosion. The strata of rock are classified ac-cording to their age, and a time scale cor-responding to this can be constructed. Themain divisions (eras) are the PALEOZOIC,MESOZOIC, and CENOZOIC. These are fur-ther subdivided into periods and epochs.

geophyte A plant whose perennatingbuds are situated below ground as, for ex-ample, in plants with bulbs, corms, tubers,or rhizomes. See Raunkiaer’s plant classifi-cation.

geotropism See gravitropism. cell

germ cell Any cell in a cell lineage thateventually produces gametes.

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germination The first outward sign ofgrowth of a reproductive body, such as aspore or pollen grain. The term is mostcommonly applied to seeds, in which ger-mination involves the emergence of theradicle, plumule, or coleoptile through thetesta. Both external conditions (e.g. wateravailability, temperature, and light) and in-ternal biochemical status must be appro-priate before germination can occur. Seedgermination may be either epigeal, inwhich the cotyledons appear aboveground, or hypogeal, in which the cotyle-dons remain below ground.

germ line The lineage of cells fromwhich gametes arise, continuous throughgenerations.

germ plasm The part of an organismthat, according to Weissmann at the begin-ning of the 20th century, passed its charac-ters on to the next generation. It is nowknown that most of this information is car-ried by DNA in the chromosomes.

giant ferns See Marattiaceae.

gibberellic acid (GA3) A common gib-berellin and one of the first to be discov-ered. Together with GA1 and GA2 it wasisolated from Gibberella fujikuroi, a fun-gus that infects rice seedlings causing ab-normally tall growth. Gibberellic acid is aterpenoid synthesized from mevalonicacid. Some growth retardants are thoughtto act by blocking this synthesis pathway.See gibberellin.

gibberellin A plant hormone involvedchiefly in shoot extension. Gibberellins are

diterpenoids chemically related to gibberel-lic acid; their molecules have the gibbaneskeleton. More than thirty have beenisolated, the first and one of the most com-mon being GIBBERELLIC ACID, GA3. Gib-berellins stimulate elongation of shoots ofvarious plants, especially the extension tonormal size of the short internodes of ge-netically dwarf pea or maize plants. This isachieved by increasing cell-wall expansibil-ity, but it requires the presence of auxin,too. Such dwarf varieties may be used inbioassays for gibberellin. Increased gib-berellin levels can mimic or mediate theeffect of long days. Thus they stimulate in-ternode extension and flowering in LONG-DAY PLANTS such as lettuce and spinach.They are also effective in inhibiting tuberdevelopment or breaking tuber dormancy,e.g. in the potato, and breaking bud dor-mancy in woody species. Gibberellins caninduce PARTHENOCARPY, and are used in theproduction of seedless fruits. They may beproduced in both shoots and roots andtravel in both xylem and phloem. Theywork antagonistically with ABSCISIC ACID,and in some situations work cooperativelywith AUXINS as, for example, in the controlof sex expression in dioecious plants.

gill One of many thin platelike spore-producing structures radiating outwardfrom the stalk on the undersurface of thecap (see pileus) of agaric fungi such asmushrooms and toadstools (see Basid-iomycota). The spores (basidiospores) areproduced on the outer layer (hymenium).

Ginkgophyta (ginkgoes) A phylum ofdeciduous gymnosperms with fan-shaped,stalked leaves that have dichotomouslybranching veins, a feature unique amongliving gymnosperms. There is a single class,order, genus and species, the maidenhairtree or ginkgo (Ginkgo biloba) fromChina, known only in cultivation. Fossilremains of other species have been found inmany parts of the world, dating back to theMiddle Jurassic period. The ginkgo is dioe-cious.

glabrous Smooth, with no hairs or otherprojections.

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Gibberellic acid: the skeleton of the molecular structure

gland A specialized cell or group of cellsconcerned with the secretion of varioussubstances produced as byproducts ofplant metabolism. The secretions may passto the exterior or be contained in cavitiesor canals in the plant body.

GLC See gas–liquid chromatography.

gley (glei) A waterlogged soil lacking inoxygen, in which raw humus accumulatesas a result of lack of decomposition by bac-teria. The defining feature is the gley hori-zon below the humus – blue-grey claywhose color is due to ferrous iron com-pounds that have been reduced by mi-croorganisms, flecked with localized areasof rust-colored oxidized ferric compounds.The formation of a gley is known as gley-ing. Gley soils are typical of tundra, mead-ows, and boggy areas.

global warming See greenhouse effect.

globulin A kind of globular protein thatdissolves in dilute salt solutions but is rela-tively insoluble in pure water. Globulinsare found in plant seeds as storage pro-teins, particularly in the seeds of legumes(Fabaceae).

glucan A polysaccharide made up en-tirely of glucose units, e.g. cellulose, starch.See polysaccharide.

gluconeogenesis The synthesis of glu-cose from noncarbohydrate precursors,such as pyruvate, amino acids, and fattyacids (which are converted to glucose bythe KREBS CYCLE and GLYOXYLATE CYCLE),glycerol, and intermediates of the Krebscycle. In plants, glucose is also formed byphotosynthesis.

glucose A monosaccharide (C6H12O6)occurring widely in nature as a storagecompound: it occurs as glucose units in su-crose (which is hydrolyzed to glucose andfructose) and GLYCOGEN (which is hy-drolyzed to glucose), as well as in STARCH

and CELLULOSE. It is the main energy sourceof most organisms, and the starting point

for synthesis of carbohydrates and nu-cleotide sugars. Glucose is synthesized di-rectly from carbon dioxide in the Calvincycle, and from fatty acids via b-oxidationto acetyl CoA, which then enters the GLY-OXYLATE CYCLE (a synthetic pathway ingerminating oil-rich seeds). Glucose is oxi-dized in respiration through glycolysis andthe KREBS CYCLE. See glycolysis; photosyn-thesis.

glume See bract.

glutamic acid See amino acids.

glutamine An amino acid formed by theaddition of ammonia to glutamic acid,using energy from ATP. Glutamine is im-portant in the storage and neutralization offree ammonia. It also donates an aminogroup in synthesis reactions, e.g. the syn-thesis of purines and pyrimidines. Seeamino acids.

gluten A mixture of proteins found inwheat flour. It is composed mainly of twoproteins (gliaden and glutelin), the proteinsbeing present in almost equal quantities.Certain people are sensitive to gluten(celiac disease) and must have a gluten-freediet.

glycan 1. A POLYSACCHARIDE made up ofa single type of sugar unit (i.e. >95% ). Asa class the glycans serve both as structuralunits (e.g. cellulose in plants and chitin infungi) and energy stores (e.g. starch). Themost common homoglycans are made upof D-glucose units and called GLUCANS.2. Any polysaccharide.

glyceraldehyde (CHOCH(OH)-CH2OH)A simple monosaccharide bearing an alde-hyde group. The phosphory- lated form,glyceraldehyde 3-phosphate is derivedfrom fructose 1,6-bisphosphate duringGLYCOLYSIS, and from glycerate 3-phos-phate in the Calvin cycle (See photosynthe-sis). Glyceraldehyde 3-phosphate isinvolved in many metabolic pathways.

glycerate 3-phosphate (phosphoglyceric

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glycerate 3-phosphate

acid) The phosphorylated form of glyc-eric acid, a 3-carbon atom molecule. Glyc-erate 3-phosphate is the first product of thedark reactions of PHOTOSYNTHESIS, formedby the carboxylation and cleavage of ribu-lose bisphosphate. It is also formed fromglyceraldehyde 3-phosphate in GLYCOLYSIS,with the release of ATP, after which it maybe converted to phosphoenol pyruvate.Glycerate 3-phosphate is a precursor of theamino acid serine.

glyceride See acylglycerol.

glycerol (glycerin; 1,2,3-propanetriol)An alcohol with three OH groups. Glyc-erol is biologically important as the alcoholinvolved in LIPID formation (these particu-lar lipids being called acylglycerols). It isalso used to protect living tissues such ascorneas and red blood cells for freezing,and as a mounting medium in microscopy.See acylglycerol; phosphoglyceride.

glycerolipid Lipids derived from GLYC-EROL. Glycerolipids are synthesized fromglycerol 3-phosphate by the addition offatty acid chains and a head group. Plantssynthesize glycerolipids in mitochondria,plastids and the endoplasmic reticulum.Glycerolipids include triacylglycerols (glyc-erol linked to three fatty acid chains), im-portant storage compounds in seeds, beingbroken down with the aid of lipases intofatty acids, which can then undergo b-oxidation. Compare glycolipid.

glycine (aminoethanoic acid; aminoaceticacid) The simplest AMINO ACID, with theformula NH2CH2COOH. It may be syn-thesized directly from ammonia, carbondioxide, and a donated methyl group; de-rived from serine; or derived from glycer-ate 3-phosphate by TRANSAMINATION ofglyoxylate. It is broken down by a reversalof the first two pathways, or by deamina-tion to glyoxylate, followed by conversionto malic acid and further breakdown in theKrebs cycle. Glycine is necessary for thesynthesis of purines, and is a basic compo-nent of PORPHYRINS.

glycogen A polysaccharide composed ofmany GLUCOSE units, with a branchingstructure similar to that of amylopectin. Itis used as a storage compound in fungi andcertain bacteria as well as in animals, beinghydrolyzed to glucose with the aid of am-ylase.

glycolipid (glycosyldiacylglycerol) Anacylglycerol containing a carbohydrategroup or an amino sugar. Glycolipids arethe major lipids in chloroplasts. See glyc-erolipid.

glycolysis (Embden–Meyerhof–Parnaspathway) The conversion of glucose intopyruvate, with the release of some energyin the form of ATP. Glycolysis occurs incell cytoplasm. In ANAEROBIC RESPIRATION,

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Glycolysis

breakdown proceeds no further and pyru-vate is converted into ethanol (or lacticacid in animals and some bacteria) for stor-age or elimination. In AEROBIC RESPIRATION,glycolysis is followed by the KREBS CYCLE.Glycolysis alone yields only two moleculesof ATP per molecule of glucose in anaero-bic respiration. In aerobic respiration thereis a net yield of six (the conversion ofNADH back to NAD yields a further fourATP molecules, and can occur only whenoxygen is present). The main regulatoryenzyme in glycolysis is phosphofructoki-nase, which catalyzes the phosphorylationof fructose 6-phosphate to fructose 1,6-bis-phosphate, and is inhibited by high con-centrations of ATP. See also respiration;alcoholic fermentation.

glycoprotein A conjugated proteinformed by the combination of a proteinwith carbohydrate side chains. Certainantigens, enzymes, and hormones are gly-coproteins. One of the most important

plant glycoproteins is extensin, a structuralcomponent of cell walls, in which the sidechains are tetrasaccharides of arabinose.

glycoside A derivative of a pyranosesugar (e.g. glucose) in which there is agroup (R) attached to the carbon atom that is joined to the –CHO group. In a gly-coside the C–OH is replaced by C–OR.The linkage –O– is a GLYCOSIDIC BOND,joining monosaccharides in polysaccha-rides. Glycosides are formed by the reac-

tion of a pyranose sugar with an alcohol orphenol.

glycosidic bond (glycosidic link) Abond formed between monosaccharides byremoval of water (condensation reaction).The most common glycosidic bond occursbetween the carbon-1 of one sugar and thecarbon-4 of the other (a 1-4 glycosidicbond); sucrose has a 1-2 glycosidic bondbetween the carbon-1 of glucose and car-bon-2 of fructose. If the bond lies belowthe plane of the glucose ring, the bond istermed an a-glycosidic bond (e.g. starch), ifabove this plane, a b-glycosidic bond (e.g.cellulose).

glycosyldiacylglycerol See glycolipid.

glyoxylate cycle A modification of theKREBS CYCLE occurring in plants, protoc-tists, and some microorganisms, in regionswhere fats are being rapidly metabolized,e.g. in germinating fat-rich seeds. Acetylgroups formed from the fatty acids arepassed into the glyoxylate cycle, with theeventual formation of mainly carbohy-drates. The glyoxylate cycle bypasses thecarbon dioxide-releasing steps of the Krebscycle, allowing carbohydrates to be synthe-sized from fatty acids via succinate. See il-lustration overleaf.

glyoxysome See microbody.

Gnetophyta A phylum of gymnospermswith compound cones, cells in the sec-ondary xylem almost identical to an-giosperm vessels, and the tendency for thetip of the nucellus to be drawn out into amicropylar tube; there are no resin canals.The embryo has two cotyledons. Oftenmore than one embryo is produced ini-tially, but usually only one reaches matu-rity. There are three very different orders,each with only one genus: Gnetales (Gne-tum), Ephedrales (Ephedra or joint-firs),and Welwitschiales (Welwitschia).

Golgi apparatus (Golgi body; Golgi com-plex) An organelle of eukaryotic cellsdiscovered by Camillo Golgi in 1898. It isassociated with the endoplasmic reticulum

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Glycosidic bond: the glycosidic bond in sucrose

but lacks ribosomes. It consists of stacks offlattened membrane-bounded sacs (cister-nae) that bud off vesicles (Golgi vesicles)containing the products of the secretory ac-tivity in the Golgi apparatus. The Golgiapparatus is rich in secretory cells. The cis-ternae are spread randomly (as dic-tyosomes) in plant cells but form a singlenetwork in most animal cells. In the cister-nae, materials (e.g. enzymes and polysac-charides) are processed and leave in Golgivesicles for transport, often to the plasmamembrane for secretion. If the vesicles re-lease their contents on the cell surface, theirmembranes become part of the plasmamembrane; thus the Golgi apparatus con-tributes to cell membrane synthesis.

graft The transplantation of an organ ortissue in plants and animals. In plants,grafting is an important horticultural tech-nique in which an above-ground part (thescion) of one individual is united with theroot of another (the stock) of the same or adifferent species. (Where more than twoparts are grafted together, the middle pieceis called the interstock.) Usually the shoot

or bud of the scion is grafted onto thelower part of the stock. The cambia ofscion and stock merge, allowing the vascu-lar tissues of the two plants to combine.Production of callus around the graft areahelps to secure the union. Grafting is usu-ally successful only between closely relatedspecies. It is often used to propagate vari-eties derived from mutations or hybridiza-tion that cannot be grown from seed,especially woody species such as apples,pears, and roses.

graft hybrid See chimera.

grain See caryopsis.

Gram, Hans Christian Joachim (1853–1938) Danish bacteriologist. He devisedthe important method of staining bacteriathat bears his name (see Gram’s stain) inBerlin in 1884, building on the method ofPaul Erlich. The discovery that some bacte-ria retain the stain (Gram positive), whileothers do not (Gram negative), has beenimportant in identifying and classifying

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Glyoxylate cycle

bacteria and also in deciding upon thetreatment of bacterial diseases.

Gramineae See Poaceae.

Gram’s stain A stain containing thebasic dye crystal violet or gentian violetthat forms the basis for the division intoGram positive and Gram negative bacteria.The bacteria are first killed and stainedwith the violet dye, then treated with an or-ganic solvent such as acetone or ethanol.Gram positive bacteria (e.g. Actinobacte-ria) retain the deep purple color of crys-tal violet; Gram negative bacteria (e.g.Cyanobacteria) are decolorized, and maybe counterstained with other dyes, such assafranin, carbol fuchsin, or neutral red. Anumber of physiological differences are as-sociated with the difference in staining, in-cluding nutrition, and susceptibility toantibiotics.

granum (pl. grana) A stack of mem-branes (resembling a pile of coins) in aCHLOROPLAST. With the light microscopethese stacks are just visible as grains(grana). The photosynthetic pigments andaccessory pigments are bound to thesemembranes, which also contain the com-ponents of the electron-transport chainand enzymes needed for the light-depen-dent reactions of photosynthesis.

grape sugar See glucose.

grassland A major world plant com-munity (biome) dominated by grasses(Poaceae). Grasslands occur in many tem-perate regions where annual rainfall is250–500 mm and in tropical regions with750–1500 mm. They are widespread in theinteriors of continents, where there is a dis-tinct rainy season in spring and summer.Many grasslands, especially in the wetterareas, are naturally maintained by grazingmammals and, to some extent, fire.

graticule See micrometer.

gravireceptor A receptor that can sensegravity. This is usually an object that

moves in response to a gravitational field (aSTATOLITH). Starch-filled plastids (AMYLO-PLASTS) are thought to act as gravirecep-tors, moving in response to gravity andpressing on the endoplasmic reticulumnear the outer edge of specialized cellscalled STATOCYTES located in the root capor young root, and in a layer (termed theSTARCH SHEATH) adjacent to the vasculartissues. Other possible gravireceptors arestretch-activated ion channels in mem-branes, which may sense movement of theprotoplasm in response to gravity. Suchchannels may be involved in the responseof the endoplasmic reticulum to movingamyloplasts. See gravitropism. See alsophotoreceptor.

gravitropism (geotropism) A directionalgrowth movement of part of a plant in re-sponse to gravity. Primary roots (tap roots)grow vertically toward gravity (positivegravitropism) whereas primary shootsgrow vertically away from gravity (nega-tive gravitropism), though the direction ofshoot growth may also be modified bylight. Dicotyledon leaves and some stemstructures (e.g. rhizomes and stolons) growhorizontally (diatropism). Secondary (lat-eral) roots and stem branches may grow atan intermediate angle with respect to grav-ity (plagiotropism). Gravitropic responsesare believed to involve hormones, maybeauxins or gibberellins. See also gravirecep-tor; klinostat; tropism.

green algae See Chlorophyta; Gamo-phyta.

greenhouse effect The rise in tempera-ture of the atmosphere, analogous to thatin a greenhouse. Solar (short-wave) radia-tion passes easily through the atmosphere(or glass in a greenhouse) and is absorbedby the earth’s surface. It is re-emitted in theform of infrared (long-wave) radiation,which is absorbed by water vapor, carbondioxide, methane, chlorofluorocarbons(CFCs), and other so-called greenhousegases in the atmosphere with a consequentincrease in the atmospheric temperature.Many scientists believe that increasing at-

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mospheric pollution by carbon dioxide,mainly due to the burning of fossil fuels,and many other gases is leading to a rise inglobal temperatures (global warming),which will eventually affect other aspectsof climate and have profoundly damagingeffects on natural ecosystems and agricul-ture, as well as raising sea level.

Grew, Nehemiah (1641–1712) Britishphysician and botanist. Grew carried outimportant work in plant anatomy and wasa contemporary of MALPIGHI. His work cul-minated in his publication of The Anatomyof Plants, in which he was among the firstto observe chloroplasts and stomata, de-scribing the latter as ‘breathing pores’.

ground layer The lowermost layer of a plant community, usually comprisingmosses, lichens, fungi, and low-growingherbs, especially rosette plants and creep-ing, prostrate forms. Compare field layer.

ground meristem The central region ofthe apical meristem from which the groundtissues of pith, cortex, medullary rays, andmesophyll differentiate. In root meristem itis also called periblem.

ground tissues The unspecialized tis-sues that are found in any region of theplant not occupied by the specialized tissueof vascular bundles, cambium, epidermis,etc. The pith and cortex of the root andstem are ground tissue, as are the meso-phyll layers of the leaf.

growth An irreversible increase in sizeand/or dry weight. It excludes certain de-velopmental processes that involve no sizechange, such as cleavage and uptake ofwater by seeds (IMBIBITION). Growth in-volves cell division and cell expansionthrough synthesis of new materials, and isclosely related to subsequent developmen-tal processes. If some measure of growth ofan organism, such as height or weight, isplotted throughout its life, a characteristicS-shaped (sigmoid) growth curve is ob-tained for most organisms. In some organ-isms, including many plants growth never

stops entirely, though it may become ex-tremely slow (indeterminate or indefinitegrowth). This allows plants to continuegrowing and many plants grow laterallyshowing clonal growth. The result is thatsome can be extremely long-lived, withclones, such as those of the creosote bush(Larrea tridentata) known to have lived forover 10 000 years.

growth factor Any substance that af-fects the growth of a plant. This includesGROWTH SUBSTANCES (HORMONES) andGROWTH INHIBITORS, as well as food re-serves and minerals.

growth inhibitor A substance thatslows the growth of a plant, for examplethe plant hormones ABSCISIC ACID and ETH-YLENE (ethene), which act at very low con-centrations.

growth ring See annual ring.

growth substance See hormone.

GSC Gas–solid chromatography. Seegas chromatography.

guanine A nitrogenous base found inDNA and RNA. Guanine has a purine ringstructure. The nucleoside guanosine tri-phosphate (GTP) is derived from guanine,and supplies energy for certain enzyme-catalyzed metabolic reactions.

guanosine (guanine nucleoside) A nu-cleoside present in DNA and RNA andconsisting of guanine linked to D-ribose viaa b-glycosidic bond.

guard cell A specialized kidney-shapedepidermal cell, located to the side of aSTOMA. Two guard cells together encircleeach stoma and control the opening andclosing of the stomatal aperture. The con-trol is effected through changes in turgid-ity. The wall of the guard cell bordering thepore is heavily thickened while the oppo-site wall is comparatively thin. Thus whenthe guard cell is turgid the thin wall be-comes distended, bulging out away from

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the pore, and causes the thickened wall,which cannot distend, to be drawn out-wards with it. This results in an aperturebeing formed between adjacent guard cells.When osmotic pressure of the guard celldrops the pore closes. Most plants exhibita stomatal rhythm whereby the stomataopen in light and close in darkness.

gum Any substance that swells in waterto form a gel or a sticky solution. Similarcompounds that produce slimy solutionsare called mucilages. Gums and mucilagesare not distinguishable chemically. Mostare heterosaccharides, being large, com-plex, flexible, and often highly branchedmolecules.

guttation Loss of water as liquid fromthe surface of a plant. Water is normallylost as vapor during transpiration but, ifthe atmosphere is very humid, water mayalso be forced from the leaves through hy-

dathodes as a result of root pressure. Dropsof guttated water are often seen at the tipsof grasses in the early morning, when thelow temperature reduces transpiration. It isalso common in tropical plants living in en-vironments with high humidity that re-duces transpiration rates.

gymnosperm A general term for anyseed plant except the angiosperms, charac-terized by bearing naked seeds. There arefour widely different living groups: the cy-cads, ginkgo, conifers, gnetophytes, andseveral fossil groups. Gymnosperms lackendosperm and the precise double fertiliza-tion characteristic of the angiosperms, al-though some show a primitive form of it.Cycads and ginkgo have motile sperm, un-like gnetophytes and conifers, which pro-duce pollen. The female gametophyte isreduced, but larger than in angiosperms; itis not autotrophic. Most gymnosperms re-tain archegonia except for the most ad-

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habitat The place where a particular or-ganism lives, described in terms of its cli-matic, vegetative, topographic, and otherrelevant factors.

hair (trichome) An outgrowth from aplant epidermal cell. Plant hairs may take awide variety of forms, often characteristicof particular species or genera. They maybe unicellular or multicellular, simple,forked, stellate (starlike), downy, silky,long, or short. Some, such as the hairs onthe leaves of geraniums (Pelargonium) areglandular. See also root hair.

halophyte A plant that grows in soilswith a high concentration of salt, as foundin salt marshes or other littoral habitats.Examples are species of Spartina. Halo-phytes are adapted to obtain water fromsoil water with a higher osmotic pressurethan normal soil water, so they need to beable to accumulate a high concentration ofsalts in their root cells. Many have a succu-lent growth form with swollen stems orleaves.

halosere A plant community in a SUC-CESSION that starts on land periodically in-undated by the sea. Haloseres include salt-and flooding-tolerant pioneer communitieson mudflats and saltmarshes.

haplobiontic Describing life cycles inwhich only one type of somatic body isformed, which may be either haploid ordiploid, i.e. either the sporophyte or the ga-metophyte generation is missing. For ex-ample, in the unicellular Chlamydomonasthe vegetative cell is haploid, and the onlydiploid cells are the zygospores, which ger-minate to produce haploid zoospores. Indiatoms the vegetative cells are diploid,

and are formed directly by the fusion of ga-metes. Haplobiontic is thus a collectiveterm for haplontic and diplontic. Comparediplobiontic. See alternation of genera-tions.

haploid (monoploid) Describing an or-ganism, cell, or nucleus containing onlyone representative from each of the pairs ofhomologous chromosomes found in thenormal DIPLOID cell. Haploid chromosomesare thus unpaired and the haploid chromo-some number (n) is half the diploid number(2n). Meiosis, which usually precedesspore or, sometimes, gamete formation,halves the chromosome number to producehaploid gametes. The diploid condition isrestored when the nuclei of two gametesfuse to give the zygote. Gametes may de-velop without fertilization, or meiosis maysubstantially precede gamete formation,leading to the formation of haploid organ-isms, or haploid phases in the life cycles oforganisms.

haplont A haploid organism that repre-sents the vegetative stage in life cycles inwhich diploidy is restricted to the zygote.Haplontic life cycles are typical of the fila-mentous green algae. Compare diplont.

haplostele A type of protostele having asolid strand of stele with the xylem on theinside encircled by phloem, pericycle, andendodermis. This type of stele was presentin the earliest known vascular plants, suchas Rhynia.

hapteron (pl. haptera) 1. See holdfast.2. In species of Equisetum (horsetails), theouter wall of the spore, which forms an X-shaped structure, the arms of which arewrapped around the spore. On drying out,

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the arms, or haptera, act in similar mannerto the elaters of liverworts, and are thustermed elaters. See elater.

Haptomonada (Haptophyta) A phy-lum of the PROTOCTISTA that containsmostly unicellular algae, most of whichhave a threadlike structure called a hap-tonema between their two undulipodia.The haptonema is thought to be used fortemporary anchorage rather than for loco-motion and may be coiled while the cell ismotile. Haptomonads are common in ma-rine plankton and freshwater.

haptonasty (haptonastic movements) ANASTIC MOVEMENT in response to contact.An example is the sensitive plant (Mimosapudica), whose leaflets progressively col-lapse, sometimes throughout the wholeplant, once one leaflet is touched. See pul-vinus.

haptophyta See Haptomonada.

haptotropism See thigmotropism; tro-pism.

hardpan 1. A hardened soil horizon,usually in the middle or lower part of a soilprofile, A hardpan is typical of PODSOLS,where iron compounds leached from theupper layers accumulate in the B horizon,staining it red-brown and forming an im-permeable layer (iron pan) that can preventdrainage, resulting the in waterlogging ofhigher horizons.2. Also called calcrete, a hardened layer inor on a soil formed on calcareous sub-strata, formed in arid and semiarid regionsas a result of fluctuating climatic condi-tions by calcite that precipitates out of so-lution as water evaporates at the surface.

Hardy–Weinberg equilibrium The sit-uation in a large, randomly mating, stablepopulation under specific conditions, inwhich the proportion of dominant to re-cessive genes remains constant from onegeneration to the next. If a pair of alleles, A and a, have the frequencies p and q in a population, then the Hardy-Weinberg

equilibrium is described by the equation p2 + 2pq + q2 = 1, where p2 and q2 are thefrequencies of the double dominant anddouble recessive respectively, and 2pq isthe frequency of the heterozygote. TheHardy–Weinberg equation p2 + 2pq + q2 =1 is obtained from the expansion of (p +q)2, the total of the frequencies making upthe gene pool being unity. The law was for-mulated in 1908 and disproved the thencurrent theory that dominant genes alwaystend to increase in a population at the ex-pense of their equivalent recessive alleles.The equilibrium holds only when the pop-ulation is sufficiently large to avoid chancefluctuations of allele frequencies in thegene pool (GENETIC DRIFT) and providingthere is no mutation, natural selection, ormigration into or out of the population.The fact that allele frequencies may be seento change fairly rapidly in large popula-tions that show minimal mutation and mi-gration emphasizes the important rolenatural selection must play. Until theHardy–Weinberg law was formulated, theextent of natural selection was not fully ap-preciated.

Hartig net See ectomycorrhiza.

Hatch–Slack pathway See C4 plant.

haustorium (pl. haustoria) A special-ized outgrowth of many parasitic fungi andcertain parasitic plants such as dodder(Cuscuta) and broomrape (Orobanche),that penetrates into and withdraws foodmaterial from the cells of the host plant.

heartwood (duramen) The hard centralregion of a tree trunk made up of xylemvessels that are no longer involved in watertransport. Such vessels are often blockedby substances (e.g. resins and tannins) thatgive the wood a darker color. Comparesapwood.

heath An area dominated by dwarfshrubs of the heath family (Ericaceae).Legumes (family Fabaceae) may also becommon components of heath floras.Heaths normally develop on poor, sandy,

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well-drained soils in temperate climates,and are usually subclimax communitiesmaintained by grazing, burning, and some-times, cutting.

heavy metal tolerance Biochemicaland physiological adaptation to concentra-tions of heavy metals, such as copper, lead,and zinc, that would prevent the growth ofmost plant species or genotypes. Suchadaptation usually involves converting themetal to a harmless form; some species ex-clude the metal from the plant altogether,while others confine it to the roots, thusprotecting the more sensitive shoots. Cer-tain species or strains have evolved suchtolerance, and some, for example certainstrains of bent grass (Agrostis capillaris),benefit from the reduced competition insoil containing heavy metal ions.

helical thickening See spiral thicken-ing.

helicase Any of a group of enzymes thatcatalyze the unwinding of double-strandednucleic acids such as DNA, using energyfrom ATP.

heliophyte A plant typical of sunnyhabitats.

heliotropism A form of PHOTOTROPISM

in which flowers move to point toward thesun, tracking it as it moves across the sky.It is a daily movement, mainly in springflowers that are pale and cup-shaped likewood anemones and buttercups.

helophyte A perennial marsh plant thathas its overwintering buds under water. Anexample is bulrush (Typha).

hematoxylin A blue dye derived fromthe logwood tree (Haemotoxylon cam-pechianum), which when oxidized tohematin stains nuclei and cellulose cellwalls blue. See staining.

heme (haem) An iron-containing por-phyrin that is the prosthetic group in cy-tochromes. See porphyrins.

hemicellulose One of a group of sub-stances that make up the amorphous ma-trix of plant cell walls together with pecticsubstances (and occasionally, in maturecells, with lignin, gums, and mucilages).They are heteropolysaccharides, i.e. POLY-SACCHARIDES built from more than one typeof sugar, mainly the hexoses (mannose andgalactose) and the pentoses (xylose andarabinose). Galacturonic and glucuronicacids are also constituents. They varygreatly in composition between species.The hemicelluloses and pectic substancesinteract with the cellulose microfibrils ofthe cell wall to give extra strength and atthe same time retain some plasticity. Insome seeds (e.g. the endosperm of dates)hemicelluloses are a food reserve. See cellwall.

hemicryptophyte A perennial plant,usually nonwoody, with its over-winteringbuds at or just below soil level. The budsare protected by the soil or surface litter.See also Raunkiaer’s plant classification.

hemiparasite A parasitic plant that haschlorophyll and can photosynthesize, butdepends on another plant for its water andmineral salts, e.g. mistletoe.

hemizygous Describing genetic materialthat has no homologous counterpart and isthus unpaired in the diploid state. Both sin-gle genes and chromosome segments maybe hemizygous; for example, the X chro-mosome in the heterogametic sex, andwhole chromosomes in aneuploids.

Hepatophyta (Marchantiopsida; liver-worts) A phylum containing leafy andprostrate thallose forms, commonly knownas liverworts. Thallose liverworts are pros-trate and dichotomously branched withthalli several cells thick. There are five or-ders of liverworts, including the Metzgeri-ales, e.g. Pellia, and the Marchantiales, e.g.Marchantia.

herbaceous perennial See perennial.

heritability The proportion of pheno-

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typic variation due to genetic factors. It canbe estimated from measurements of indi-viduals from different generations, and isused in plant and animal breeding to pre-dict how successful genetic selection will bein improving a particular trait.

hermaphrodite (bisexual; monoclinous)A plant bearing stamens and carpels in thesame flower. In many such plants, self-fertilization and inbreeding are preventedby specific self-incompatibility systems.Compare monoecious; dioecious.

hesperidium (pl. hesperidia) A type ofberry with a leathery epicarp, as in orangesand lemons. Each carpel forms a segmentof the fruit, filled with fluid-filled tri-chomes. See berry.

heteroecious Denoting rust fungi thatrequire two host species to complete theirlife cycle. An example is Puccinia graminis,the stem rust of cereals and grasses, whichoverwinters on the barberry. Compare au-toecious.

heterogametic sex The sex with dissim-ilar SEX CHROMOSOMES, one (in mammalsthe Y chromosome) being shorter than theother (the X chromosome).

heterogamy See anisogamy.

heterokaryosis The presence of two ormore nuclei with differing genotypeswithin a single cell (termed a hetero-karyon). Fusion of fungal hyphae of differ-ing genetic complements leads to hetero-karyosis. Heterokaryons often grow betterthan homokaryons, since any genetic defi-ciencies, for example in the ability to syn-thesize certain compounds, in one type ofnucleus may be overcome by alleles presentin the other nuclei.

heteromorphism The existence of morethan one form, used especially with refer-ence to life cycles in which the alternatinggenerations are markedly different mor-phologically, as in ferns, bryophytes, and

algae. Compare isomorphism; polymor-phism. See alternation of generations.

heterophylly Denoting plant speciesthat have more than one form of foliageleaf on the same individual. Examples in-clude certain species of Lycopodium andSelaginella, which have two rows of lateralleaves and one or two rows of smallerabaxial or adaxial leaves.

heterosis (hybrid vigor) The conditionin which the expression of a characteristicis greater in the heterozygous offspringthan in either of the homozygous parents.The effect arises from an accumulation ofdominant genes in the F1. Thus, if height iscontrolled by two genes, A and B, and talland short forms are determined by domi-

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Heterophylly: examples of heterophylly in twodifferent aquatic plants

nant and recessive alleles respectively, thenthe cross AAbb ´ aaBB would give an F1AaBb, containing both dominant genes fortallness. Usually the more unlike the par-ents are the more hybrid vigor is released,but the effect diminishes in subsequentgenerations as more recessive homozygotesreappear. For crop plants, hybrid vigor isparticularly important for characteristicssuch as growth rate, yield, and disease re-sistance.

heterospory The production of two different sizes of spore: MICROSPORES andMEGASPORES. The larger megaspores usu-ally contain more food reserves. Comparehomospory.

heterostyly A DIMORPHISM in which thestyles of flowers of the same species are ofdifferent lengths, thus dividing the speciesinto groups. An example is the primrose,which is divided into pin-eyed and thrum-eyed types. Pin-eyed plants have long stylesand short stamens and thrum-eyed plantsconversely have short styles and long sta-mens. Such differences promote pollina-tion between groups.

heterothallism A condition found inalgae and fungi in which sexual reproduc-tion occurs only between genetically differ-ent self-incompatible mating types (strains)of the same species. The strains may vary

morphologically or in the size of the ga-metes they produce. When heterothallismis purely physiological, morphologicallyidentical strains exist (often designatedplus and minus strains). Compare homoth-allism.

heterotrophism A type of nutrition inwhich the principal source of carbon is or-ganic, i.e. the organism cannot make all itsown organic requirements from inorganicstarting materials. Most heterotrophicorganisms are chemotrophic (i.e. showchemoheterotrophism); these comprise allanimals and fungi, most bacteria, andparasitic plants. A few heterotrophic or-ganisms are phototrophic (i.e. show photo-heterotrophism). The nonsulfur purplebacteria, for instance, require organic mol-ecules such as ethanol and acetate. Com-pare autotrophism. See chemotrophism;phototrophism.

heterozygous Having two different al-leles at a given locus. Usually only one ofthese, the dominant allele, is expressed in the phenotype. On selfing or crossingheterozygotes some double recessives mayappear, giving viable offspring. Selfing het-erozygotes halves the heterozygosity, andthus outbreeding maintains heterozygosityand produces a more adaptable popula-tion. Compare homozygous.

heterospory

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Heterostyly: example in Primula

hexose A SUGAR that has six carbonatoms in its molecules.

hexose monophosphate shunt See pen-tose phosphate pathway.

high-energy phosphate bond A phos-phate linkage that hydrolyzes sponta-neously, releasing energy, for example thelast two phosphate bonds of ATP. ATP isthe main form in which chemical energy istransported from one part of the cell to an-other.

Hill reaction The reaction, first demon-strated by Robert Hill in 1937, by whichisolated illuminated chloroplasts bringabout the reduction of certain nonphysio-logical electron acceptors with accompany-ing evolution of oxygen. For example, theblue dye dichlorophenol indophenol(DCPIP) may be reduced to a colorless sub-stance. The reaction involves part of thenormal light reaction of photosynthesis.Electrons from water involved in noncyclicphotophosphorylation are used to reducethe added substance. It provided supportfor the idea that a light reaction precededreduction of carbon dioxide in photosyn-thesis. See cyclic photophosphorylation.

hilum (pl. hila) 1. A scar on the testa ofa seed marking the point at which it was at-tached to the ovary wall by the funicle. It isa feature that distinguishes seeds fromfruits.2. The center of a starch grain aroundwhich the layers of starch are deposited.3. A small projection at the base of a ba-sidiospore near its attachment to thesterigma.

hip A type of PSEUDOCARP (false fruit)consisting of a cup-shaped receptacle con-taining achenes, each of which bears smallhooklike hairs. Hips are typical of thegenus Rosa.

histidine A basic AMINO ACID found onlyrarely in proteins. It is formed from ATPand 5-phosphoribosyl pyrophosphate in a

complex biosynthetic pathway. Glutamateis an intermediate in its breakdown.

histochemistry The location of particu-lar chemical compounds within tissues bythe use of specific staining techniques, forexample phloroglucinol to stain lignin.

histogen theory A theory, proposed byHanstein, in which the apical meristem isconsidered to consist of three main zones,the dermatogen, periblem, and plerome,which differentiate into the epidermis, cor-tex, and stele respectively. This concept hasnow been replaced, for stem apices, by thetunica–corpus theory. In roots, however,the concept is still applied, and in some an-giosperm roots a fourth histogen zone isrecognized, the calpytrogen, which givesrise to the root cap. Compare tunica–corpus theory.

histology The study of tissues and cellsat microscopic level.

histone One of a group of relativelysmall proteins found in chromosomes ofeukaryotic cells, where they organize andpackage the DNA. When hydrolyzed, theyyield a large proportion of basic aminoacids, as they are rich in the basic aminoacids arginine and lysine. They dissolvereadily in water, dilute acids, and alkalisbut do not coagulate readily on heating.

hnRNA (heterogeneous nuclear RNA)See messenger RNA.

Hofmeister, Wilhelm Friedrich Bene-dict (1824–77) German botanist whofirst demonstrated the ALTERNATION OF

GENERATIONS in lower plants. Hofmeisteralso showed that the gametophytes andsporophytes of seed-bearing plants werehomologous with those of ferns andmosses and that the gymnosperms formeda plant group between the latter and theangiosperms.

holdfast (hapteron) In algae, the cell ororgan that attaches the plant to the sub-strate. It is often disklike, sometimes di-

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vided into many fingerlike processes, and isparticularly prominent in the brown algae(e.g. Fucus).

Holocene (Recent) The present epoch inthe geological time scale, being the secondepoch of the Quaternary period, datingfrom the end of the last glaciation, about10 000 years ago, to the present day.

holoenzyme A catalytically active com-plex made up of an APOENZYME and a CO-FACTOR (nonprotein prosthetic group). Theformer is responsible for the specificity ofthe holoenzyme whilst the latter deter-mines the nature of the reaction. Someholoenzymes, such as pyruvate dehydroge-nase, are highly complex, with several co-factors. See coenzyme.

holophytic The type of nutrition inwhich complex organic molecules are syn-thesized from inorganic molecules usinglight energy. It is another term for pho-totrophic.

holozoic (heterotrophic) Designatingorganisms that feed on other organisms orsolid organic matter, i.e. insectivorous andparasitic plants, fungi, many bacteria andmost animals. Compare holophytic.

homeobox A segment of DNA found inmany so-called homeotic genes concernedwith controlling the development of organ-isms. It consists of 180 base pairs, and thesequence of bases is remarkably similaracross a wide range of eukaryotic species,from yeasts to human beings. This suggestsit arose early in evolutionary time and hasbeen little changed since. The sequence en-codes the amino acids of a peptide se-quence (the homeodomain) that enablesthe parent protein to bind to DNA. This isconsistent with the suggested role ofhomeotic proteins as genetic switches,binding to genes to control their expres-sion. See differentiation.

homeotic gene (homeogene) Any of aclass of genes that are crucial in determin-ing the DIFFERENTIATION of tissues in differ-

ent parts of the body during development.They encode proteins that regulate the ex-pression of other genes by binding to DNA.This binding capability can be pinpointedto a characteristic base sequence known asa HOMEOBOX. Such genes often occur incomplexes in which the physical order ofthe genes corresponds to the order in whichthey are expressed from anterior to poste-rior in the developing embryo.

homogametic sex The sex with ho-mologous SEX CHROMOSOMES, in mammals,designated XX.

homogamy The condition in flowers inwhich the anthers and stigmas ripen at thesame time, so encouraging self-pollination.Homogamy occurs in the closed cleistoga-mous flowers that appear late in the seasonin certain plants. Compare dichogamy.

homologous 1. Describing structuresthat, though in different species, are be-lieved to have the same origin in a commonancestor. Thus the stamens of angiospermsand the MICROSPOROPHYLLS of lower vascu-lar plants are said to be homologous, beingderived from a leaflike structure bearingthe MICROSPORANGIA (anthers). Compareanalogous.2. See isomorphism.

homologous chromosomes Chromo-somes that pair at meiosis. Each carries thesame genes as the other member of the pairbut not necessarily the same alleles for agiven gene. One member of each pair ofmaternal origin, the other of paternal ori-gin. With the exception of the sex chromo-somes (X and Y) they are morphologicallysimilar, being of similar size and shapewith centromeres at the same location.During the formation of the germ cells onlyone member of each pair of homologs ispassed on to the gametes. At fertilizationeach parent contributes one homolog ofeach pair, thus restoring the diploid chro-mosome number in the zygote.

homospory The production of only onekind of asexual spore, which then develops

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into a hermaphrodite gametophyte. In thevascular plants the condition is seen inmost ferns, psilophytes, and Lycopodiumspecies. Compare heterospory.

homostyly The usual condition foundin plants in which the styles of all flowersof the same species are about the samelength. Compare heterostyly.

homothallism A condition found insome algae and fungi in which each thal-lus is self-compatible. Homothallic speciesmay produce distinctly different sizes ofgametes that can fuse with each other andare thus effectively hermaphrodite. Com-pare heterothallism.

homozygous Having identical allelesfor any specified gene or genes. A homozy-gote breeds true for the character in ques-tion if it is selfed or crossed with a similarhomozygote. An organism homozygous atevery locus produces offspring identical toitself on selfing or when crossed with agenetically identical organism. Homo-zygosity is obtained by inbreeding, andhomozygous populations may be welladapted to a certain environment, but slowto adapt to changing environments. Com-pare heterozygous. See pure line.

honey guides Lines or spots on petalsthat direct a pollinating insect to the nec-taries.

Hooke, Robert (1635–1703) Englishphysicist. In 1665 he published his Micro-graphia, a beautifully illustrated book thatdescribed both his theories of the nature oflight and the investigations he had madewith his improved microscope. While ob-serving cork, he described it as resemblinga honeycomb and was the first to refer tothe cavities as ‘cells’.

Hooker, Sir Joseph Dalton (1817–1911) Plant taxonomist and explorer.Son of the British botanist William JacksonHooker (1785–1865), the first director ofKew Gardens, Hooker studied medicine atGlasgow University. His famous voyage to

the Antarctic (1839–43) on HMS Erebusresulted in a six-volume flora of Antarc-tica, New Zealand, and Tasmania that at-tracted the attention of Charles DARWIN.Hooker was one of those in whom Darwinconfided his theory and was instrumentalin encouraging Darwin to present his jointpaper with Wallace on the theory of evolu-tion by natural selection.

In 1865 he succeeded his father as di-rector of Kew Gardens, which became aninternational center for botanical research.With George BENTHAM, he produced theworld flora Genera Plantarum.

hormone (growth substance; plant hor-mone; phytohormone) One of a group ofessential organic substances produced inplants. They are effective in very low con-centrations and control growth, develop-ment, and responses to external stimuli bytheir interactions. They affect physiologi-cal processes, but are not actually part ofmajor metabolic pathways. The same hor-mone may have many different effects, de-pending upon the balance between theconcentrations of different hormones, theparticular tissues concerned, and the age ofthe plant. See auxin; gibberellin; cytokinin;abscisic acid; ethylene.

horsetails See Sphenophyta.

host An organism used as a source ofnourishment by another organism, the par-asite, which lives in or on the body of thehost. See parasitism.

humidity The moisture content of theatmosphere. The absolute humidity is theamount of water vapor present in a unitvolume of air (usually 1 m3 of air). The rel-ative humidity is the ratio of the amount ofwater vapor actually present in the air tothe greatest amount that the air could holdat the same temperature, and is usually ex-pressed as a percentage.

humus The nonliving finely divided or-ganic matter in soil derived from thedecomposition of animal and plant sub-stances by soil bacteria. Humus consists of

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60% carbon, 6% nitrogen, and smallamounts of phosphorus and sulfur, and isvalued by horticulturalists and farmers asit improves the fertility, water-holding ca-pacity, and workability of the soil. Humushas colloidal properties that enable it to re-tain water, so it can improve the moisturecontent of sandy soils. It aids the formationof crumbs in the soil, and is often added toclay soil to increase the particle size, pro-moting drainage and aeration.

hyaline cells Any leaf cell with nochlorophyll that is normally used for stor-age of water or solutes.

hyaloplasm (cell matrix; ground sub-stance) See cytoplasm.

hybrid An organism derived from cross-ing genetically dissimilar parents. Thusmost individuals in an outbreeding popula-tion could be called hybrids, and geneti-cists apply the term to the F1 generationresulting from a monohybrid cross, wherethe offspring differ in the alleles of a singlegene. However, the term is usually reservedfor the product of a cross between individ-uals that are markedly different. If two dif-ferent species are crossed, the offspring isoften sterile, owing to the nonpairing of thechromosomes necessary for gamete forma-tion. In plants this is sometimes overcomeby the doubling of the chromosome num-ber, giving an allopolyploid. By contrast,hybrids derived from different varieties ofthe same species are often more vigorousthan their parents (hybrid vigor), and areselected and propagated by vegetativemeans by agriculturists and horticulturists.See chimera; heterosis.

hybrid sterility The reduced ability ofsome HYBRIDS to produce viable gametes. Itis due to the lack of homologous pairs ofchromosomes, so chromosomes cannotpair in meiosis, and the resulting gametesmay have extra or missing chromosomes.Hybrid sterility is sometimes overcome bypolyploidy. Compare heterosis.

hybrid vigor See heterosis.

hydathode A specialized secretory struc-ture involved in the removal of excesswater from plants. It may be a modifiedstoma with the guard cells permanentlyopen so that water is lost passively byhydrostatic pressure, as in sea lavenders(Limonium), or a glandular hair fromwhich water is actively secreted, as in run-ner bean (Phaseolus). Hydathodes arefound at the leaf tips or along the leaf mar-gins.

hydrocarbon An organic compoundthat contains carbon and hydrogen only,e.g. acetylene, butane. Many of the compo-nents of natural gas, petroleum, and coalare hydrocarbons.

hydrogen An essential element in livingtissues. It enters plants, with oxygen, aswater and is used in building up complexreduced compounds such as carbohydratesand fats. Water itself is an importantmedium, making up 70–80% of the weightof organisms, in which chemical reactionsof the cell can take place. Hydrogenatedcompounds, particularly fats, are rich inenergy and on breakdown release energyfor driving living processes.

hydrogen bond A type of bond occur-ring between molecules. Hydrogen bond-ing takes place between oxygen, nitrogen,or fluorine atoms on one molecule, and hy-drogen atoms joined to oxygen, nitrogen,or fluorine on the other molecule. The at-traction is due to electrostatic forces. Hy-drogen bonding is responsible for theproperties of water. It is important in manybiological systems for holding together thestructure of large molecules, such as pro-teins and DNA.

hydrolase An enzyme that catalyzes aHYDROLYSIS reaction. Glycosidases, pepti-dases, and phosphatases are examples.Hydrolases play an important part in ren-dering insoluble food material into a solu-ble form, which can then be transported insolution. They are important in the mobi-lization of stored food, and in the uptake of

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nutrients by fungi and other heterotrophicorganisms.

hydrolysis The breakdown of a com-pound due to a reaction with water (H+ orOH– ions).

hydrophilic Describing a molecule orsurface that has an affinity for water. Suchmolecules are usually polar, for exampleproteins. Compare hydrophobic.

hydrophily Pollination in which watercarries the pollen from anther to stigma.The pollen may be transported on thewater surface or through the water. Pollenthat floats is light and water repellent, andmay have a coating of oil that alters thesurface tension of the water.

hydrophobic Describing a molecule orsurface that has no affinity for water. Suchmolecules are nonpolar, e.g. benzene.Compare hydrophilic.

hydrophyte A plant found growing inwater or in extremely wet areas. Hy-drophytes show certain adaptations tosuch habitats, notably development ofaerenchyma, reduction of cuticle, root sys-tem, and mechanical and vascular tissues,and divided leaves. Large intercellular airspaces in leaves, stems, and roots allowoxygen to diffuse through the plant. Hy-drophytes with floating leaves can ex-change gases with the atmopshere; manysubmerged hydrophytes, such as spikedwater milfoil (Myriophyllum spicata) haveno stomata, absorbing water and gasesover their entire surface. Compare meso-phyte; xerophyte.

hydroponics (water culture) The growthof plants in liquid culture solutions ratherthan soil. The solutions must contain thecorrect balance of all the essential mineralrequirements. The method is used commer-cially, especially for glasshouse crops, andalso in experimental work in determiningthe effects of mineral deficiencies.

hydrosere Any plant community in aSUCCESSION that starts in fresh water.

hydrotropism (hydrotropic movement)A TROPISM in which the stimulus is water. Itis a special kind of chemotropism. Rootsare positively hydrotropic and hypocotylsnegatively hydrotropic. If water is in shortsupply, the stimulus of water is strongerthan the stimulus of gravity in determiningresponse.

hygroscopic Able to absorb water fromthe surroundings, including moisture fromthe atmosphere.

hymenium (pl. hymenia) A layer of thefruiting body of certain ascomycete andbasidiomycete fungi in which the asci orbasidia are borne. The hymenium may bedirectly exposed to the air, as in the gills ofthe mushroom (a basidiomycete) or mayopen into a flask-shaped cavity, as in theperithecia of certain ascomycetes.

hyperplasia Enlargement of a tissue dueto an increase in the number of its cells. Inplants this usually occurs in response to adisease-causing organism.

hypertonic Designating a solution withan osmotic pressure greater than that of aspecified other solution, the latter beinghypotonic. When separated by a selectivelypermeable membrane (e.g. a cell mem-brane) water moves by osmosis into the hy-pertonic solution from the hypotonicsolution. Compare hypotonic; isotonic.

hypertrophy Enlargement of a tissue ororgan due to an increase in the size of itscells. An example is the swelling of roots of plants from the Brassicaceae familyinfected with club root (Plasmodiophorabrassicae). Compare hyperplasia.

hypha (pl. hyphae) In fungi, a fine nonphotosynthetic tubular filament thatspreads to form a loose network termed amycelium or aggregates into fruiting bod-ies (e.g. toadstools). Hyphae may bebranched or unbranched and may or may

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not have cross walls (septa) dividing theminto cells. Many fungi have incompletesepta. Hyphae are parasitic or saprophyticand the tips secrete enzymes to digest andpenetrate the food supply. The hyphalwalls of most species of fungi differ fromthose of plants in being composed of mi-crofibrils of a nitrogenous compoundcalled chitin or a form of fungal cellulose.Hyphae also differ in lacking plastids, anypigment being contained in the walls, cyto-plasm, or oil globules.

hypocotyl The stem below the cotyle-dons, occupying the region between thecotyledon stalks and the point where lat-eral roots arise. Rapid elongation of thehypocotyl after germination pushes thecotyledons above ground in plants show-ing epigeal germination. Compare epicotyl.

hypodermis One or more layers of cellsthat may be found immediately below theepidermis of plants. It may be composed ofthin-walled colorless cells and functions as

water-storing tissue as in certain succulentleaves and the aerial roots of epiphytes.

hypogeal germination Germination inseed plants during which the cotyledons re-main underground, as in broad bean (Viciafaba). The cotyledons thus act only as foodstorage organs and not as photosyntheticorgans and the hypocotyl does not elon-gate. Compare epigeal germination.

hypogyny The simple arrangement offlower parts in which the receptacle is ex-panded at the top of the pedicel in such away that the androecium and the periantharise from beneath the gynoecium giving asuperior ovary. Compare epigyny; perig-yny.

hypolimnion The lower noncirculatingwater in a thermally stratified lake.

hyponasty (hyponastic movements) Thecurving of a plant organ upward and to-ward the axis, caused by greater growth on

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IAA (indole acetic acid) A naturally oc-curring auxin. See auxin.

ICBN See International Code of Botani-cal Nomenclature.

Ice Age A period in the latter part of thePleistocene characterized by successivecoolings and warmings of the earth. In atleast four major glaciations (cold periods),ice caps spread south from the Arctic andnorth from the Antarctic.

idioblast Any specialized plant cell thatis dispersed among cells of a very differentkind. Idioblasts may contain a variety ofmaterials, e.g. tannins, oils, crystals, andwaste products. Isolated sclereids can becalled idioblasts.

idiogram See karyogram.

imbibition The phenomenon in which asubstance absorbs a liquid and swells, butdoes not necessarily dissolve in the liquid.The process is reversible, the substancecontracting on drying. Water is imbibed bymany biological substances: cellulose,hemicelluloses, pectic substances, lignin(all plant cell wall constituents); starch;certain proteins, especially in seeds; etc.Dry seeds absorb water by imbibition, ini-tially via the testa, and as seed volumeincreases great imbibitional pressures de-velop.

immunity The ability of plants and ani-mals to withstand harmful infective agentsand toxins. In most plants this is achievedby physical barriers preventing entry ofpathogens and by physiological reactionsto isolate the pathogen and its effects.

immunoelectrophoresis See electro-phoresis.

inbreeding Breeding between closely re-lated individuals. The most extreme formof inbreeding is self-fertilization, which oc-curs in some plants and algae (e.g. Spir-ogyra). Inbreeding increases homozygosityso that deleterious recessive genes are ex-pressed more often in the phenotype, anddecreases heterozygosity and hence the po-tential genetic variability of the popula-tion. There is also a general lowering ofvigor in inbred stock (inbreeding depres-sion), which is especially pronouncedamongst normally outbreeding popula-tions. Compare outbreeding. See cleis-togamy.

incipient plasmolysis The condition ofa cell that results when it is surrounded bya solution having the same osmotic pres-sure as the cell contents. See plasmolysis.

incompatibility 1. The rejection ofgrafts, transfusions, or transplants betweenplants or animals of different genetic com-position.2. A mechanism in flowering plants thatprevents fertilization and development ofan embryo following pollination by thesame or a genetically identical individual.It results in self-sterility, thus preventinginbreeding. It is due to interaction betweengenes in the pollen grain and those in thestigma, in such a way that the pollen is ei-ther unable to grow or grows more slowlyon the stigma.3. A genetically determined mechanism insome fungi that prevents sexual fusion be-tween individuals of the same race or strain(e.g. Mucor). See also heterothallism.

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incomplete dominance (co-dominance)The situation in which two different allelesare equally dominant. If they occur to-gether the resulting phenotype is inter-mediate between the two respectivehomozygotes. For example, if white antir-rhinums (AA) are crossed with red antir-rhinums (A¢A¢) the progeny (AA¢) willbe pink. Sometimes one allele may beslightly more dominant than the other(partial or incomplete dominance) inwhich case the offspring, though still inter-mediate, will resemble one parent morethan the other.

indefinite growth (indeterminategrowth) A form of GROWTH that has nofinite limit: the plant or organ continues togrow indefinitely. All plants are somewhatindeterminate, as they have meristems thatcan differentiate into almost any part giventhe right conditions.

indehiscent Describing a fruit or fruit-ing body that does not open at maturity torelease the seeds or spores. The fruit walleither decays releasing the seeds in situ, orthe fruit is eaten by an animal and the seedspass out intact in its droppings. Comparedehiscent.

independent assortment The law, for-mulated by MENDEL (Mendel’s SecondLaw), that genes segregate independentlyat meiosis so that any one combination ofalleles is as likely to appear in the offspringas any other combination. The work of T. H. Morgan later showed that genes arelinked together on chromosomes and sotend to be inherited in groups. The law ofindependent assortment therefore appliesonly to genes on different chromosomes.The term can also be applied to wholechromosomes. See linkage; Mendel’s laws.

indeterminate growth See indefinitegrowth.

indicator A substance used to test foracidity or alkalinity of a solution by a colorchange. Examples are litmus and phe-nolphthalein. A universal indicator shows

a range of color changes over a wide rangefrom acid to alkaline, and can be used toestimate the pH.

indicator species An organism that canbe used to measure the environmental con-ditions that exist in a locality. For example,lichen species are indicators of levels ofpollution, as different species are sensitiveto different levels and types of pollutants.

indigenous Describing an organism thatis native to an area, rather than introduced.

indole acetic acid (IAA) A naturally oc-curring auxin. See auxin.

inducer A substance that activates astructural gene or block of genes by com-bining with a REPRESSOR. See operon.

inducible enzyme (adaptive enzyme) Anenzyme that is produced by a cell only inthe presence of high concentrations of itssubstrate or a structurally similar sub-stance.

indusium The flap of tissue, derivedfrom the placenta, that encloses the devel-oping sporangia in the sorus of a fern.

inferior Below. In botany, the term isgenerally applied to the position of theOVARY of a flower in which the sepals,petals, and stamens arise above it. Theovary appears to have sunk into and fusedwith the cup-shaped receptacle. This con-dition occurs only in epigynous flowers. Ina floral formula, an inferior ovary is de-noted by a line above the gynoecium sym-bol and number. Compare superior. Seealso epigyny.

inflorescence A collection of flowerssharing a common stalk, the peduncle andusually subtended by a bract. Each flowerusually arises in the axil of a small bract, orbracteole, and may or may not be borne onits own individual stalk, called a pedicel.The development of an inflorescence mayrepresent the end of vegetative growth ofthat apex (definite growth), or allow it to

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continue (indefinite growth). There aremany types of inflorescence, determinedmainly by the method of branching. If thestem ends in a flower and subsequentgrowth is then from lateral buds below theapex, which themselves form flowers andmore lateral shoots, the inflorescence istermed a cymose inflorescence or cyme,e.g. lime (Tilia). If one shoot develops be-hind each axis a monochasial cyme, ormonochasium, is formed, as in avens(Geum). Variations in the inflorescencearrangement occur, according to the direc-tion of the lateral branches, for example inforget-me-not (Myosotis), all the branchesarise on the same side of the parent stemwhile in buttercup (Ranunculus), thebranches arise on alternate sides of the par-ent stem. If two shoots develop below eachaxis this gives a dichasial cyme, or dicha-sium, as in the campions and catchflies (Si-lene). Cymose inflorescences typically openfrom the apex downwards; in flat-toppedforms the oldest flowers are in the center.In a raceme (racemose inflorescence) theapex continues growing, and subsequentflowers develop in sequence up the stem,

e.g. foxglove (Digitalis). In a compoundraceme each branch of the inflorescencebears a smaller raceme of flowers, e.g. fes-cues (Festuca). If the lateral branches of araceme are themselves branched, as inmany grasses, the inflorescence is called apanicle. This term is often applied to anysort of branched racemose inflorescence,for example the horse chestnut, in whicheach branch is actually a cyme (Aesculus).A spike is a type of racemose inflorescencehaving sessile flowers (they have nopedicels) borne on an elongated axis, as inwheat (Triticum). The catkin and spadixare modifications of the spike. A catkin is ashort, densely packed raceme bearing uni-sexual flowers with highly reduced or ab-sent perianth, e.g. oak (Quercus). A spadixis a type of inflorescence found in the fam-ily Araceae, e.g. cuckoopint (Arum macu-latum). It is a modified spike with a largefleshy axis on which are borne small her-maphrodite or, more usually, unisexualflowers. The inflorescence is enclosed by alarge bract, the spathe, which may be fo-liose or petalloid and has been shown to at-tract insects in certain species. In a

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Inflorescence: types of inflorescence

capitulum (pl. capitula), as in many Aster-aceae, e.g. daisy (Bellis), the inflorescencecomprises many unstalked florets insertedon the flattened disklike end of the pedun-cle and surrounded by a ring of sterilebracts, the involucre. The internodes arenot recognizable. Each floret may or maynot be borne in the axil of a bract on thedisk. Many species of composite have twodistinct types of floret in the capitulum:disk florets in the center are tubular floretsending in five short teeth, while ray floretshave a straplike extension to the tube andoccur around the edge of the capitulum,rather like petals. Some species, such asthistles, have only ray florets, others haveonly disk florets, e.g. chicory (Cichorium),and many have both disk and ray florets,e.g. sunflower (Helianthus). In racemoseinflorescences the flowers typically openfrom below upwards. An umbel is a type ofinflorescence in which the stem axis is notelongated and individually stalked flowersappear to arise from same point on thestem. These flowers are massed on oneplane, giving the appearance of an um-brella, with the oldest flowers on the out-side and the youngest in the middle. Theumbel is typical of the carrot family (Api-aceae). Umbels may be grouped into acompound umbel, composed of umbels ofsmaller umbels, as in onion (Allium). Acorymb is an inflorescence with flowerstalks of different lengths, the lowest beingthe longest. This gives a flat-topped clusterof flowers at the same level that is charac-teristic of many brassicas, e.g. candytuft(Iberis). Umbels, corymbs, and capitulacan in turn be grouped into cymose inflo-rescences, e.g. onion, Viburnum, andscabious (Scabiosa) respectively.

initial A cell permanently in a MERISTEM,actively dividing and adding new cells tothe plant body. An initial never becomesdifferentiated. One of its daughters re-mains in the meristem as an initial, whilethe other differentiates to form a non-meristematic cell. Two basic groups exist:apical initials at root and shoot apices, andlateral meristem initials whose positiondepends on the location of the meristem,

e.g. intrafascicular CAMBIUM between xylemand phloem. Apical initials are rarely singlecells except in certain lower plants, and ini-tiate more than one structure, e.g. lateralshoots, flowers, and leaves, as well as newstem tissues. Vascular cambium possessestwo distinct types of initials, ray initialsforming medullary rays, and fusiform ini-tials producing xylem and phloem el-ements.

initiation factor See translation.

inositol An optically active cyclic sugaralcohol, synthesized from glucose 6-phos-phate. A derivative of iniositol, phytic acid,is an important storage compound inseeds, which is believed to be convertedduring germination to inositol, and thenceto glucuronic and uronic acids for the syn-thesis of cell-wall components.

insectivorous plant See carnivorousplant.

integument A layer surrounding the nu-cellus in the ovules of gymnosperms andangiosperms. Most angiosperm ovules pos-sess two integuments, while gymnospermsusually have only one. Enclosure of the nu-cellus by the integuments is incomplete, themicropyle remaining to allow access to theembryo sac or, in gymnosperms, to thearchegonium.

intercalary meristem A region of ac-tively growing primary tissue clearly sepa-rate from the apical meristem. Intercalarymeristems occur at the internode and leaf-sheath bases (joints) of many monocotyle-dons including grasses, and at theinternodes of horsetails, where they serveto increase longitudinal growth.

intercellular Describing materials foundand processes occurring between cells.Compare intracellular.

interfascicular cambium A single layerof actively dividing cells between the vas-cular bundles in stems. It is formed whenparenchyma cells resume meristematic

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activity. The interfascicular cambium andINTRAFASCICULAR CAMBIUM (within the vas-cular bundles) link into a complete cam-bium cylinder that cuts off secondaryxylem tissue to the inside, secondaryphloem tissue to the outside, and paren-chyma cells both sides (forming medullaryrays). See vascular cambium.

International Code of Botanical No-menclature (ICBN) A set of rules forthe scientific naming of wild plants, algae,fungi, and slime molds, which allocates asingle unambiguous name to each taxon.Naming of families and lower taxa is basedon the first valid name published, the start-ing-point being taken as the publication ofSpecies Plantarum, by LINNAEUS, in 1753.But if a plant is later assigned to differenttaxa or rank, a different name may begiven. Thus a plant may legitimately havetwo names, according to which genus dif-ferent authors place it in. If several taxa arecombined into one, that TAXON takes onthe name of the former taxon with the old-est name. Different starting dates apply tofossil plants and bacteria. There is a sepa-rate code for cultivated plants, the Interna-tional Code of Nomenclature of CultivatedPlants (ICNCP). See classification.

International Union for the Conserva-tion of Nature and Natural Resources(IUCN) See World Conservation Union.

internode The region of the stem be-tween two NODES. See intercalary meris-tem.

interphase The stage in the CELL CYCLE

when the nucleus is not in a state of divi-sion. Interphase is divisible into variousstages each characterized by a differingphysiological activity.

intine (endosporium) See pollen.

intracellular Describing the materialenclosed and processes occurring interiorto the cell membrane. Compare intercellu-lar.

intrafascicular cambium (fascicularcambium) The part of the VASCULAR

CAMBIUM within the vascular between thexylem and phloem.

intraspecific selection Natural selec-tion acting on individuals of the samespecies.

introgression (introgressive hybridiza-tion) The introduction of genetic mater-ial from one gene pool to another byhybridization and subsequent back-cross-ing to one or other of the parents. It formsthe basis of the evolution of most cropplants, e.g. sunflower (Helianthus annuus).Where the ranges of two species overlap,and fertile hybrids can be produced, thesehybrids will tend to backcross with themost abundant species, so the offspringdown many generations come to resemblethe most abundant parents.

introgressive hybridization See intro-gression.

intron A noncoding DNA sequence thatoccurs between coding sequences (exons)in many eukaryote genes. Messenger RNA(mRNA) does not contain introns, thesebeing removed during the transcriptionprocess. Intron removal is now thought tobe an autocatalytic process in which theRNA acts as its own enzyme (see ri-bozyme). However, in the case of mRNAin the nucleus the process is regulated by acomplex of proteins called a spliceosome.Compare exon.

introrse Denoting anthers in which de-hiscence lines are toward the center of theflower. Pollen is consequently shed towardthe carpels, which favors self-pollination.Compare extrorse.

intussusception The incorporation ofcellulose molecules into the existing cellwall, giving an increase in wall area. Theprocess is usually preceded by water up-take, increasing the pressure on the cellwall, which stretches. The new wall is thenthickened by APPOSITION.

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inulin A polysaccharide food reserve ofsome higher plants, particularly the Aster-aceae, e.g. Dahlia root tubers. It is a poly-mer of fructose.

inversion See chromosome mutation.

invertase See sucrase.

invert sugar See sucrose.

in vitro Literally ‘in glass’; describingexperiments or techniques performed inlaboratory apparatus rather than in the liv-ing organism. Cell tissue culture is an ex-ample. Compare in vivo.

in vivo Literally ‘in life’; describingprocesses that occur within the living or-ganism. Compare in vitro.

involucre 1. A protective structure con-sisting of a ring of bracts arising below theINFLORESCENCE in angiosperms with con-densed inflorescences (e.g. the capitulumand umbel).2. A sheathlike outgrowth of tissue inbryophytes protecting the archegonia orantheridia in certain liverworts.3. A tubular extension of the thallus ofhornworts that rises up and surrounds thebase of the sporophyte.

iodine See staining.

ion-exchange chromatography A typeof CHROMATOGRAPHY in which the com-pounds are separated according to theiracidity.

ionic bond A bond formed when anelectron passes from one atom to another.The atom that loses the electron becomes apositively charged ion, and the receivingatom becomes negatively charged, so thetwo ions become bonded by a strong elec-trostatic force. Compare covalent bond;hydrogen bond.

iron A MICRONUTRIENT essential forplant growth. It is found in porphyrins,and is essential for chlorophyll synthesis.

Lack of iron leads to CHLOROSIS, especiallyin young leaves. It is also found in cy-tochromes, which are important compo-nents of the electron-transport chains ofrespiration and photosynthesis.

isoelectric focusing A technique usedin ELECTROPHORESIS to separate amphotericmolecules (able to combine with eitheracids or bases).

isoelectric point The value of pH in amedium at which a molecule has no elec-tric charge. Repulsion between moleculesceases, so they precipitate. This principle isused to separate mixtures of amino acids orproteins. See isoelectric focusing.

isoenzyme (isozyme) An enzyme thatoccurs in different structural forms withina single species. The isomeric forms allhave the same molecular weight but differ-ing structural configurations and proper-ties, usually due to different combinationsof their component subunits. They areformed by different alleles, and are a valu-able tool for detecting genetic variation.

Isoetales (quillworts) An order of theLYCOPHYTA containing two genera – Iso-etes, found in most parts of the world, andStylites, found only in the Peruvian Andes.Quillworts are heterosporous, mainlyaquatic perennials, with a dense rosette oftubular quill-like leaves arising from ashort, stout rootstock.

isogamy The sexual fusion of gametesof similar size and form. It occurs in fungiand some protoctists. Compare anisog-amy.

isolating mechanism Structural, physi-ological, behavioral, genetic, geographical,or other factors that restrict the interbreed-ing of one population with another, hencerestricting gene flow. The development ofisolating mechanisms promotes the forma-tion of new varieties and species.

isoleucine A nonpolar AMINO ACID, syn-thesized from threonine.

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isomerase An enzyme that catalyzes theconversion of a molecule from one iso-meric form to another. For example, in gly-colysis, glucose phosphate isomerasecatalyzes the conversion of glucose 6-phos-phate to fructose 6-phosphate.

isomorphism A condition seen in cer-tain algae (e.g. Ulva, the sea lettuce) inwhich the alternating generations of thelife cycle are morphologically identical(HOMOLOGOUS). Compare heteromorphism.

isotonic Designating a solution with anosmotic pressure or concentration equal tothat of a specified other solution, usuallytaken to be within a cell. It therefore nei-

ther gains nor loses water by osmosis.Compare hypertonic; hypotonic.

isotopic dating See radiometric dating.

isozyme See isoenzyme.

IUCN See World Conservation Union.

Ivanovsky, Dmitri Iosifovich (1864–1920) Russian botanist. In 1862, whileworking on tobacco mosaic disease at the Technical Institute in St Petersburg,Ivanovsky noticed that healthy plantscould be infected with the sap of diseasedtobacco plants. He also noticed that thesap filtrate contained tiny crystalline par-ticles that he believed were causative

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Jacob–Monod model A model of regu-lation of gene activity in bacteria proposedby F. Jacob and I. Monod in 1960. Usingthe concept of the OPERON, it proposed thatmetabolism is regulated and coordinatedby controlling messenger RNA synthesis(i.e. TRANSCRIPTION) and hence the produc-tion of enzymes. A similar but more com-plex system is known to operate ineukaryotes. See inducible enzyme.

Johannsen, Wilhelm Ludwig (1857–1927) Danish botanist and geneticist. Jo-hannsen became interested in the Theoryof Heredity, a work by the British anthro-pologist Sir Francis Galton, which statedthat natural selection does not work on theoffspring of self-fertilizing plants. Jo-hannsen experimented with the Princessbean and found that if the self-fertilizingparent plants were from a mixed popula-tion, then selection did indeed work. Hedescribed the offspring of a single parent asa ‘pure line’ and stated that they were ge-netically identical. In 1905 he coined theterms ‘phenotype’ to describe the outwardappearance of an individual and ‘genotype’to describe the genetic makeup of an indi-vidual. Following the rediscovery ofMENDEL’S LAWS, he published his Elementsof Heredity in 1905 and used the word‘gene’ for Mendel’s factors of inheritance.His application of statistical methods tocharacteristics of populations proved to bea cornerstone of modern genetics.

J-shaped growth curve See exponen-tial growth.

Juncaceae (rushes) A family of mono-

cotyledonous plants, mostly herbs, withlong, narrow grasslike or cylindrical leavesspirally arranged, but usually arising closeto the ground. They are found in temperateand cold regions, and on tropical moun-tains, forming an important part of thevegetation in poorly drained areas.

junk DNA The noncoding part ofDNA, i.e. the part that does not code forproteins. Its function is not yet understood,but it appears that part of it contains in-tructions that help control where and whenthe different proteins are expressed. Com-pare repetitive DNA; selfish DNA.

Jurassic The middle period of the Meso-zoic era, 215–145 million years ago. Theclimate was warm and humid, and therewas widespread expansion of forests,which gave rise to thick deposits of coal.The Jurassic has been called the ‘Age of Cy-cads’. The seed-ferns declined, and therewas a great increase in the number and di-versity of cycads, palmlike gymnosperms,and conifers. Fossils of monocotyledonouspollen and stems of palms suggest that theangiosperms evolved during this period.See also geological time scale.

Jussieu, Antoine-Laurent de (1748–1836) French plant taxonomist. Work-ing at the Jardin du Roi in Paris, Jussieubased his classification system on manyplant characteristics rather than the smallnumber used by LINNAEUS. Using the col-lections of Banks, Linnaeus, and PhilibertCommesson, he established 15 classes and100 families of plants in his classification.

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karyogamy The fusion of two nucleithat exist within a common cytoplasm, asoccurs in the formation of the zygote fromtwo gametes. In the more advanced fungi,especially the Basidiomycota, nuclei mayfuse some considerable time after fusion ofhyphae of different mating strains, forminga dikaryon.

karyogram (idiogram) The formalizedlayout of the KARYOTYPE of a species, oftenwith the chromosomes arranged in a cer-tain numerical sequence. This usually takesthe form of drawings or photographs of theindividual chromosomes, often arranged inhomologous pairs in numerical sequence.

karyokinesis During cell division, theprocess of nuclear division that precedescytoplasmic division. There are two maintypes of nuclear division: MITOSIS, whichresults in daughter cells identical to theirparents; and MEIOSIS (reduction division),which produces daughter cells which havehalf the number of chromosomes of theirparent cells, and in which the genetic ma-terial has been recombined. Compare cy-tokinesis.

karyotype The physical appearance ofthe chromosome complement of a givenspecies. A species can be characterized byits karyotype since the number, size, andshape of chromosomes vary greatly be-tween species but are fairly constant withinspecies. See karyogram.

kelp See Laminariales.

a-ketoglutaric acid A five-carbon di-carboxylic acid that is an intermediate inthe KREBS CYCLE. It is synthesized by oxida-tion of isocitrate, coupled to the reduction

of NAD. In the Krebs cycle it undergoes de-carboxylation to form succinyl CoA. a-ketoglutarate is also involved in aminoacid metabolism, especially TRANSAMINA-TION.

ketohexose A ketose SUGAR with sixcarbon atoms, e.g. fructose.

ketopentose A ketose SUGAR with fivecarbon atoms, e.g. ribulose.

ketose A monosaccharide SUGAR con-

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Ketose: typical ketose sugars

taining a ketone (=CO) or potential ketonegroup, e.g. dihydroxyacetone.

kinase (phosphotransferase; phosphory-lase) 1. Any enzyme that transfers aphosphate group, usually from ATP.2. An enzyme that activates the inactiveform of other enzymes. For instance, whentrypsinogen, the inactive form of trypsin,comes in contact with enterokinase, activetrypsin is released.

kinetin See cytokinin.

kinetosome (basal body) A barrel-shaped body found at the base of all eu-karyote UNDULIPODIA (cilic and flagella)and identical in structure to the CENTRIOLE.It is essential for formation of undulipodia.

kingdom Formerly, the highest rankingcategory in most classification systems.Most taxonomists today recognize the

rank of domain as being higher than that ofkingdom. See Five Kingdoms classification.

klinostat (clinostat) An apparatus usedin tropism experiments to remove the uni-directional influence of a stimulus on aplant organ.

Kramer, Paul Jackson (1904– )American plant physiologist. Working atDuke University, Kramer made a signifi-cant contribution to the study of water-uptake mechanisms in plants. Using radio-actively labeled elements he demonstratedthat the region of maximum absorption ofwater is a few centimeters behind the roottip and that quantities of minerals are ab-sorbed passively in the transpiration stream.

Kranz anatomy The specific arrange-ment of the photosynthetic tissues in theleaves of C4 PLANTS. The cells of the bundlesheath that surround the vascular tissuesare large and contain specialized elongated

kinase

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chloroplasts that do not contain grana, butform starch grains. The chloroplasts of themesophyll, by contrast, appear normal andcontain grana, but do not form starchgrains. In these chloroplasts carbon diox-ide combines with phosphoenolpyruvate toform oxaloacetic acid, which is trans-ported to the bundle sheath cells, where thecarbon dioxide is released, then fixed bythe enzyme ribulose bisphosphate carboxy-lase to form glycerate 3-phosphate, thefirst step in the Calvin cycle. See photosyn-thesis.

Krebs, Sir Hans Adolf (1900–81) Ger-man-born British biochemist best knownfor his elucidation of the KREBS CYCLE. Heshared the 1953 Nobel Prize for Physiol-ogy or Medicine with Fritz Lipmann(1899–1986).

Krebs cycle (citric acid cycle; TCA cycle;tricarboxylic acid cycle) A complex cycleof reactions in which pyruvate, producedby glycolysis, is oxidized to carbon dioxideand water, with the production of large

amounts of energy. It is the second stage ofAEROBIC RESPIRATION, requires oxygen, andoccurs in the matrix of mitochondria. 2-carbon acetate (acetyl coenzyme A), de-rived from pyruvate by decarboxylation,reacts with 4-carbon oxaloacetate to form6-carbon citrate, which is then decarboxy-lated in a series of steps to reconstitute ox-aloacetate. Some ATP is produced bydirect coupling with Krebs cycle reactions,but most production is coupled to the ELEC-TRON-TRANSPORT CHAIN. The operation ofthis depends on the generation of reducedcoenzymes, NADH and FADH2, by theKrebs cycle. These reduced cofactors pro-vide the energy for ATP formation in therespiratory electron-transport chain andOXIDATIVE PHOSPHORYLATION. Consideringthe Krebs cycle and electron-transportchain together, each pyruvate moleculeyields 15 ATP molecules. Since two pyru-vate molecules enter the cycle from glycol-ysis, 30 ATP are produced in all. Theintermediates in the Krebs cycle may beused directly or indirectly in the synthesisof amino acids.

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labeling The technique of using isotopes(usually radioactive isotopes) or other rec-ognizable chemical groups to investigatebiochemical reactions. For instance, a com-pound can be synthesized with one of theatoms replaced by a radioactive isotope ofthe element. The radioactivity can then beused to follow the course of reactions in-volving this compound.

Labiatae See Lamiaceae.

lactic acid A 3-carbon hydroxy-acidformed as a result of fermentation by cer-tain bacteria, e.g. Lactobacillus.

lactic acid bacteria A group of bacteriathat ferment carbohydrates in the absenceof oxygen, with lactic acid always a majorend product, e.g. Lactobacillus spp., Strep-tococcus lactis. They have a high toleranceof acid conditions. Lactic acid bacteria areinvolved in the formation of yoghurt,cheese, sauerkraut, and silage. They canoccur as spoilage organisms and some arepathogenic causing infections.

lacuna (pl. lacunae) 1. An intercellularcavity in plant tissues resulting from cellbreakdown, tissue splitting, or organizedformation. In hydrophytes a well-arrangedsystem of these air-filled cavities may existto provide buoyancy. A large lacuna occu-pies the center of many stems.2. See leaf gap.3. A depression in a lichen thallus.

lag phase See exponential growth.

lamella (pl. lamellae) 1. A layer withinthe cytoplasm or inside an organelle, whichis formed from a flattened membrane-bounded vesicle or tube. The layer thus

consists of two membranes lying close to-gether with a cavity between them. An ex-ample is the internal system of lamellae(thylakoids) in CHLOROPLASTS.2. A general term applied to any thin plate-like structure, e.g. the blade of a leaf.

Lamiaceae (Labiatae) A dicotyledonousfamily of herbs and low-growing shrubs,most of which have stems of square cross-section and simple leaves with stipulesarranged in opposite pairs. There are about6700 species, distributed worldwide butespecially numerous in the Mediterraneanregion. The family includes many herbs,such as basil (Hyssopus officinalis), marjo-ram (Origanum), mint (Mentha), sage(Salvia officinalis), savory (Satureja), andthyme (Thymus vulgaris).

lamina (pl. laminae) 1. (of angiosperms)A thin usually flat foliage leaf blade, com-monly attached to the stem by a petiole. Inmost plants the leaf laminae are the mainphotosynthetic organs. A lamina is termedsimple if complete, and compound if di-vided into leaflets. Internal organizationshows a wide photosynthetic mesophylllayer permeated by veins and bounded byan epidermis. Leaf variation is mainly dueto the structural diversity of the lamina.2. (of algae) The bladelike part of the thal-lus of certain algae, notably the Phaeo-phyta (brown algae), e.g. kelps (Laminaria,Macrocystis, Nereocystis) and bladder-wrack (Fucus vesiculosus).

Laminariales (kelps) An order of thePHAEOPHYTA (brown algae) that containsthe kelps (e.g. Macrocystis and Nereocys-tis) and is distinguished by its hetero-morphic ALTERNATION OF GENERATIONS inwhich a large sporophyte alternates with a

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microscopic gametophyte, and in whichgrowth takes place in a special meristem re-gion (see meristoderm) on the surface ofthe thallus.

laminarin The chief carbohydrate foodreserve of the Phaeophyta (brown algae).

lateral meristem A MERISTEM that liesparallel to the sides of the organ in which itoccurs and gives rise to secondary tissuesthat increase the girth of the organ. Seecambium; secondary growth. Compareapical meristem.

lateral root Any ROOT that arises fromthe PERICYCLE of another root.

laterite A hard crust that may form onthe surface of the SOIL in tropical regionswith alternating wet and dry seasons.

latex A liquid found in some floweringplants contained in special cells or vesselscalled laticifers (or laticiferous vessels). It isa complex variable substance that maycontain terpenes (e.g. rubber), resins, tan-nins, waxes, alkaloids, sugar, starch,enzymes, crystals, etc. in solution or sus-pension. It is often milky in appearance, as in dandelion (Taraxacum spp.) and let-tuce (Lactuca sativa) but may be colorless,orange, or brown. Commercial rubbercomes from the latex of the rubber plantsFicus elastica and Hevea brasiliensis.Opium comes from alkaloids found in thelatex of the opium poppy (Papaver som-niferum).

laticifers See latex.

Lauraceae A family of trees and shrubs(bays and laurels) usually with simple,leathery, evergreen alternate or oppositeleaves without stipules. There are about2500 species, mainly in the tropics andsubtropics, especially in Amazonia andSoutheast Asia.

Law of Independent Assortment SeeMendel’s laws.

Law of Segregation See Mendel’s laws.

layering A method of vegetative propa-gation in which RUNNERS or STOLONS arepegged down to the soil surface. Where anode touches the soil, adventitious rootsdevelop and a shoot arises from the LAT-ERAL MERISTEM. If the intervening runner orstolon rots or is cut away, the daughterplants becomes independent.

leaching The removal of humus and soilnutrients in solution by water movingdown the soil profile. It makes soils moreacid since cations (e.g. potassium and mag-nesium) are replaced by hydrogen ions.Leaching leads to the formation of pod-solized and lateritic soils.

leaf A flattened appendage of the stemthat arises as a superficial outgrowth fromthe apical meristem. Leaves are arranged ina definite pattern, have buds in their axils,and show limited growth. Most foliageleaves are photosynthetic, bilaterally sym-metrical, and externally differentiated intoLAMINA, MIDRIB, and PETIOLE. Many varia-tions occur. Not all foliage leaves are pho-tosynthetic as they may be modified whollyto bud scales and spines or partly to formtendrils. See illustration overleaf.

leaf buttress A leaf primordium, ap-pearing as a protuberance on the side of thestem apex, produced by periclinal divisionof the tunica and corpus, and associatedwith a procambium. It is the earliest stagein the development of a leaf, and forms theleaf base as the leaf grows out from it. Theleaf axis arises from the leaf buttress andmaintains procambial continuity in thecentral region. This differentiates into thevascular tissue of the developing leaf.

leaf curl Any disease of plants thatcauses an increase in numbers of cells thatresults in curling and puckering of theleaves. It may be caused by a fungus (e.g.Taphrinia deformans causes peach leafcurl), or by viruses (e.g. tobacco and cottonleaf curl) transmitted by insects such aswhiteflies (Bemisia).

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Leaf margins

leaf fall The shedding of leaves, usuallyassociated with a fall in AUXIN concentra-tion. In both evergreen and deciduous treesold leaves are continually shed throughoutthe growing season, but the remainingleaves are also shed at the onset of winteror a dry season in deciduous species. Seeabscission.

leaf gap (lacuna) A region of paren-chyma differentiated in the stem vascularcylinder immediately above a divergingLEAF TRACE. Lateral connections insure nobreak in the vascular system.

leaf mosaic 1. The nonoverlapping pat-tern of leaves on a plant that results mainlyfrom the genetically programmed arrange-ment of leaves and branches, and in somespecies also by the twisting of the petiolesdue to EPINASTY. It ensures that the maxi-mum amount of light is intercepted by theleaves. See phyllotaxis.2. A virus disease of plants characterizedby yellow and green or light- and dark-green mottling of the leaves.

leaf trace A vascular bundle or group ofvascular bundles connecting the vascularsystems of leaf and stem. It stretches fromthe leaf base to the vascular ring in the stemaxis.

lecithin (phosphatidyl choline) One of agroup of phospholipids that contain glyc-erol, fatty acid, phosphoric acid, andcholine that are found widely in higherplants and animals, particularly as a com-ponent of cell membranes.

legume 1. (pod) A dry dehiscent single-or many-seeded fruit formed from a singlecarpel, that dehisces mechanically by split-ting, often explosively, along both sides.The opposite tensions set up by obliquefibers in the drying pericarp cause dehis-cence. The valves may continue twistingand remove any remaining seeds. It is thetypical fruit of the FABACEAE family, and isfound in certain other families. Comparelomentum.2. Any plant of the family Fabaceae.

lemma See bract.

lenticel See bark.

leptosporangiate Describing the condi-tion, found in certain ferns (e.g. Filicales),in which the sporangium develops from asingle initial cell. The wall of a leptospo-rangium is usually only one cell thick, andas it dries out, it flicks the spores away ex-plosively. Compare eusporangiate.

leptotene See meiosis; prophase.

leucine An amino acid synthesized frompyruvate. It can be broken down to yieldacetoacetate. See amino acids.

leukoplast A colorless PLASTID, i.e. onenot containing chlorophyll or any otherpigment. Leukoplasts are common in thecells of roots and underground stems andstorage organs.

liana (liane) A long-stemmed woodyclimbing plant that grows from groundlevel to the canopy of trees, and oftenhangs down freely from branches, e.g. cu-rare (Strychnos toxifera). Lianas may havestems up to 100 meters (330 feet) long, andmay climb over the canopy to gain maxi-mum sunlight. Many flower only whenthey reach the top of the canopy. Up to aquarter of all the woody growth in somerainforests may be lianas.

lichens Symbiotic associations betweenan alga or cyanobacterium (the photo-biont) and a fungus (the mycobiont). Theyare slow-growing but can colonize areastoo inhospitable for other plants. Usuallythe fungus is an ascomycete but occasion-ally it is a basidiomycete. Reproduction inlichens may be asexual by soredia (algalcells enclosed by fungal hyphae) or by sex-ual fungal spores, which can survive only ifsome algal cells are also present. Fragmentsof the thallus containing both phycobiontand mycobiont cells may also grow intonew lichens, a form of asexual reproduc-tion. Examples include Peltigesa and Xan-thoria.

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life cycle

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life cycle The sequence of developmen-tal changes making up the span of an or-ganism’s life from the fertilization ofgametes to the same stage in the subse-quent generation. See diplobiontic; haplo-biontic.

ligase An enzyme that catalyzes thebond formation between two substrates atthe expense of the breakdown of ATP orsome other nucleotide triphosphate. Thedegree of bond formation by ligases is pro-portional to the amount of ATP availablein the cell at a particular instant.

light green A stain used in light mi-croscopy to show up cytoplasm and cellu-lose. It is commonly used as a counterstainwith safranin to highlight cytoplasm. Seestaining.

light microscope See microscope.

light reactions The light-dependent re-actions of PHOTOSYNTHESIS that convertlight energy into the chemical energy ofNADPH and ATP, which are used in thedark reactions for carbon dioxide fixation.Light absorbed by two groups of pigmentsin the chloroplast – Photosystems I and II.The absorbed light energy excites chloro-phyll a molecules, causing them to emithigh-energy electrons that pass down anELECTRON-TRANSPORT CHAIN, releasing en-ergy to synthesize ATP (photophosphory-lation) and NADPH. The light reactionstake place on the thylakoid membranes ofchloroplasts.

light saturation point See compensa-tion point.

lignin One of the main structural mate-rials of vascular plants. Together with cel-lulose it is one of the main constituents ofwood, where it imparts high tensile andcompressive strengths, making it ideal forsupport and protection. Lignified tissuesinclude SCLERENCHYMA and XYLEM. Ligninis deposited during secondary thickeningof cell walls. The degree of lignificationvaries from slight in protoxylem to heavy

in sclerenchyma and some xylem vessels,but values of 25–30% lignin and 50% cel-lulose are average. It is a complex variablepolymer, derived from sugars via aromaticalcohols.

ligule 1. A scalelike outgrowth, varyingin shape and size, of certain angiospermleaves. In grasses the ligule occurs at thejunction of the leaf sheath and lamina. Al-though membranous in most species, itmay be only a fringe of hairs. Ligules areimportant taxonomic features in grasses.2. A toothed, strap-shaped structureformed by the extension of one side of thecorolla tube in the florets of certain speciesof the ASTERACEAE. The teeth indicate thenumber of fused petals. In some species,e.g. dandelion (Taraxacum spp.), all floretsof a capitulum are ligulate but in others e.g.daisy (Bellis perennis), only the ray floretshave ligules.3. A very small tongue-shaped flap of tissueinserted on the upper surface of the leaves(microsporophylls) and sporophylls of cer-tain clubmosses (e.g. Selaginella) and quill-worts (Isoetes).

Liliaceae (lilies) A large family of mono-cotyledonous plants. Their taxonomy ismuch in dispute, with groups that were for-merly sections now regarded with newevidence as being separate families. Theyhave long, narrow parallel-veined leaves,whorled or alternate, arising at the base ofthe stem or along it. Most are herbs, e.g.lilies (Lilium) and tulips (Tulipa); some aresucculents or dwarf shrubs, e.g. butcher’sbroom (Ruscus aculeatus), and a few areclimbers (e.g. Smilax). Many species haveperennating organs such as bulbs, corms,or rhizomes.

liming The addition of lime to a soil toreduce its acidity, improve crumb struc-ture, or increase its calcium content. Seeflocculation.

limiting factor Any factor in the envi-ronment that governs the behavior ormetabolic activity of an organism or sys-tem by being above or below a certain

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level. In general, a number of different fac-tors (e.g. light intensity, temperature, car-bon dioxide concentration) may belimiting, but at any one time the factor thatis closest to its critical minimum is the lim-iting factor. For instance, the rate of pho-tosynthesis rises with increasing lightintensity as long as there is sufficient car-bon dioxide available, but at high light in-tensities carbon dioxide may become thelimiting factor instead.

limiting layer See meristoderm.

limnology The scientific study of fresh-water and its flora and fauna. Limnologyalso includes studying the chemical andphysical aspects of inland water. It may bedivided into the study of standing waterhabitats, e.g. lakes and ponds, and runningwater habitats, e.g. rivers and streams.

linear Describing leaves that are long,narrow, and parallel-sided for most oftheir length.

linkage The occurrence of genes to-gether on the same chromosome so thatthey tend to be inherited together and notindependently. Groups of linked genes aretermed linkage groups and the number oflinkage groups of a particular organism isequal to its haploid chromosome number.Linkage groups can be broken up byCROSSING OVER at meiosis to give new com-binations of genes. Two genes close to-gether on a chromosome are more stronglylinked, i.e. there is less chance of a crossover between them, than two genes furtherapart on the chromosome. Linked genesare symbolized Ab...Y/aB...y, indicatingthat Ab...Y are on one homolog whileaB...y are on the other homolog. Linkage isindicated when the associated inheritanceof two or more nonallelic genes is greaterthan would be expected from independentassortment. The genes on a single chromo-some form one linkage map. See chromo-some map; independent assortment.

Linnaeus, Carolus (1707–76) Swedishbotanist. While working at Uppsala Uni-

versity he began to investigate the sexualityof plants and by 1730 he had begun to setout a classification system based on thenumber and arrangement of stamens andpistils. After traveling through Laplandand discovering many new species, he set-tled in Holland and published his SystemaNaturae, in which he set out a classifica-tion of animals, plants, and minerals. Heplaced flowering plants into classes basedon the number of stamens and subdividedthe classes into orders based on the numberof their pistils.

Linnaeus’s most famous contributionto taxonomy was his system of BINOMIAL

NOMENCLATURE, in which he gave plantstwo Latin names. The first name is thegeneric name and the second is the specific(or species) name.

linoleic acid A common unsaturated18-carbon fatty acid occurring in glyc-erides in linseed oil, cottonseed oil, andother vegetable oils. It is an essential nutri-ent in the diet of mammals.

linolenic acid An unsaturated 18-car-bon fatty acid occurring commonly inplants as the glyceryl ester, for example inlinseed oil and poppy-seed oil. Linoleicacid is the predominant fatty acid in thechloroplast. It is an essential nutrient in thediet of mammals.

lipase Any of various enzymes that cat-alyze the hydrolysis of fats to fatty acidsand glycerol. Lipases are important in ger-minating seeds, e.g. castor oil (Ricinuscommunis) where they help to break downstorage fats.

lipid A collective term used to describe agroup of substances in cells characterizedby their solubility in organic solvents suchas ether and benzene, and their absence ofsolubility in water.

The group is rather heterogeneous interms of both function and structure. Theyencompass the following broad bands ofbiological roles: (1) basic structural unitsof cellular membranes and cytologicallydistinct subcellular bodies such as chloro-

limiting layer

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plasts and mitochondria; (2) compartmen-talizing units for metabolically active pro-teins localized in membranes; (3) a store ofchemical energy and carbon skeletons; and(4) primary transport systems of nonpolarmaterial through biological fluids. Plantssynthesize a wide range of lipids. Long-chain fatty acids (often 16–18 carbonatoms) are esterified to glycerol for use inmembranes, and PHOSPHOGLYCERIDES areparticularly important components of mem-branes; esters of long-chain monohydric al-cohols and fatty acids form the WAXES ofthe cuticle and the SUBERIN of the endoder-mis; and triacylglycerols are importantstorage lipids in seeds, especially in suchspecies as the castor oil seed (Ricinis com-munis), where they are stored in oil bodies,as small lipid droplets coated with lipidsand protein. The fatty acid components ofplant lipids come mainly from acetyl CoA,produced in plastids (including chloro-plasts) and mitochondria. GLYCOLIPIDS aresynthesized in the endoplasmic reticulum,mitochondria, and plastids.

lipopolysaccharide A conjugated poly-saccharide in which the noncarbohydratepart is a lipid. Lipopolysaccharides are amajor component of the cell walls ofGram-negative bacteria.

lipoprotein Any conjugated proteinformed by the combination of a proteinwith a lipid. Most membranes, includingthe plasma membrane, are composedmostly of lipoprotein. The lipid compo-nents are mainly phosphoglycerides andglycolipids.

litmus paper Red or blue acid–alkali in-dicator papers. In acids blue litmus paperturns red and in alkalis red litmus paperturns blue. Litmus solution has a pH rangefrom 4.5–8.3.

littoral 1. The zone of the seashore be-tween the high and low tide mark. Theterm is also applied to organisms living inthis zone. Since tidal ranges vary continu-ally, the zone is often defined in terms ofthe upper and lower limits of certain

species of organism. Compare benthic;sublittoral.2. The zone between the water’s edge anda depth of about six meters in a pond orlake, where light reaches the bottom sedi-ments. Rooted hydrophytes, both emer-gent and submergent, are found in thiszone. Compare profundal; sublittoral.

liverworts See Hepatophyta.

living fossil Modern organisms withanatomical or physiological features foundelsewhere only in extinct species. An exam-ple is the maidenhair tree (Ginkgo biloba),discovered in Japan in the 17th century,and later in China, but found only in culti-vation. Fossil members of the Ginkgophytaare common from rocks of Mesozoic age.

loam A medium-textured soil contain-ing a mixture of large and small mineralparticles. Loams are easy soils to work andcombine the good properties of sandy andclay soils.

locule (loculus) An air-filled compart-ment in an ovary in which the ovules de-velop. The term locule can also be appliedto any other cavity in an organ in whichother structures develop, for example thelocules of anthers in which pollen isformed. When all the carpel edges meet atthe middle of a syncarpous ovary there areas many locules as carpels, and the ovary isdesignated bi-, tri-, quadri-, or multilocularaccording to number. Adjoining carpelwalls form septa that separate locules. Aunilocular condition occurs in mono-carpellary ovaries and syncarpous ovarieslacking septa.

loculus (pl. loculi) See locule.

locus (pl. loci) The position of a gene ona chromosome. Alleles of the same gene oc-cupy the equivalent locus on homologouschromosomes.

loess A fine-textured, fertile, often cal-careous uniform soil comprising mainlyquartz particles about 0.015–0.005 mm in

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diameter. Widespread in central Europe,southern Russia, China, the central USA,and Argentina, it is derived from wind-blown clay and silt particles originally de-posited at the edge of the ice sheets at theend of the last ICE AGE. See chernozem.

log phase (logarithmic phase) See expo-nential growth.

lomasome An infolding of the plasmamembrane found particularly in fungal hy-phae and spores, and also in some algaeand higher plants.

lomentum (pl. lomenta) A dry dehiscentfruit formed from a single carpel and bear-ing more than one seed, as in sainfoin(Onobrychis viciifolia). It resembles alegume or siliqua but is divided by falsesepta into single-seeded compartments thatrupture at maturity.

long-day plant (LDP) A plant thatflowers in response to a dark periodshorter than a critical maximum, e.g.spinach (Spinacea oleracea), which flowersin summer, when days are longer andnights shorter. See photoperiodism; criticalday length.

lumen The central space that remains,surrounded by cell walls, in a cell that haslost its living contents (e.g. in xylem el-ements).

lutein The commonest of the xantho-phylls, an orange pigment found in greenleaves and certain algae, e.g. the Rho-dophyta. See photosynthetic pigments.

lyase An enzyme that catalyzes the sepa-ration of two parts of a molecule with theformation of a double bond in one of them.For example, fumarase catalyzes the inter-conversion of malic acid and fumaric acid.The reaction does not involve hydrolysis.

Lycophyta (Lycopodophyta; Lycopsida)A phylum of spore-bearing vascular plantscontaining about 1000 living species and 6genera, mostly found in tropical regions.

Most lycopods belong to the genera Ly-copodium and Selaginella. There are fiveorders, three of which – the Lycopodiales,Selaginellales, and Isoetales – contain bothliving and fossil representatives. The re-maining orders – Lepidodendrales andPleuromeiales – are represented only byfossils. The extinct trees of the genus Lepi-dodendron were once distributed widelyand contributed largely to the coal seamsof the Carboniferous.

Lycopodiales An order of the LYCO-PHYTA containing the homosporous club-mosses or ground pines. They are mainlytropical, some of them epiphytic, but alsooccur in temperate regions, especially inmontane habitats. Lycopodium and re-lated genera comprise the common club-mosses or (North America) ground pines.

Lycopodophyta See Lycophyta.

Lycopsida See Lycophyta.

Lysenko, Trofim Denisovich (1898–1976) Ukrainian agriculturalist. Whileworking at the Kiev Agricultural Institute,Lysenko claimed to have invented VERNAL-IZATION, the cold treatment of grain to pro-mote next-season flowering in species thatwould normally take two years to flower.However, this had long been a commonpractice in agriculture. Lysenko went fur-ther by stating that vernalized plants couldpass this ‘acquired characteristic’ to theiroffspring, so that no further treatment wasnecessary – a return to the discredited ideasof Lamarck. In the 1930s he achieved greatinfluence in the Soviet Union under Stalinand used this to enforce his unorthodoxideas. In particular, he rejected the chro-mosome theory of inheritance, acting todiscredit those scientists who followed thisview and even changing school textbooksand courses to suit his own beliefs. He fi-nally fell from power in 1964.

lysine A basic amino acid synthesizedfrom aspartic and pyruvic acids, and bro-ken down through acetyl CoA and the

log phase

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Krebs cycle. Lysine is a precursor of certainalkaloids. See amino acids.

lysis (degeneration) The death and sub-sequent breakdown of a cell. See also au-tolysis; lysogeny.

lysogeny 1. The formation of an inter-cellular space in plants by dissolution ofcells. Compare schizogeny.2. A stable phage–bacteria relationship inwhich lysing of the bacteria does not occur.The phage (known as a temperate phage)penetrates the host cell and its nucleic acidbecomes integrated into the bacterialDNA. In this state the phage is termed aprophage; most of the viral genes arerepressed and both bacteria and phagereproduce together, producing infected

daughter cells. In a process called induc-tion, certain environmental factors cancause the phage to leave the host DNA andresume the lytic cycle.

lysosome An organelle of plant and an-imal cells that contains a range of digestiveenzymes whose destructive potential neces-sitates their separation from the rest of thecytoplasm. Lysosomes are bounded by asingle membrane and have homogeneouscontents that often appear uniformly graywith the electron microscope. They areusually spherical and about 0.5 m m in di-ameter, although lysosomal compartmentsmay range from small Golgi vesicles tolarge plant vacuoles. Lysosomes may beformed directly from endoplasmic reticu-lum or by budding off of Golgi vesicles

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macromolecule A very large molecule,usually a polymer, having a very high mol-ecular weight (10 000 or more atoms).Proteins and nucleic acids are examples.

macronutrient A nutrient required inmore than trace amounts by an organism.It may be an organic or inorganic com-pound. See essential element. Compare mi-cronutrient.

macrosclereids Elongated rod-shapedSCLEREIDS that form a close outer protectivelayer in the seed testas and fruit walls ofsome plants.

magnesium An element essential forplant and animal growth. It is contained inthe chlorophyll molecule and is thus essen-tial for photosynthesis. It is an importantcomponent of membranes, and an essentialcofactor for certain phosphate-transferringenzymes, e.g. phosphohydrolase and phos-photransferase. High concentrations ofmagnesium ions, Mg2+, are needed to main-tain ribosome structure. Deficiency ofmagnesium may result in CHLOROSIS,, NE-CROSIS, stunted growth, and in somespecies, whitening and puckering of theleaf edges. It may be remedied by addingmagnesium sulfate or magnesium oxide tothe soils.

Magnoliaceae A family of primitive di-cotyledons that includes Magnolia and thetulip trees (Liriodendron spp.). They aremostly trees and shrubs of warm temperateto tropical parts of east Asia and tropicalNorth America.

maidenhair ferns See Adiantaceae.

male sterility A condition in which

pollen production is prevented by the mu-tation of one or more of the genes involved.Male sterility is used by plant breeders toensure cross-pollination takes place. It isalso used in genetic engineering to ensurethat genetically modified varieties do notrelease pollen containing modified genes.

malic acid (2-hydroxybutanedioic acid)A colorless crystalline 4-carbon carboxylicacid, which occurs in acid fruits such asgrapes and gooseberries. In biologicalprocesses malate ion is an important inter-mediate in the KREBS CYCLE.

Malpighi, Marcello (1628–1694) Ital-ian histologist. Often described as thefather of microscopy, Malpighi, a philoso-pher and professor of medicine, initiatedthe detailed study of plant and animal tis-sues. He is best known for his investiga-tions into mammalian blood and alsostudied many body organs, including theskin. He described the stomata on theabaxial surfaces of leaves, the annual ringsof woody plants, and also made a distinc-tion between monocotyledonous and di-cotyledonous plants.

maltose A sugar found in germinatingcereal seeds. It is a disaccharide composedof two glucose units linked by an a-1,4–GLYCOSIDIC BOND. Maltose is an im-portant intermediate in the enzymehydrolysis of starch. It is further hy-drolyzed to glucose.

manganese A metallic MICRONUTRIENT

needed by plants for growth. Manganeseions are cofactors in various enzymes, e.g.kinases, IAA oxidase, and certain enzymesinvolved in the biosynthetic pathway forchlorophyll. Manganese is required for the

M

light reactions of Photosystem II (see pho-tosynthesis). Deficiency of manganese maycause dwarfing, mottling of the upperleaves, CHLOROSIS between the leaf veins,‘blight’ of sugar cane, and ‘gray speck’ ofoats.

mangrove Any of a range of trees andshrubs that form forests and dense thicketsalong muddy coasts and estuaries and insalt marshes, some of which have proproots. Adventitious roots arch downwardto the mud and send up new trunks. Theprop roots trap sediment, building up the land and aiding SUCCESSION. Manymangrove species have PNEUMATOPHORES,or ‘breathing knees’, that rise up out of themud. They have abundant lenticels,through which air diffuses to supply thewaterlogged roots below the mud.

mannitol A soluble sugar alcohol (car-bohydrate) found widely in plants andforming a characteristic food reserve of thePhaeophyta (brown algae). It is also themain soluble sugar in fungi and lichens.Mannitol is a hexahydric alcohol, i.e. eachof the six carbon atoms has an alcohol (hy-droxyl) group attached. It may be derivedfrom mannose or fructose.

mannose A simple sugar found in manypolysaccharides. It is an aldohexose, iso-meric with glucose. In some plants, such asmembers of the family Fabaceae (legumes).It is the main unit of storage polysaccha-rides, forming polymers called mannans. Itis also a component of some hemicellu-loses. Reduction of mannose yields manni-tol, a major sugar in fungi, lichens, andsome algae.

mantle See mycorrhiza.

maquis A kind of Mediterranean scrubfound on poor soils in regions with a pro-nounced dry season. It is made up of thick-ets of evergreen sclerophyllous (havingthick leathery leaves) or spiny bushes andshrubs up to about 3 meters (9 ft) tall, e.g.gorse (Ulex), broom (Cytisus, Genista),laurels (Laurus), and members of the Eri-

caceae (heath family), interspersed witharomatic herbs and smaller shrubs, such asthyme (Thymus), Cistus, and myrtle (Myr-tus), and scattered small trees such as olive(Olea europaea) and figs (Ficus). Similarvegetation in California is called CHAPAR-RAL.

Marattiaceae (giant ferns) A family oflarge tropical EUSPORANGIATE ferns, some-times called the giant ferns, that dates backto the Carboniferous period. They havestout erect stems and large compoundfronds, in some species, e.g. Angiopterisevecta, up to 4.5 m (15 ft) long.

Marchantiales An order of thallose liv-erworts with flat, ribbonlike, dichoto-mously branching thalli that lie close to theground. Marchantia, a common species ofgardens, riverbanks, and wet moorland,has umbrellalike, separate male and femalereproductive structures on long uprightstalks.

marker gene A gene of known locationand function that can is used to establishthe relative positions and functions ofother genes. During gene transfer, amarker gene may be linked to the trans-ferred gene to determine whether or not thetransfer has been successful. See chromo-some map; genetic engineering.

marsh See swamp.

Marsileaceae A family of highly spe-cialized heterosporous water ferns. Theleaves are borne on creeping rhizomes, andmay be cylindrical and pointed, as in Pilu-laria, or divided into two or four leaflets, asin Marsilea and Regnelidium.

mass flow A hypothesis put forward byMünch (1930) to explain the mechanismof phloem transport. The movement ofsubstances is believed to be the result ofchanges in osmotic pressure. Thus in an ac-tively photosynthesizing region (source)where sugars are being produced the os-motic pressure is high and water is takenin. Conversely, in regions (sinks) where

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photosynthetic products are being used upor converted to storage compounds there isa lowering of osmotic pressure and water islost. A system is then set up in which thereis mass flow of water and dissolved sugarsfrom source to sink.

medulla 1. See pith.2. In fungal fruiting bodies, the layer oflongitudinal hyphae.3. In lichens, the layer of loosely organizedhyphae lying below the cortex and algallayer.4. In multicellular algal thalli, the inner lay-ers of the thallus, comprising nonpig-mented cells that often form a storagetissue.

medullary ray A linear group of livingparenchyma cells in the secondary xylemof a woody plant. They extend from thevascular cambium for various distancesinto the wood.

megaphyll (macrophyll) A foliage LEAF

with a branched system of veins in theblade and with the leaf trace leaving a leafgap in the stele. It is typical of ferns andseed-bearing plants. The large often pin-nately divided megaphyll of ferns, oftentermed a frond, contrasts with the gener-ally much smaller leaves (MICROPHYLLS) ofclubmosses and horsetails. The scale- orneedlelike leaves of conifers have charac-teristics of both types since they have a sin-gle vein like a microphyll but leave a leaftrace like a megaphyll.

megasporangium (pl. megasporangia) Asporangium that produces megaspores. InSelaginella the megasporangium is borne inthe axis of a sporophyll located in a stro-bilus. Usually all the spore mother cells de-generate except one, which forms a tetradof cells. One or more of these may developinto MEGASPORES, which, when shed, de-velop into the female gametophyte. Theovule of seed plants may be consideredequivalent to the megasporangium of the pteridophytes. Compare microsporan-gium. See heterospory.

megaspore The larger of the two typesof spores in heterosporous pteridophytesand seed plants that produces the femalegametophyte. In some pteridophytes themegaspores are released from the sporo-phyte, in others they are retained and fer-tilized on the sporophyte; in the seed plantsthey are retained. The megaspores of pteri-dophytes and gymnosperms produce a fe-male gametophyte called a PROTHALLUS

that forms two or more archegonia eachcontaining a haploid female gamete. In an-giosperms the megaspore becomes the em-bryo sac, lacking an obvious prothallus butcontaining, most commonly, eight nuclei,one of which is organized as the female ga-mete. Other angiosperms have four or six-teen or more nuclei in the embryo sac.Compare microspore.

megasporophyll A leaf, modified leaf,or leaflike structure that bears the mega-sporangium. Simple megasporophylls in-clude the fertile ligulate leaves of hetero-sporous lycopods (e.g. Selaginella). Theyare usually grouped in a STROBILUS thatmay also contain MICROSPOROPHYLLS orvegetative leaves. The carpel of angio-sperms and the ovuliferous scale of gym-nosperms are modified megasporophylls.

meiosis The process of cell divisionleading to the production of daughter nu-clei with half the genetic complement of theparent cell. Cells formed by meiosis giverise to gametes and fertilization restoresthe correct chromosome complement.

Meiosis consists of two divisions duringwhich the chromosomes replicate onlyonce. Like mitosis the stages PROPHASE,METAPHASE, and ANAPHASE can be recog-nized. However during prophase homolo-gous chromosomes attract each other andbecome paired forming bivalents. At theend of prophase genetic material may beexchanged between the chromatids of ho-mologous chromosomes. Meiosis also dif-fers from mitosis in that after anaphase,instead of nuclear membranes forming,there is a second division, which may be di-vided into metaphase II and anaphase II.The second division ends with the forma-

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tion of four haploid nuclei, which developinto gametes. Compare mitosis. See alter-nation of generations; telophase.

membrane A structure consisting mainlyof lipid and protein (lipoprotein) surround-ing all living cells as the plasma membrane,or plasmalemma, and also found sur-rounding organelles within cells. Mem-branes function as selectively permeablebarriers, controlling passage of substancesbetween the cell and its organelles, andtheir internal environment, either activelyor passively. Membranes are typically7.5–10 nm in thickness with two regularlayers of lipid molecules (a bilayer) con-taining various types of protein molecules.Some proteins penetrate through the mem-brane, while others are associated with oneside; some float freely over the surfacewhile others remain stationary. Some areenzyme controlling, for instance, in the ac-tive transport of molecules or ions throughthe membrane. Larger molecules or parti-cles can pass from one side of a membraneby ENDOCYTOSIS or EXOCYTOSIS.

The lipids are mostly phospholipids.These are polar molecules: one end (thephosphate end) is hydrophilic (water-loving) and faces outward, while the otherend (consisting of two fatty acid tails) ishydrophobic (water-hating) and faces in-ward. Short chains of sugars may be asso-ciated with the proteins or lipids formingglycoproteins and glycolipids. The particu-lar types of carbohydrates, lipids, and pro-teins determine the characteristics of the

membrane, affecting, for example, cell–cellrecognition (as in embryonic developmentand immune mechanisms), permeability,and hormone recognition. Membranesmay contain efficient arrangements of mol-ecules involved in certain metabolicprocesses, e.g. electron transport and phos-phorylation (ATP production) in mito-chondria and chloroplasts. See osmosis;freeze fracturing; plasma membrane.

membrane potential The potential dif-ference that exists across a membrane as aresult of the action of PROTON PUMPS, whichmaintain an imbalance of positive and neg-ative ions on either side of the membrane.Such pumps are usually coupled to ATP:energy is required to maintain the potentialdifference.

Mendel, Gregor Johann (1822–84)Austrian plant geneticist. Having devel-oped an interest in horticulture as a childgrowing up on a peasant farm, Mendel wasable to carry out many plant breeding ex-periments after entering an Augustinianmonastery at Brünn (now Brno in theCzech Republic). He used the pea plant inhis experiments and selected seven charac-teristics of the plant to observe. First, heisolated his selected pure-bred parentplants and carried out his crosses by trans-ferring pollen from one to another. He thencollected and counted the seeds and grewthem on, patiently observing and carefullyrecording the characteristics and their fre-quency in the next and often in successive

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Meiosis: the differences in chromosome behavior in meiosis and mitosis

generations. Mendel concluded from hisresults that each character is controlled bytwo ‘factors’ in the somatic cells, but eachgamete can pass on only one of these fac-tors to the next generation. He also discov-ered that the factors were distributed atrandom in the gametes and that each pairof factors segregates independently of oneanother (see Mendelism; Mendel’s laws).Mendel’s ‘factors’ of inheritance are, infact, genes and he is recognized as being thefirst person to provide genetics with amathematical basis.

Mendel published his results in a localscientific journal and his paper was widelycirculated, but unfortunately ignored.When Mendel became abbot of themonastery, he had to abandon his scientificstudies. It was not until 1900, some 16years after his death, that his work was re-discovered by DE VRIES and others and itsaccuracy and ingenuity appreciated.

Mendelian ratio The ratio of contrast-ing alleles present in the offspring of a crossinvolving genes that behave according toMENDEL’S LAWS.

Mendelism The theory of inheritanceaccording to which characteristics are de-termined by particulate ‘factors’, or genes,that are transmitted by the germ cells. It isthe basis of classical genetics, and isfounded on the work of MENDEL in the1860s. See Mendel’s laws.

Mendel’s laws Two laws formulated byMENDEL to explain the pattern of inheri-tance he observed in plant crosses. The firstlaw, the Law of Segregation, states that anycharacter exists as two factors, both ofwhich are found in the somatic cells butonly one of which is passed on to any onegamete. The second law, the Law of Inde-pendent Assortment, states that the distrib-ution of such factors to the gametes israndom; if a number of pairs of factors isconsidered, each pair segregates indepen-dently.

Today Mendel’s ‘characters’ are termedgenes and their different forms (factors) arecalled alleles. It is known that a diploid cell

contains two alleles of a given gene, each ofwhich is located on one of a pair of ho-mologous chromosomes. Only one ho-molog of each pair is passed on to agamete. Thus the Law of Segregation stillholds true. Mendel envisaged his factors asdiscrete particles but it is now known thatthey are grouped together on chromo-somes. The Law of Independent Assort-ment therefore applies only to unlinkedgenes. See linkage.

meristem A distinct region of actively orpotentially actively dividing cells primarilyconcerned with growth. Numerous meris-tems occur in plants. In active meristemsseparation occurs between the cell that re-mains meristematic (initial) and the cell ul-timately being differentiated. Two basicmeristematic groups are the primary apicalmeristems at root and shoot apices, and thesecondary lateral meristems, which includevascular and cork cambia. See also inter-calary meristem; secondary growth.

meristoderm (limiting layer) The outer-most cellular layer of the thallus of certainPhaeophyta (brown algae). It consists ofsmall densely packed rectangular cells con-taining brown pigmented plastids and cov-ered by a mucilaginous layer to preventdesiccation. The meristoderm maintains itsmeristematic activity and assists the outercortical layers in adding to the thickness ofthe thallus and replacing tissues wornaway by tidal action.

mesarch Denoting a stele or part of astele in which the protoxylem is sur-rounded by metaxylem. Compare cen-trarch; endarch; exarch.

mesocarp See pericarp.

mesophilic Designating microorganismswith an optimum temperature for growthbetween 25–45° C. Compare psychro-philic; thermophilic.

mesophyll Specialized tissue located be-tween the epidermal layers of the leaf andcomposed mainly of photosynthetic cells

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(CHLORENCHYMA). Veins, supported bysclerenchyma and collenchyma, are em-bedded in the mesophyll. Palisade meso-phyll consists of cylindrical cells, at rightangles to the upper epidermis, with manychloroplasts and small intercellular spaces.It is the main photosynthesizing layer in theplant. Spongy mesophyll, adjacent to thelower epidermis, comprises interconnect-ing irregularly shaped cells with fewchloroplasts and large intercellular spacesthat communicate with the atmospherethrough stomata allowing gas exchangebetween the cells and the atmosphere. Thedistribution of mesophyll tissue varies indifferent leaves depending on the environ-ment in which the plant lives.

mesophyte A plant that is adapted togrow under adequate conditions of watersupply and has no particular adaptationsto withstand environmental extremes. Indrought conditions wilting is soon ap-parent as the plants have no specialmechanisms to conserve water. Most an-giosperms are mesophytes. Compare hy-drophyte; xerophyte.

Mesozoic The middle era in the most re-cent (Phanerozoic) eon of the geologicaltime scale, dating from about 230–66 mil-lion years ago. It is divided into three mainperiods: the Triassic, Jurassic, and Creta-ceous. During the Triassic period seed-ferns predominated, but as the climatewarmed the cycads came to dominate, andthe Mesozoic is sometimes called ‘The Ageof Cycads’. In the Jurassic period, tropicaland temperate regions were dominated byforests of palmlike cycads and conifers,and the dawn redwoods made their firstappearance. Ginkgoes were also wide-spread. The first angiosperms arose in theCretaceous period, and rapidly diversified,becoming the dominant vegetation by theend of the Mesozoic as the climate cooledand dried. See also geological time scale

messenger RNA (mRNA) The form ofRNA that transfers the information neces-sary for protein synthesis from the DNA inthe nucleus to the ribosomes in the cyto-

plasm. One strand of the double helix of DNA acts as a template along whichcomplementary RNA nucleotides becomealigned and joined together by the enzymemRNA polymerase. These form a polynu-cleotide identical to the other DNA strand,except that the thymine bases are replacedby uracil. This polynucleotide is called het-erogeneous nuclear RNA (hnRNA) and itcontains both coding and noncoding se-quences (see exon; intron). In the nucleolusPOST-TRANSCRIPTIONAL PROCESSING occurs:adenine polynucleotide (ply A) is added toone end and a guanine derivative to theother, and (in eukaryotic cells) the intronsare then removed to produce mRNA(sometimes called mature mRNA). Thewhole process is termed transcription. Thenew mRNA molecule thus has a copy ofthe genetic code, which directs the forma-tion of proteins in the ribosomes. Comparetransfer RNA.

metabolism The chemical reactionsthat take place in cells. The molecules tak-ing part in these reactions are termedmetabolites. It is metabolic reactions, par-ticularly those producing energy, that keepcells alive. Metabolic reactions characteris-tically occur in small steps, comprising ametabolic pathway. Metabolic reactionsinvolve the breaking down of molecules to provide energy (CATABOLISM) and thebuilding up of more complex moleculesand structures from simpler molecules (AN-ABOLISM).

metabolite See metabolism.

metaphase The stage in mitosis andmeiosis when the chromosomes becomealigned along the equator of the nuclearspindle. Metaphase follows prophase andprecedes anaphase.

metaphloem The primary PHLOEM formedfrom the PROCAMBIUM after the PRO-TOPHLOEM. It is found behind the zone ofelongation below the meristem, and ismore durable than the protophloem. Inplants showing no secondary thickeningthe metaphloem is responsible for the

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transport of most of the organic materialsin the plant, but this function is taken overby the secondary phloem in regions wheresecondary tissues have differentiated.

metaxylem The primary XYLEM el-ements that are differentiated from thePROCAMBIUM after the PROTOXYLEM. Theyare found some distance behind the apicalmeristem beyond the zone of elongation.The secondary cell walls show reticulateand scalariform thickening and are thus in-extensible.

methionine A nonpolar sulfur-contain-ing AMINO ACID synthesized from aspartate.Methionine can be broken down to suc-cinyl CoA, which is further oxidized in theKrebs cycle. The ATP-activated derivativeof methionine, S-adenosyl methionine, do-nates its methyl group to many differentmolecules in various metabolic pathways.

methylene blue A blue dye used in lightmicroscopy to stain bacterial protoplasm.It is also used to stain nuclei blue. See stain-ing.

microbiology The study of microscopicorganisms (e.g. bacteria and viruses), in-cluding their interactions with other organ-isms and with the environment.

microbody A common organelle ofplant and animal cells, bounded by a singlemembrane, spherical, and usually about0.0–1.0 m m in diameter. Microbodiesoriginate from the ENDOPLASMIC RETICULUM

and contain oxidative enzymes such ascatalase, which decomposes hydrogen per-oxide, a toxic waste product of the activi-ties of other enzymes in the microbody.

GLYOXYSOMES contain enzymes of theglyoxylate cycle, transaminases, and en-zymes associated with b-oxidation of fattyacids. They play a major role in conversionof lipids to sucrose in fatty or oily seedlingtissues, e.g. the endosperm of castor-oilseeds. PEROXISOMES occur in certain animaltissues, and are numerous in photosyn-thetic cells of plant leaves, where they areconcerned with glycolate metabolism in

photorespiration and contain high levels ofglycolate oxidase and other associated en-zymes. Glycolate comes from chloroplastsand products such as glycine are passed tomitochondria. Hence these three organellesoften appear close together. See photores-piration.

microevolution EVOLUTION on a scalewithin a species, which results from the ac-tion of natural selection on the genetic vari-ation between individuals of a givenpopulation.

microfibril A ribbonlike structure atleast 60 nm long and about 45 nm in di-ameter, made of parallel chains of cellulosemolecules linked by hydrogen bonds. Mi-crofibrils are found in the CELL WALLS ofplants and some algae. In secondary cellwalls they are packed more closely, leavingless room for water between them.

microfilament A minute filament, about0.4–0.7 nm wide, found in eukaryotic cellsand having roles in cell motion and shape.Microfilaments are made of two helicallytwisted strands of globular subunits of theprotein ACTIN, almost identical to the actinof muscle. They can undergo rapid exten-sion or shortening by subunit assembly ordisassembly, or form complex three-dimensional networks. Their occasionalassociation with myosinlike protein (as inmuscle) suggests they may also be contrac-tile. They often occur in sheets or bundlesjust below the plasma membrane and at theinterface of moving and stationary cyto-plasm. They are involved in CYTOKINESIS,CYCLOSIS, cell movements, e.g. ameboidmovement, and movement of subcellularcomponents, e.g. pinocytotic vesicles.

micrograph A photograph taken withthe aid of a microscope. Photomicrographsand electron micrographs are producedusing optical microscopes and electron mi-croscopes respectively.

micrometer1 In microscopy, a devicefor measuring the size of an object underthe microscope. An eyepiece micrometer

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(graticule) of glass or transparent film,with a scale etched or printed on it, isplaced in the eyepiece so that both the ob-ject to be measured and the scale are infocus. The scale of the graticule changes atdifferent magnifications and it must there-fore be calibrated against a stage microme-ter, which is a glass slide with a scaleetched into it, and placed on the micro-scope stage.

micrometer2 (micron) Symbol: m m Aunit of length equal to 10–6 meter (one mil-lionth of a meter). It is often used in mea-surements of cell diameter, sizes ofbacteria, etc. Formerly, it was called themicron.

micron See micrometer.

micronutrient (trace element) A nutri-ent required in trace amounts by an organ-ism. For example, a plant can obtainsufficient of the essential trace elementmanganese from a solution containing 0.5parts per million of manganese. Many actas enzyme cofactors or are components ofpigments. They include boron, cobalt, cop-per, manganese, molybdenum, and zinc.See deficiency disease.

microphyll A foliage leaf that has a sin-gle unbranched vein running from base toapex and no leaf gap associated with theleaf trace: the stele remains entire when aleaf trace branches off to a leaf. Micro-phylls are the typical leaves of clubmossesand horsetails. Compare megaphyll.

micropropagation The production oflarge numbers of plants under laboratoryconditions. It usually involves the use ofTISSUE CULTURE to produce large numbersof genetically identical plants (clones), butcan involve seeds, e.g. of orchids. It is usedto produce disease-free plants, and also tomultiply genetically engineered plants, andto increase rare plant populations for con-servation purposes.

micropyle A pore leading to the nucel-lus formed by incomplete integument

growth around the apex of the ovule.Pollen tubes usually pass through it priorto fertilization. In most seeds the micropyleforms a small hole in the testa throughwhich water is absorbed, but in some seedsit is closed.

microscope An instrument designed tomagnify objects and thus increase the reso-lution with which one can view them. Res-olution is the ability to distinguish betweentwo separate adjacent objects. Radiation(light or electrons) is focused eitherthrough the specimen by a condenser lensor onto it. The resulting image is magnifiedby further lenses. Since radiation must passthrough the specimen in transmission mi-croscopy, it is usual to cut larger specimensinto thin slices of material (sections) with amicrotome. Biological material has littlecontrast and is therefore often stained. Ifvery thin sections are required the materialis preserved and embedded in a supportingmedium.

The light microscope (optical micro-scope) uses light as a source of radiation.With a compound microscope the image ismagnified by two lenses, an objective lensnear the specimen, and an eyepiece, wherethe image is viewed, at the opposite end ofa tube. Its maximum magnification is lim-ited by the wavelength of light: it can dis-tinguish between two points only 0.3 m mapart. Its main use is the observation ofthin sections of tissues and organs, and thestudy of fine detail of microorganisms. Forviewing relatively large specimens and fordissection, the lower resolution stereo-scopic binocular microscope is used: it usesincident light and has two eyepieces, givinga three-dimensional image. Various modi-fications of the light source and back-ground illumination allow different aspectsof cell structure to be observed, e.g. bright-field illumination, DARK-GROUND ILLUMI-NATION, PHASE CONTRAST MICROSCOPY,interference microscopy, and CONFOCAL

MICROSCOPY. Much greater resolution be-came possible with the introduction of theelectron microscope, which uses electronsas a source of radiation, because electronshave much shorter wavelengths than light.

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However, only dead material can be ob-served because the specimen must be in avacuum and electrons eventually heat anddestroy the material. The electron micro-scope uses electromagnetic lenses to focusa parallel beam of electrons onto an objectand produce an image of the resulting elec-tron scattering projected onto a photo-graphic plate or fluorescent screen. Theprocess takes place in a vacuum to mini-mize unwanted electron scattering. Sincethe wavelength of an electron beam is0.005 nm, an electron microscope hasabout 300 times greater resolution than alight microscope. In theory, this micro-scope can distinguish between objects0.0025 nm apart, but in practice, resolu-tions of less than about 1 nm are rarely at-tained. Electron microscopes are of twomain types, the transmission electron mi-croscope (TEM) and the scanning electronmicroscope (SEM). The former producesan image by passing electrons through thespecimen. With the scanning microscopeelectrons scan the surfaces of specimensrather as a screen is scanned in a TV tube,allowing surfaces of objects to be seen withgreater depth of field and giving a three-dimensional appearance to the image. Thesurface of entire objects can be scanned,and material to be viewed is often given anelectron-reflecting coating. Scanning mi-croscopes cannot operate at such highmagnifications as transmission micro-scopes, but can still resolve to at least 2 nm.Of growing importance is the scanningprobe microscope (atomic force micro-scope), in which a highly sensitive probemounted on a cantilever is moved to andfro over a surface, and responds to theminute intermolecular distance betweenthe probe and the surface, which causes theprobe to be deflected from the surface. Thedeflection of the probe is measured with alaser beam, providing a topographical mapof the surface without penetrating it. Theprobe may be dragged over the surface ofthe object, tap the object at intervals, ortravel just above the surface. The scanningprobe microscope can achieve resolutionsof less than 1 nm in x-, y-, and z-directions,and can produce images of individual mol-

ecules such as myosin and DNA. In non-touching mode it provides a tool for thestudy of real-time changes in the surfacewithout interfering with processes causingthe changes, e.g. the change in structure ofstarch as it is digested by an enzyme. Mod-ifications of the technique can give furtherinformation; it is used, for example, tomeasure the volume of chromosomes.

microsomes Fragments of endoplasmicreticulum and Golgi apparatus in the formof vesicles formed during homogenizationof cells and isolated by high-speed centrifu-gation. Microsomes from rough endoplas-mic reticulum are coated with ribosomesand can carry out protein synthesis in thetest tube.

microsporangium (pl. microsporangia)In heterosporous plants, the sporangiumthat produces the MICROSPORES, located on the microsporophyll. The microspo-rangium wall splits to disperse the maturemicrospores. Microsporangia are found insome pteridophytes (e.g. Selaginella, waterferns) and are represented by the pollensacs in gymnosperms and angiosperms.Compare megasporangium.

microspore The smaller of the twotypes of spores produced in large numbersby seed plants and heterosporous pterido-phytes. In pteridophytes, microsporesdevelop into the male gametophyte genera-tion, but in gymnosperms and angio-sperms, the microspores (POLLEN grains)develop into a very reduced gametophyterepresented by the pollen tube and the veg-etative nucleus (pollen-tube nucleus) andthe generative nuclei. The latter are themale gametic nuclei. Compare megaspore.

microsporophyll A leaf or modifiedleaf on which the microsporangium isborne. Simple microsporophylls includethe scales found in the male cones of gym-nosperms and the fertile photosyntheticleaves of clubmosses, usually grouped in astrobilus. In gymnosperms the male scale isthe microsporophyll, while in angiosperms

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the stamen is a highly modified micro-sporophyll. Compare megasporophyll.

microtome An instrument for cuttingthin sections (slices a few micrometersthick) of biological material for micro-scopic examination. The specimen is usu-ally embedded in wax for support and cutby a steel knife. Alternatively it is frozenand a freezing microtome, which keeps thespecimen frozen while cutting, is used(cryo-microtomy). For electron micros-copy, extremely thin (20–100 nm) sectionscan be cut by an ultramicrotome. Here thespecimen is embedded in resin or plastic forsupport and mounted in an arm that ad-vances slowly, moving up and down, to-ward a glass or diamond knife. As sectionsare cut they float off on to the surface ofwater contained in a trough behind theknife.

microtubule A thin cylindrical un-branched tube of variable length found ineukaryotic cells, either singly or in groups.Its walls are made of the globular proteintubulin. Microtubules have a skeletal role,helping cells to maintain their shape. Theyoccur freely in the cell, and also form partof the structure of the SPINDLE during celldivision, bringing about chromosomemovement. Microtubules also help to ori-entate materials and structures in the cell,e.g. cellulose fibrils during the formation ofplant cell walls, and may be involved in thetransport of materials with and betweencells, e.g. phloem transport. They are alsopart of the centrioles found in many fungaland algal cells, and the basal bodies andundulipodia of many motile unicells.

middle lamella In mature plant tissues,the material cementing the walls of adja-cent cells. It is derived from the first-formed layer of the primary cell wall,which is laid down by the PHRAGMOPLAST

(cell plate), and is composed mainly of cal-cium and magnesium pectates.

midrib 1. The thick vein running downthe middle of the leaf from the petiole tothe leaf tip of a plant.

2. Any thickened linear structure runningdown the center of an algal thallus, such asa seaweed, e.g. Fucus spp.

mildew A fungal disease of plants inwhich the hyphae appear on the surface ofthe plant.

Mississippian In North America, theLower Carboniferous period, dating from345 to 310 million years ago. See Car-boniferous; Pennsylvanian.

mitochondrial DNA (mt-DNA) TheDNA found in mitochondria. It is a circu-lar molecule not associated with histonesor other proteins and independent of nu-clear DNA. It codes for specific RNA com-ponents of ribosomes, and also for certainrespiratory enzymes that occur in the mito-chondria and are synthesized on mitochon-drial ribosomes. Plant mt-DNA evolvesvery slowly (unlike animal mt-DNA), andcan break apart and recombine. It is usu-ally transmitted from females to their off-spring in the cytoplasm of the ova, butmitochondria do not usually pass into malegametes.

mitochondrion (pl. mitochondria) Anorganelle of all plant and animal cellschiefly associated with aerobic respiration.It is surrounded by two membranes sepa-rated by an intermembrane space; the innermembrane forms fingerlike processescalled cristae, which project into the gel-like matrix. Mitochondria are typicallysausage-shaped, but may assume a varietyof forms, including irregular branchingshapes. The diameter is always about0.5–1.0 m m, and the length averages 2m m. Mitochondria contain the enzymesand cofactors of AEROBIC RESPIRATION andtherefore are most numerous in active cells(up to several thousand per cell). The reac-tions of the KREBS CYCLE take place in thematrix and those of electron transport cou-pled to oxidative phosphorylation (i.e. therespiratory chain) on the inner membrane.Within the membrane the components ofthe respiratory chain are highly organized.The matrix is also involved in amino acid

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metabolism via Krebs cycle acids andtransaminase enzymes, and in fatty acidoxidation. Like chloroplasts, mitochondriacontain their own ribosomes and DNA,which is circular in form, like that of bac-teria, and reproduce by binary fission. It isbelieved that mitochondria and chloroplas-tsmay be the descendants of once indepen-dent organisms that early in evolutioninvaded eukaryotic cells, leading to an ex-treme form of symbiosis. See endosym-biont theory. See also electron-transportchain.

mitosis (karyokinesis) The ordered pro-cess by which the cell nucleus and cyto-plasm divide in two during the division ofbody (i.e. nongermline) cells. The chromo-somes replicate prior to mitosis to formtwo sister chromatids, which are then sep-arated during mitosis in such a way thateach daughter cell inherits a genetic com-plement identical to that of the parent cell.Although mitosis is a continuous process itis divided into four phases; prophase,metaphase, anaphase, and telophase. Telo-phase may be followed by cytokinesis.Compare meiosis; amitosis; endomitosis.

mitosporic fungi See Fungi Anamor-phici.

molecular systematics A branch of bi-ology that compares functionally equiva-lent macromolecules from differentorganisms as a basis for classification. Se-quences of amino acids in proteins (e.g. en-zymes) or of nucleotides in nucleic acids(e.g. ribosomal RNA) are determined usingautomated techniques, and compared sta-tistically using sophisticated computerprograms. Essentially, how closely two or-ganisms are related in evolutionary terms isreflected in the degree of similarity of theirmacromolecules.

molybdenum A MICRONUTRIENT (traceelement) needed for plant growth. It formspart of the enzymes nitrate reductase,which is involved in nitrate reduction, andnitrogenase, the key enzyme in nitrogen

fixation. Deficiency of molybdenum maylead to CHLOROSIS between the leaf veins.

Monera An alternative name for thekingdom BACTERIA (Prokaryotae).

monochasial cyme (monochasium) Seeinflorescence.

Monocotyledonae A class of the flow-ering plants (phylum Anthophyta) derivingtheir name from single cotyledon in the em-bryo (though some dicotyledons have this,too). They are usually herbaceous plantsand most do not show secondary growth.Examples of monocotyledons are thegrasses (Poaceae) and lilies (Liliaceae).

monoculture The growing of a singlespecies or variety of crop over a large area.The economic reasons for this are the abil-ity to grow crops of uniform height for har-vesting, and economies of scale in theharvesting process. The major disadvan-tage is increased susceptibility to pests andpathogens, especially if a single variety isgrown.

monoecious Denoting plants in whichthe male and female reproductive organsare borne on separate structures on thesame individual plant. Monoecious flower-ing plants bear separate unisexual maleand female flowers, e.g. maize and manytemperate trees such as birch, hazel, andoak. Many monoecious plants are wind-pollinated. Compare dioecious; hermaph-rodite.

monohybrid An indiviudal heterozy-gous at one locus (i.e. for a single gene), ob-tained by crossing homozygous parentswith different alleles at a given locus; forexample, Mendel’s cross between tall (TT)and dwarf (tt) garden peas to give a tallmonohybrid (Tt). When a monohybrid isselfed, dominant and recessive phenotypesappear in the offspring in the ratio of 3:1(the monohybrid ratio). Compare dihy-brid.

monoploid See haploid.

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monopodial Describing the system ofbranching in plants in which the main axisof the stem (the monopodium) continues togrow indefinitely by the terminal bud.Monopodial growth is also called indefi-nite or racemose branching and is typicalof the formation of a racemose inflores-cence. Compare sympodial.

monosaccharide A SUGAR that cannotbe hydrolyzed to simpler carbohydrates ofsmaller carbon content. Glucose and fruc-tose are examples. Monosaccharides maybe classified according to the number ofcarbon atoms they contain: 5-carbon pen-toses (e.g. ribose); 6-carbon hexoses (e.g.glucose, fructose). The general formula ofa monosaccharide is (CH2O)n.

monosomy See aneuploidy.

monotrichous Describing BACTERIA thatpossess one flagellum, e.g. Vibrio.

Moraceae A family of trees, shrubs,lianas, and herbs, found mainly in tropicaland subtropical regions. There are about12 000 species. The Moraceae containsmany important food plants, such as ediblefigs (Ficus carica), breadfruit (Artocarpusartilis), jackfruit (A. heterophyllus), andmulberry (Morus).

morel See Pezizales.

morphogenesis The development ofform and structure.

morphology The study of the form oforganisms. The term may be used synony-mously with ‘anatomy’ although generallythe study of external form is termed ‘mor-phology’ while the study of internal struc-tures is termed ‘anatomy’.

mosaic 1. A hybrid organism whosecells differ genetically, although they haveall arisen from a single zygote.2. A pattern of light and dark green patcheson the leaves of a plant, usually due to viralinfection.3. See leaf mosaic.

mosquito ferns See Azollaceae.

mosses In older classifications, a class ofbryophytes containing leafy plants withmulticellular rhizoids. In the FIVE KING-DOMS CLASSIFICATION, mosses comprise thephylum BRYOPHYTA. They differ from liver-worts in their greater differentiation of thegametophyte and also their complex mech-anisms of capsule dehiscence, with no for-mation of elaters in the capsule. Ordersinclude the Bryales (e.g. Funaria, Poly-trichum, Mnium) and the Sphagnales (e.g.Sphagnum).

mucilage Any of various gumlike carbo-hydrates that swell when wet, becomingslimy and jellylike. See gum.

mucopolysaccharide See polysaccha-ride.

Mucorales (pin molds) An order of theZYGOMYCOTA, containing fungi that form adense mycelium bearing sporangia or coni-dia. The tips of the spore-bearing hyphaeoften become pigmented, hence the namepin molds (e.g. Mucor mucedo). Most aresaprobes in soil, dung, and other organicdebris. Many cause spoilage of storedfood. A few are parasites on other fungi,plants, or animals.

multicellular Consisting of many cells.

multifactorial inheritance See contin-uous variation.

multiple allelism The existence of a se-ries of alleles (three or more) for one gene.Only two alleles of the series can be presentin a diploid cell. Dominance relationshipswithin an allelic series are often compli-cated. Multiple alleles are common in theincompatibility systems of plants, such asSinapis, poppy (Papaver), and clover (Tri-folium). The numbers of such alleles areoften considerable, e.g. Brussels sprouts(Brassica oleracea var. bullata) has up to40 incompatibility alleles, or S alleles.

multiple fruit (composite fruit) A type

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of pseudocarpic (‘false’) fruit that incorpo-rates parts of the inflorescence. Examplesare figs (Ficus) and pineapples (Ananus co-mosus).

mutagen Any physical or chemicalagent that induces MUTATION or increasesthe rate of spontaneous mutation. Chemi-cal mutagens include compounds that reactwith nucleotides, e.g. nitrous acid, pro-flavin; base analogues, which are incorpo-rated in place of normal bases during DNAreplication; and acridines, which causebase-pair deletions or additions. Physicalmutagens include short-wave radiationsuch as ultraviolet light, x-rays, and cosmicrays; and ionizing radiation, such as a andb particles.

mutation Mutations are the ultimatesource of all genetic variation. They are in-herited only if they occur in the cells thatgive rise to the gametes; somatic mutationsmay give rise to chimaeras and cancers.GENE MUTATIONS alter only a single gene,resulting in new allelic forms of the geneand hence new variations upon which nat-ural selection can act. Most mutations aredeleterious but are often retained in thepopulation because they also tend to be re-cessive and can thus be carried in the geno-type without affecting the viability of theorganism. The natural rate of mutation islow, and varies with different gene loci.The mutation frequency can be increasedby mutagens. See also chromosome muta-tion; mutagen; polyploid.

mutualism The close relationship be-tween two or more species of organisms inwhich all benefit from the association.There are two types of mutualism: obliga-tory mutualism, in which one cannot sur-vive without the other, for example thealgal/fungal partnership found in lichens;and facultative or nonobligatory mutual-ism, in which one or both species can sur-vive independently, for example, in theassociation between the bacterium Rhizo-bium and members of the family Fabaceae,which leads to the formation of root nod-ules in which nitrogen fixation occurs, the

bacterium can live independently of theplant; in lichens often the alga can live in-dependently of the fungus. In many mutu-alistic associations, the relationship canchange from a parasitism to a mutualismor vice versa. Compare symbiosis; com-mensalism; amensalism.

mycelium (pl. mycelia) A filamentousmass comprising the vegetative body of afungus, each filament being called a hypha.The mycelium often forms a loose mesh asin Mucor, or hyphae may anastomose toform a netlike structure, as in Ascomycotaand Basidiomycota. See hypha.

mycoplasmas (PPLO; pleuropneumonia-like organisms) A group of extremelysmall bacteria that naturally lack a rigidcell wall. They often measure less than 200nm in diameter and their cells are delicateand plastic. Mycoplasmas are implicated incertain plant diseases, such as witchesbroom of alfalfa, maize stunt, and yellowsdiseases. They can kill tissue cultures andcause serious diseases in humans and ani-mals, e.g. pleuropneumonia.

mycorrhiza (pl. mycorrhizae) The asso-ciation between the hyphae of a fungus andthe roots of a higher plant. Two main typesof mycorrhiza exist, ectomycorrhizae (ec-totrophic mycorrhizae), in which the fun-gus forms a mantle around the smallerroots, as in many temperate and borealtrees, and endomycorrhizae (endotrophicmycorrhizae), in which the fungus growsaround and within the cortex cells of theroots, as in many herbaceous plants. Or-chids and heathers have further specializedtypes of mycorrhizae, in which there isoften a mantle and cell penetration. In ec-tomycorrhizae the fungus, which is usuallya member of the Agaricales, benefits by ob-taining carbohydrates and possibly B-group vitamins from the roots. The treesbenefit in that mycorrhizal roots absorbnutrients more efficiently than uninfectedroots, and it is common forestry practice toinsure the appropriate fungus is appliedwhen planting seedling trees. In endomyc-orrhizae the fungus is generally a species of

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Rhizoctonia and again both partners bene-fit nutritionally from the relationship. In a special form of endomycorrhiza, thevesiculo-arbuscular mycorrhiza (arbuscu-lar mycorrhiza), the fungus lives betweenthe cells of the cortex and projections of its hyphae actually penetrate the corticalcells. Sometimes the projections are finelybranched hyphae called arbuscules.

myosin A contractile protein found incells, which is involved in the movement ofcytoplasm, for example in the cytoplasmicstreaming of plant cells, the ameboidmovement of some protoctists and fungus

gametes, and the movement of slime moldplasmodia. Myosin is thought to interactwith the protein ACTIN to bring aboutmovement.

Myrtaceae A family of trees, shrubs andcreepers found in most parts of the tropicsand warm temperate regions, especiallyAustralia. The family includes many im-portant forest trees such as Eucalyptusspecies, especially in Australia, sources oftimber, tannin and oils. Some species pro-duce edible fruits, e.g. guava (Psidium gua-java), or spices, e.g. allspice (Pimentadioica) and cloves (Eugenia caryophyllus).

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NAD (nicotinamide adenine dinucleotide)A pyridine-based nucleotide that acts as acoenzyme to oxido-reductase enzymes inelectron-transfer reactions, such as the ox-idation of citrate to a-ketoglutarate in theKREBS CYCLE. The reduced form is writtenas NADH, and may act as a reducing agentor be reoxidized in the respiratory chaincoupled to ATP formation. Its main impor-tance is as an electron acceptor in oxida-tion reactions.

NADP (nicotinamide adenine dinucleo-tide phosphate) A pyridine-based nu-cleotide that acts as a COENZYME to oxido-reductase enzymes. Unlike NAD, it usuallyacts as an electron donor in enzyme-catalyzed reduction reactions.

Naegeli, Karl Wilhelm von (1817–91)Naegeli was educated at Zurich, Geneva,and Berlin, where he studied philosophybefore resuming his botanical work. Heproduced an accurate account of cell divi-sion (1842) while studying pollen forma-tion and even observed chromosomes individing nuclei, calling them ‘transitorycytoblasts’. His observations disprovedSCHLEIDEN’S theory that new cells wereformed by budding off the nuclei of exist-ing cells. Working on apical growth inplants in 1845, he concluded that therewere distinct regions of cell division (meris-tems). Naegeli also laid the foundations ofcell ultrastructure when he studied starchgrains in cells and formulated his micellartheory. Sadly, his own entrenched views onheredity and evolution, which arose fromhis studies on the hawkweed (Hieracium),led him to reject MENDEL’s very importantwork on pea plants.

nano- Symbol: n A prefix denoting one

thousand-millionth, or 10–9. For example,1 nanometer (nm) = 10–9 meter. See SI units.

nastic movements (nasties) Movementsof plant parts in which direction of move-ment is independent of the direction of thestimulus that induces them. With pho-tonasty the stimulus is light. For instance,at constant temperature Crocus and tulip(Tulipa) flowers open in the light and closein the dark because of slight growth move-ments. Opening is caused by epinasty – i.e.greater growth of the upper surface of aplant organ – and closed by hyponasty –i.e. greater growth of the lower surface of aplant organ. Similarly, at constant light in-tensity crocus and tulip flowers show ther-monasty; i.e. they open in warm air andclose in cool air. Such day–night rhythmsare examples of nyctinasty and may alsooccur in leaves, as in Oxalis. Nongrowthnastic movements also occur and are morerapid. The ‘sensitive plant’ (Mimosa pu-dica) rapidly closes its leaflets upwards andthe petioles droop in response to touch,shock (seismonasty), or injury. This plant,and many other legumes, shows non-growth nyctinastic movements. Movementis the result of osmotic changes in specialswollen groups of cells (pulvini) at thebases of the moving structures. When thestimulus is contact, the movement is hap-tonastic as in the closure movements of in-sectivorous plants. The closure of the twohalves of the Venus fly-trap leaf (Dionaeamuscipula) is a nongrowth haptonasticmovement caused by a loss of turgor in thecells along the midrib following stimula-tion of the sensitive hairs on the leaf. Seealso taxis; tropism.

nasties See nastic movements.

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natural selection The process, whichDARWIN called the ‘struggle for survival’, bywhich organisms less adapted to their envi-ronment tend to perish, and better-adaptedorganisms tend to survive. There has beenmuch debate about the level at which nat-ural selection acts: traditionally it is re-garded as the action of the environment on individual organisms that determineswhich genotypes survive and reproduce,but there are strong arguments for naturalselection acting at the level of the gene (the‘selfish gene’ hypothesis) and some authorsargue that it happens in the long term athigher levels such as population or species.In the end the less successful types will dieout. According to DARWINISM, natural se-lection acting on a varied population re-sults in evolution.

necrosis The death of a cell or group ofcells in a living plant, especially where thedead area is discolored. Necrosis is oftencaused by fungal infections.

necrotrophic Describing a parasitic or-ganism that obtains its nutrients from deadcells and tissues of its host.

nectar A sugar-containing fluid secretedby the nectaries in plants. It is mainly pro-duced in flowers and attracts insects orother animals, who feed on it and may un-intentionally transfer pollen from oneflower to another, thus facilitating cross-pollination. Floral nectar contains variableproportions of different monosaccharideand disaccharide sugars and, frequently,other substances such as amino acids andvolatile compounds as well. Some plants,particularly trees, produce nectar on stemsand leaves. This may be collected by insectssuch as ants that can protect the plant frominsect herbivores.

nectary A patch of glandular epidermalcells usually on the receptacle or the petals,but sometimes in other parts of the flowersor in specialized organs. They produce asugary liquid (nectar) that attracts insects.

negative feedback See feedback loop.

negative staining A method of prepara-tion of material for electron microscopyused for studying three-dimensional andsurface features, notably of viruses, macro-molecules (e.g. enzyme complexes), andthe cristae of mitochondria. A stain is usedthat is not taken up by certain componentsof the specimen. Usually the stain coversthe background and penetrates surface fea-tures of the specimen, but leaves the speci-men itself unstained. For example, nigrosinor Indian ink is used to make bacteria vis-ible. In electron microscopy a similar tech-nique mixes the specimen material withelectron-dense negative stains, such asphosphotungstic acid (PTA) or potassiummolybdate, which is not taken up by pro-teinaceous material in the specimen. Theelectron-transparent proteinaceous mater-ial shows up as dark areas against a lightbackground on the resulting electron mi-crograph. See staining.

neo-Darwinism DARWIN’s theory ofevolution through NATURAL SELECTION,modified and expanded by genetic studiesarising from the work of MENDEL and hissuccessors. This fusion of Darwin’s theoryof natural selection with Mendel’s geneticswas called the neo-Darwinian synthesis inthe 1930s. The inclusion of genetics in evo-lutionary studies answered many questionsthat Darwin’s theory raised but could notadequately explain because of lack ofknowledge at the time it was formulated.Notably, genetics has revealed the sourceof variation on which natural selection op-erates, namely mutations of genes andchromosomes, and provided mathematicalmodels of how alleles fluctuate in naturalpopulations, thereby quantifying the pro-cess of evolution. More recent discoveriesin molecular biology have added to our un-derstanding of the causes of variation andthe nature of evolution at the molecularlevel.

neritic See sublittoral.

neutral theory The theory that mostevolutionary changes are caused by ran-dom GENETIC DRIFT (random changes in thefrequencies of alleles in a population)

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rather than by NATURAL SELECTION. GeneticMUTATIONS arise at random. Many are neu-tral – they do not affect the fitness or sur-vival of the carrier. They therefore survivein the genome and changes in their fre-quencies are due to the breeding systemand chance rather than to natural selec-tion. See gene frequency; variation.

niacin See nicotinic acid.

niche See ecological niche.

nicotinic acid (niacin) A carboxylicacid that is one of the water-soluble B-group of VITAMINS. It also forms part of thealkaloid nicotine, hence its name. It issythesized from various precursors, but es-pecially from tryptophan. As nicotinamide,it is a constituent of two coenzymes, NADand NADP, which operate as hydrogenand electron-transfer agents and play avital role in metabolism. In humans, defi-ciency of nicotinic acid causes pellagra.Diets based on maize, which is particularlypoor in nicotinic acid, tend to lead to an in-crease in pellagra in the population.

Nidulariales The bird’s nest fungi – anorder of the BASIDIOMYCOTA in which thebasidioma is nestlike, usually cup-shaped(e.g. splash cups, Cyathus striatus) or fun-nel-shaped (e.g. white-egg bird’s nest, Cru-cibulum laeve), containing several separatespore cases (the ‘eggs’) that are exposedwhen the covering membrane dies back.The spores are dispersed by rain splash.Bird’s nest fungi may be found on theground, or on wood or dung.

ninhydrin A reagent used to test for thepresence of proteins and amino acids. Acolorless aqueous solution turns blue in thepresence of a-amino acids in solution.When dissolved in an organic solvent it is used as a developer to color amino acidson chromatograms. If a chromatogramtreated with ninhydrin is heated stronglythe amino acids appear as purple spots thatcan be identified by measuring the Rfvalue. Ninhydrin is carcinogenic. See alsopaper chromatography.

nitrification The oxidation of ammoniato nitrite, and/or nitrite to nitrate, carriedout by certain nitrifying bacteria in the soil.The chemosynthetic bacteria Nitrosomo-nas and Nitrobacter carry out the first andsecond stages respectively of this conver-sion. The process is important in the NI-TROGEN CYCLE since many plants assimilatenitrate as their source of nitrogen. Com-pare denitrification.

nitrifying bacteria See nitrification.

nitrogen An essential element found inall amino acids and therefore in all pro-teins, and in various other important organic compounds, e.g. nucleic acids.Gaseous nitrogen forms about 80% of theatmosphere but is unavailable in this formexcept to a few nitrogen-fixing bacteria.Nitrogen is incorporated into plants as thenitrate ion, NO3

– or, especially in acidsoils, the ammonium anion NH4

+, ab-sorbed in solution from the soil by roots.Plants suffering from nitrogen deficiencytend to develop CHLOROSIS and become eti-olated, the effects being seen first in theoldest parts. See also nitrogen cycle.

nitrogenase See dinitrogenase.

nitrogen cycle The circulation of nitro-gen between organisms and the environ-ment. Atmospheric gaseous nitrogen canbe used directly only by certain nitrogen-fixing bacteria (e.g. Clostridium, Nostoc,Rhizobium). They convert nitrogen to am-monia, nitrites, and nitrates, which are re-leased into the soil by excretion and decay.Some are free-living, while others formsymbiotic associations with plants (see ni-trogen fixation). Another method by whichatmospheric nitrogen is fixed is by light-ning, which causes nitrogen and oxygen tocombine. The oxides so produced dissolvein rain to form nitrous and nitric acids; inthe soil these acids combine with mineralsalts to form nitrites and nitrates. Whenplants and animals die, the organic nitro-gen they contain is converted back into ni-trate in the process termed nitrification.Apart from uptake by plants, nitrate may

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also be lost from the soil by denitrificationand by leaching. The use of nitrogen fertil-izers in agriculture and the emission of ni-trous oxides in car exhaust fumes haveinfluenced the nitrogen cycle and con-tributed to urban air pollution and acidprecipitation. See acid rain.

nitrogen fixation The formation of ni-trogenous compounds from atmosphericnitrogen. In nature this may be achieved byelectric discharge in the atmosphere or bythe activities of certain microorganisms.For example, symbiotic bacteria of thegenus Rhizobium are associated with legu-minous plants (FABACEAE), causing the rootcortex to form root nodules that house thebacteria. These bacteria contain the nitro-genase enzyme that catalyzes the fixationof molecular nitrogen to ammonium ions,which the plant can assimilate. In returnthe legume supplies the bacteria with car-bohydrate.

node The point of leaf insertion on astem. At the apex of the stem the nodes arevery close together but become separatedin older regions of the stem by intercalarygrowth, which forms the internodes. Incertain monocotyledons such as thoseforming bulbs, the nodes are very closelyspaced on a condensed stem.

noncompetitive inhibition A form ofenzyme inhibition in which the inhibitorbinds to the enzyme at a site other than theactive site, altering the conformation of theenzyme and affecting its ability to bindwith the substrate.

noncyclic photophosphorylation Seephotosynthesis.

nondisjunction The failure of homolo-gous chromosomes to move to separatepoles during anaphase I of meiosis, bothhomologs going to a single pole. This re-sults in two of the four gametes formed attelophase missing a chromosome (i.e. beingn – 1). If these fuse with normal haploid (n)gametes then the resulting zygote is mono-somic (i.e. 2n – 1). The other two gametes

formed at telophase have an extra chromo-some (i.e. are n + 1) and give a trisomic zy-gote (i.e. 2n + 1) on fusion with a normalgamete. If two gametes deficient for thesame chromosome fuse then nullisomy (2n– 2) will result, which is almost alwayslethal, and if two gametes with the sameextra chromosomes fuse, tetrasomy (2n +2) results. All these abnormal chromosomeconditions are collectively referred to asaneuploidy.

nonsense triplet A triplet of bases(CODON) in DNA or RNA that does notcode for an amino acid. Some nonsensecodons (stop or termination codons) spec-ify the termination of polypeptide synthesisduring TRANSLATION. See genetic code.

nucellus (pl. nucelli) The parenchyma-tous tissue core of an ovule, enclosing themegaspore or egg cell (or in angiospermsthe EMBRYO SAC). The pollen tube gainsentry to the nucellus through a gap in thesurrounding integuments called the MI-CROPYLE. In some angiosperm species thenucellus persists as the perisperm, provid-ing nourishment for the developing em-bryo.

nuclear membrane (nuclear envelope)See nucleus.

nuclease See endonuclease; exonucle-ase.

nucleic acid hybridization (DNA hy-bridization) The pairing of a single-stranded DNA or RNA molecule withanother such strand, forming a DNA–DNA or RNA–DNA hybrid. In order toachieve hybridization the base sequences ofthe strands must be complementary. Thisphenomenon is exploited in many tech-niques, notably in GENE PROBES, which aredesigned to bind to particular complemen-tary base sequences among a mass of DNAfragments.

nucleic acids Organic acids whose mol-ecules consist of chains of alternating sugarand phosphate units, with nitrogenous

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bases attached to the sugar units. Essen-tially they are polymers of NUCLEOTIDES

formed by condensation reactions betweenthe component nucleotides, which arelinked by phosphodiester bonds. There aretwo forms of nucleic acids, DNA (deoxyri-bonucleic acid) and RNA (ribonucleic acid).In DNA the sugar is deoxyribose; in RNAit is ribose. Nucleic acids occur in the cellsof all organisms, and may be single- ordouble-stranded.

nucleolar organizer See nucleolus.

nucleolus (pl. nucleoli) A more or lessspherical structure found in nuclei of eu-karyote cells, and easily visible with a lightmicroscope, staining densely with basicdyes. There may be one to several per nu-cleus. The nucleolus is the site of manufac-ture of ribosomal subunits and is thus mostconspicuous in cells making large quanti-ties of protein. Nucleoli disappear duringcell division. The nucleolus synthesizes ri-bosomal RNA (rRNA) and is made ofRNA (about 10%) and protein. It formsaround particular loci of one or more chro-mosomes called nucleolar organizers.These loci contain numerous tandem re-peats of the genes coding for ribosomalRNA. TRANSCRIPTION of this code requiresa specific RNA polymerase found only inthe nucleolus.

nucleoplasm The material of the NU-CLEUS, especially the material inside the nu-clear envelope.

nucleoprotein A compound consistingof a protein associated with a nucleic acid.Examples of nucleoproteins are the chro-mosomes, made up of DNA, some RNA,and histones (proteins); and the ribosomes(ribonucleoproteins), consisting of riboso-mal RNA and proteins.

nucleoside A molecule consisting of aPURINE or PYRIMIDINE base linked to asugar, either ribose or deoxyribose. Adeno-

sine, cytidine, guanosine, thymidine, anduridine are common nucleosides.

nucleosome See chromosome.

nucleotide The compound formed bycondensation of a nitrogenous base (aPURINE or PYRIMIDINE) with a sugar (riboseor deoxyribose) and phosphoric acid. ATPis a mononucleotide (consisting of a singlenucleotide), the coenzymes NAD and FADare dinucleotides (consisting of two linkednucleotides), and the nucleic acids arepolynucleotides (consisting of chains ofmany linked nucleotides).

nucleus (pl. nuclei) An organelle of eu-karyote cells containing the genetic infor-mation (DNA) and hence controlling thecell’s activities. It is the largest organelle,typically spherical and bounded by a dou-ble membrane, the nuclear envelope or nu-clear membrane, which is perforated bymany pores (nuclear pores) that allow ex-change of materials with the cytoplasm.The outer nuclear membrane is an exten-sion of the endoplasmic reticulum. In thenondividing (interphase) nucleus the ge-netic material is irregularly dispersed asCHROMATIN; during nuclear division (mito-sis or meiosis) this condenses into denselystaining CHROMOSOMES, and the nuclearenvelope disappears, as do the nucleoli thatare normally present. See nucleolus.

nullisomy See aneuploidy.

nut A dry indehiscent fruit resemblingan achene but derived from more than onecarpel. It has a hard woody pericarp andcharacteristically a single seed. Cupules,supporting the nuts, may be distinctive ofthe species. Examples are the fruits of thebeech (Fagus), hazel (Corylus), oak (Quer-cus), and sweet chestnut (Castanea). Seeachene.

nutation See circumnutation.

nyctinasty (nyctinastic movements) The

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obligate Describing an organism that re-quires specific environmental conditionsfor its survival and cannot adopt an alter-native mode of living. For example, an ob-ligate aerobe is an organism that can growonly under aerobic conditions and cannotsurvive in anaerobic conditions. Comparefacultative.

octadecanoic acid See stearic acid.

offset A short runner arising from an ax-illary bud near the base of a stem andgrowing horizontally above ground awayfrom the parent, seen in daisies (Bellis) andhouseleeks (Sempervivum). It stores nofood but turns up at the end and producesa new plant from the apical bud. Likestolons and suckers, offsets are a means ofvegetative propagation in angiosperms.

oil 1. A TRIACYLGLYCEROL that is liquidat room temperature. The commonest fattyacids in oils are oleic and linolenic acids,which are unsaturated. Storage oils, syn-thesized in the endoplasmic reticulum,make up as much as 60% of the dry weightof certain seeds, such as the castor bean(Ricinus communis), corn (Zea mays), flax(Linum usitatissimum), soybean (Glycinemax), sunflower (Helianthus), safflower(Carthamus tinctorius), and mustards(family Brassicaceae). Oils can be stored inoil bodies (elaiosomes) – oil-secretingstructures that attract ants to disperse theseeds. Oil may also make up part of thefleshy part of a fruit, as in olive, the oilpalm, and coconut palm.2. Essential oils. Any of a number ofvolatile oils secreted by aromatic plantsthat are the source of characteristic odorsor tastes. They are stored as droplets inplant glands. Most essential oils are ter-

penoids or derivatives of benzene. Someflowers, such as the vine Dalechampia (Eu-phorbiaceae) and some orchids, produceoils that are collected by pollinating bees.Some repel insects or grazing animals,while others (allelochemicals) deter en-croaching neighboring plants. Resinousoils that appear at the site of wounds helpto prevent loss of sap and protect againstthe entry of pathogens and parasites.

oil immersion A microscopic techniqueusing special high-powered objectivelenses. A drop of immersion oil (e.g. cedar-wood oil) is placed on the coverslip of a mi-croscope slide and the objective lenscarefully lowered into it. The oil has thesame refractive index as the lens glass andincreases the resolving power obtainableby letting a wider angle of rays enter theobjective lens.

oleic acid An 18-carbon unsaturatedfatty acid occurring as the glyceride in oilsand fats. Oleic acid occurs naturally inlarger quantities than any other fatty acidin plant cells except in chloroplasts, wherelinolenic acid is the dominant fatty acid. Inmany organisms oleic acid can be synthe-sized directly from stearic acid and furtherenzymatic paths exist for conversion tolinoleic acid and linolenic acid. This path-way does not occur in humans and thehigher animals so plant sources are an es-sential dietary element.

Oligocene The epoch of the Tertiary pe-riod 38–25 million years ago. The world’sclimate ranged from temperate to subtrop-ical, and the epoch was marked by an ex-pansion of grasslands at the expense offorests, a change that promoted the evolu-tion of large herbivores. Around the great

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Tethys Sea there were tropical swamps,which gave rise to extensive deposits of lig-nite in Germany and neighboring coun-tries. The angiosperms were evolving anddiversifying rapidly, and overtook thegymnosperms in their abundance. Flowersand insects were evolving together, andcomplex coevolution was taking place aspollination mechanisms diversified. Seealso geological time scale.

oligosaccharide See polysaccharide.

oligotrophic Describing lakes and pondsthat are low in nutrients and consequentlylow in productivity. With little decayingvegetation, the oxygen content of the wateris adequate for fish and other animal life.Compare eutrophic.

one gene–one enzyme hypothesis Thetheory that each gene controls the synthe-sis of one enzyme, which was advanced fol-lowing studies of nutritional mutants offungi. Thus by regulating the production ofenzymes, genes control the biosynthetic re-actions catalyzed by enzymes and ulti-mately the character of the organism.Genes also code for proteins, or polypep-tides that form proteins, other than en-zymes, so the idea is perhaps moreaccurately expressed as the one gene–onepolypeptide hypothesis.

ontogeny The course of development ofan organism from fertilized egg to sexualmaturity and the production of gametesand the next generation. Occasionally on-togeny is used to describe the developmentof an individual structure.

oogamy Sexual reproduction involvingthe fusion of two dissimilar gametes. Themale gamete is usually motile and smallerthan the female gamete, which is usuallynonmotile, contains a food store, and maybe retained by the parent. The term is gen-erally restricted to descriptions of plants,particularly those that produce female ga-metes in oogonia. It is an extreme form ofanisogamy. Compare isogamy. See anisog-amy.

oogonium (pl. oogonia) The female re-productive organ of certain algae andfungi, often distinctly different in shapeand size from the male reproductive organ(the antheridium). This initially unicellulargametangium contains one or more largenonmotile haploid eggs called oospheres.These may be liberated prior to fertiliza-tion, e.g. Fucus, or remain within the oogo-nium, e.g. Pythium.

Oomycete See Oomycota.

Oomycota (Oomycete) A phylum offunguslike protoctists that includes thewater molds, downy mildews, and whiterusts. Most oomycetes live in fresh wateror soil. They are mainly parasites or sapro-phytes, and feed by extending hyphae intothe tissues of their host, releasing digestiveenzymes and absorbing nutrients throughthe hyphal walls. Some are important croppests, such as potato blight (Phytophthorainfestans), damping off fungus (Pythium),mildew of grapes (Plasmopara viticola),and Saprolegnia parasitica, a parasite ofaquarium fish.

oosphere A large nonmotile female ga-mete or egg cell. In lower plants oospheresmay be produced inside an oogonium; theymay be retained inside the oogonium andfertilized there, or released for fertilizationin the water. In angiosperms the oogoniumis enclosed in the EMBRYO SAC and pro-tected by the integuments of the ovule.

oospore A diploid zygote of certain pro-toctists, produced by fertilization of the fe-male gamete (the oosphere), by the malegamete, the antherozoid. The zygote mayform a thick coat and go through a restingperiod before germination. The term‘oospore’ distinguishes a zygote producedoogamously from a zygote produced isog-amously or anisogamously and called a zy-gospore. See zygote.

OP See osmotic pressure.

operator gene A bacterial regulatorygene that contains the genetic code for

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starting TRANSCRIPTION of one or morestructural genes. The structural genes to-gether with the relevant operator genes arelinked together in a functional unit calledan OPERON. The operator is under the con-trol of a regulator gene, which codes for asmall protein called a repressor molecule,which binds to the operator gene and pre-vents it from initiating transcription. Ifproduction of the repressor ceases or if an-other molecule, such as the substrate of anenzyme, binds with it, preventing it frombinding to the operator gene, TRANSCRIP-TION can proceed.

operculum (pl. opercula) 1. The circu-lar lid of the capsule of many mosses. Itcovers the peristome and may be forciblyblown off the capsule by pressure develop-ing in the lower portion of the capsule, orby the swelling of cells of the annulus.2. In the pollen grains of many species, a lidcovering the aperture, which is displacedby the emerging pollen tube.3. A cap covering the ostiole of the perithe-cium in certain ascomycetes.

operon A genetic unit found in prokary-otes and comprising a group of closelylinked genes acting together and coding forthe various enzymes of a particular bio-chemical pathway. They function as a unit,because either all are transcribed or none.At one end is an operator, which containsa site called the PROMOTER. For transcrip-tion of the structural genes (the genes thatactually code for the enzymes) to proceed,mRNA polymerase must first bind with thepromoter. Whether or not this happens iscontrolled by another gene outside theoperon, the regulator gene. The regulatorgene produces a regulator protein thatbinds with the operator, renders it inoper-ative, and so prevents enzyme production.The presence of a suitable substrate pre-vents this binding, and so enzyme produc-tion can commence. An example is the lacoperon in the bacterium Escherichia coli,which is involved in the metabolism of lac-tose. In the absence of an inducer molecule(in this case, the substrate lactose or one ofits derivatives), no transcription occurs.

But if lactose binds to the regulator pro-tein, the regulator can no longer bind to theoperator, so transcription can proceed andenzyme synthesis is initiated. See Jacob–Monod model; repressor.

Ophioglossales (moonworts andadder’s-tongue ferns) An order of euspo-rangiate ferns (see Filicinophyta) charac-terized by fronds that are divided into adistinct sterile green blade and a fertilespike bearing sporangia sunken into itssides. Unlike all other ferns, the fronds arenot rolled up as they push up through theground. They are found in temperate andtropical regions.

opiate An ALKALOID derived from theopium poppy (Papaver somniferum) in alatex that oozes from cut seed capsules.Opiates are highly addictive drugs that re-semble endorphins, chemicals naturallyproduced in the mammalian body that sup-press pain and enhance mood by occupy-ing certain receptor sites on nerve cells thataffect transmission of impulses. Opiatesoccupy the same receptor sites. They arethe source of codeine, morphine, heroin,and other drugs, many of which have medicinal uses in the treatment of severepain.

Orchidaceae The orchid family com-prising monocotyledonous perennial herbs.The family includes terrestrial forms, epi-phytes, and full saprophytes with no greenparts (e.g bird’s-nest orchid (Neottia nidus-avis) and coralroot, (Corallorhiza spp.)). Itis the largest family of flowering plantscontaining over 25,000 species. See illus-tration overleaf.

order A collection of similar families.The Latin names of plant orders generallyend in ales (e.g. Liliales). Orders may be di-vided into suborders. Similar orders consti-tute a class. See International Code ofBotanical Nomenclature.

Ordovician The second oldest period ofthe Paleozoic era, some 510–440 millionyears ago. Algae were abundant in the

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oceans, especially mat-forming algae andalgae in symbiotic relationships in thecorals of the reefs dominating the manyshallow, warm seas of the time. Some algaemay also have been terrestrial, formingmats on wet ground. See also geologicaltime scale.

organ A part of an organism that ismade up of a number of different tissuesspecialized to carry out a particular func-tion. Examples include the stem, leaf,flower, and root.

organ culture The maintenance orgrowth of living organs, usually embry-onic, in vitro. Mature plant organs, no-tably roots, may be cultured indefinitely in suitable media. Leaves, embryos, meris-tems, and ovules have also been success-fully cultured.

organelle A discrete membrane-enclosed subcellular structure with a par-ticular function. The largest organelle is

the nucleus; other examples are CHLORO-PLASTS, MITOCHONDRIA, VACUOLES, and RI-BOSOMES. Organelles allow division oflabor within the cell.

organochlorines Organic compoundscontaining chlorides, used in pesticidessuch as DDT, and in other chemicals usedto control plant pests, which are not read-ily broken down and persist in tissues ofanimals, affecting breeding success andcausing other problems. They are bannedfrom use in most countries, but are stillused in some, especially where insect-bornediseases such as malaria are prevalent.

origin of life The development of livingorganisms from inorganic matter. Geolog-ical evidence strongly suggests that lifeoriginated on earth about 4600 millionyears ago. The earliest evidence of life isbacterialike fossils in rocks over 3500 mil-lion years old. The basic components of or-ganic matter – water, methane, ammonia,and related compounds – were abundant in

organ

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Orchidaceae: front view of an orchid flower

the atmosphere, which had much greaterreducing properties than today’s atmos-phere. Until recently, it was widely ac-cepted that life evolved in warm lagoons orhot springs, where energy from the sun(cosmic rays) and lightning storms causedsimple molecules to recombine into in-creasingly complex organic molecules thateventually showed the characteristics ofliving organisms. However, recent discov-eries suggest that life may have evolveddeep underground in fissures in hot rocks –a habitat in which today vast numbers ofthermophilic bacteria live. Life probablyhad its origins in organic self-replicatingmolecules that consumed the chemicalsaround them to duplicate themselves.These molecules were probably a form ofRNA.

ornithine A nonprotein AMINO ACID thatoccurs in the cell walls of certain bacteria.It is an important intermediate in the syn-thesis of the amino acid arginine, and thesynthesis of many alkaloids.

orthogenesis An early theory of evolu-tion in which evolutionary change was en-visaged as occurring in a definite directionand along a predetermined route, irrespec-tive of natural selection.

orthotropism See tropism.

orthotropous (atropous) See ovule.

osmium tetroxide A stain used in elec-tron microscopy because it contains theheavy metal osmium. It is fat-soluble, andunsatured fats will reduce it to black os-mium dioxide. It therefore stains lipids, in-cluding membranes, particularly intensely.It also acts as a fixative.

osmometer An instrument that is usedto measure OSMOTIC PRESSURE (WATER PO-TENTIAL). In its simplest form, a porous potis made selectively peremeable by impreg-nating its pores with copper ferrocyanide.The pot is filled with an osmotically activesolution, and a piston placed on the solu-tion. The pot is then immersed in a con-

tainer of pure solvent. Solvent entering theporous pot by osmosis lifts the piston untilthe force of the water entering the potequals the hydrostatic pressure created bythe weight of the piston.

osmosis The movement of solvent froma dilute solution to a more concentrated so-lution through a semipermeable membrane(selectively permeable membrane) – onethat allows the passage of some kinds ofmolecule and not others. For example, if aconcentrated sugar solution (in water) isseparated from a dilute sugar solution by aselectively permeable membrane, watermolecules can pass through from the dilutesolution to the concentrated one by diffu-sion.

Osmosis between two solutions willcontinue until they have the same concen-tration. If a certain solution is separatedfrom pure water by a membrane, osmosisalso occurs. The pressure necessary to stopthis osmosis is called the OSMOTIC PRESSURE

(OP) of the solution. The more concen-trated a solution, the higher its osmoticpressure. Osmosis is a very important fea-ture of both plant and animal biology. Cellmembranes act as differentially permeablemembranes and osmosis can occur into orout of the cell. It is necessary for an animalto have a mechanism of osmoregulation tostop the cells bursting or shrinking. In thecase of plants, the cell walls are slightly‘elastic’ – the concentration in the cell canbe higher than that of the surroundings,and osmosis is prevented by the pressureexerted by the cell walls (wall pressure).Where the solvent is water, physiologistsnow describe the tendency for water tomove in and out of cells in terms of WATER

POTENTIAL.

osmotic potential See water potential.

osmotic pressure (OP) The force (pres-sure) that must be applied to a solution toprevent the passage of water or anotherpure solvent through a selectively perme-able membrane separating the solvent fromthe solution (i.e. to prevent OSMOSIS occur-ring). Osmotic pressure rises as the concen-

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tration of the solution rises. The osmoticpressure is equal to the diffusion pressuredeficit plus the turgor pressure. This term isbecoming obsolete, as it ignores capillaryand imbibitional forces that are significantin certain situations. A better concept isthat of WATER POTENTIAL.

Osmundales (royal ferns) An order offerns often large, retaining some primitivefeatures, whose whose sporangia combineeusporangiate and leptosporangiate char-acters (see Filicinophyta).

ostiole A small pore through whichspores or gametes are released from the re-productive bodies of certain fungi andalgae. Examples include the CONCEPTACLES

of wracks (Fucus spp.).

outbreeding Breeding between individ-uals that are not closely related. In plantsthe term is often used to mean cross-fertilization, and various methods exist topromote it, e.g. stamens maturing beforepistils, SELF-INCOMPATIBILITY. The most ex-treme form – crossing between species –usually results in sterile offspring and thereare various mechanisms to discourage it.Outbreeding increases heterozygosity, giv-ing more adaptable and more vigorouspopulations. Compare inbreeding. See het-erostyly; protandry; protogyny.

ovary The swollen base of the CARPEL inthe gynoecium of plants, containing one ormore ovules, each attached by a funiculus.The gynoecium of angiosperms may con-sist of more than one carpel that fuses incertain species forming a complex ovary.Where the fused carpel walls remain intact,this is a multilocular ovary; where theybreak down it becomes a unilocular ovary.If the ovary lies below the other floral or-gans in a flower, it is said to be an inferiorovary, as in Rosa. In such cases, the ovaryusually appears sunk into and fused with acup-shaped receptacle. In a floral formula,an inferior ovary is denoted by a line abovethe carpel (gynoecium) symbol and num-ber. If the petals, sepals, etc. are insertedbelow it, it is a superior ovary, as in but-

tercup (Ranunculus). In a floral formula asuperior ovary is denoted by a line belowthe carpel number. After fertilization, theovary wall becomes the pericarp of thefruit enclosing seeds in its central hollow.

ovate Up to four times as long as broad,tapering at both ends, with the broadestpart below the middle.

ovule Part of the female reproductiveorgans in seed plants. It consists of the nu-cellus, which contains the embryo sac, sur-rounded by the integuments. The ovule issurrounded by a cuticle, and is thought tobe derived from the megasporangium of itsnonseed-bearing ancestors. After fertiliza-tion the ovule develops into the seed. In an-giosperms the ovule is contained within anovary, and attached to the placental tissueof the ovary by a stalk, the funiculus. Itmay be orientated in different ways being

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upright, inverted, or sometimes horizontal.Where the developing ovule has turnedthrough 180°, so that the micropylar endhas folded over and lies close to the base ofthe funiculus, it is termed anatropous. Thisis the most common position of the ovulein flowering plants. Where the ovule hasdeveloped vertically so that the micropylarend is directly over the funiculus, it istermed orthotropous, e.g. Polygonum.Where the funiculus appears to be attachedhalf way between the chalaza and the mi-cropyle, and the micropylar end is turnedthrough 90° relative to the orthotropouscondition so that the ovule is horizontal, itis termed campylotropous, e.g. mallow(Malva sylvestris).

In gymnosperms ovules are larger butare not contained within an ovary. Theyare borne on ovuliferous scales. Gym-nosperm seeds are thus naked while an-giosperm seeds are contained within afruit, which develops from the ovary wall.

ovuliferous scale The megasporophyllfound in the axis of the bract scale in the fe-male strobili of the Coniferophyta. It is alarge woody structure and bears ovulesand later seeds, on its upper surface. It isthus homologous with the carpel of the An-thophyta.

ovum (pl. ova) (egg cell) An unfertilizedegg cell.

oxalic acid (ethanedioic acid) A dicar-boxylic acid, that occurs in various plants,e.g. in the leaves of rhubarb (Rheum),dumbcanes (Dieffenbachia), wood sorrel(Oxalis acetosella), and the garden oxalis(Oxalis spp.). It plays a role in the removalof excess cations, such as sodium, potas-sium, and calcium, which are incorporatedinto oxalates.

oxaloacetic acid (OAA) A water-solu-ble carboxylic acid, structurally related tofumaric acid and maleic acid. Oxaloaceticacid is an intermediate of the KREBS CYCLE,and in the Hatch–Slack pathway of C4PLANTS. It is produced from L-malate in anNAD-requiring reaction and itself is a step

toward the formation of citric acid in a re-action involving pyruvate and coenzyme A,part of a series of reactions that are used toreplace Krebs cycle intermediates removedfor use in biosynthesis. When these reac-tions run in reverse order, they form apathway for glucose synthesis.

oxidase An enzyme that catalyzes a re-action involving the oxidation of a sub-strate using molecular oxygen as electronacceptor, e.g. cytochrome c oxidase in therespiratory ELECTRON-TRANSPORT CHAIN.Many oxidases are flavin-linked dehydro-genases in which the reduced COENZYME

can be reoxidized by molecular oxygen toform water and hydrogen peroxide. See de-hydrogenase; oxidative phosphorylation.

b-oxidation The main pathway for thebreakdown of fatty acids to acetyl CoA. b-oxidation involves the removal of carbonacids from fatty acid chains two at a time,forming one molecule of acetyl CoA eachtime. The acetyl CoA may enter the Krebscycle and be further oxidized, or it mayenter the glyoxylate cycle and be convertedto sugar. b-oxidation is a major energy-liberating process in germinating oil-richseeds, where the associated enzymes arelocated in the glyoxysome together withthose of the glyoxylate cycle.

oxidative phosphorylation The pro-duction of ATP from phosphate and ADPin aerobic respiration. Oxidative phospho-rylation occurs in mitochondria, the energybeing provided by steps in the electron-transport chain.

oxygen An element essential to livingorganisms both as a constituent of carbo-hydrates, fats, proteins, and their deriva-tives, and in aerobic respiration. It entersplants in both carbon dioxide and water,the oxygen from water being released ingaseous form as a byproduct of photosyn-thesis. Plants are the main, if not the only,source of gaseous oxygen and as such areessential in maintaining oxygen levels inthe air for aerobic organisms.

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ozone layer A layer consisting of ozone(O3) molecules scattered through thestratosphere roughly 15–50 km above theearth’s surface. The ozone is formed by theaction of solar radiation on oxygen mol-ecules. The ozone layer absorbs about99% of the harmful ultraviolet radiationentering the earth’s atmosphere, and henceprovides a shield for living organisms with-out which life on earth would be impos-sible. This absorption of ultravioletradiation has two consequences: it warmsthe stratosphere, helping to maintain thetemperature of the Earth’s surface; and it

stabilizes the stratosphere, preventing itfrom mixing with other layers, so reducingwinds in the lower atmosphere.

In the early 1980s it was discoveredthat the ozone layer was becoming thinner,with seasonal ‘holes’ appearing overAntarctica and elsewhere. This depletion isbelieved to be caused by increasing atmos-pheric concentrations of chlorine com-pounds, especially chlorofluorocarbons(CFCs) – very stable chemicals used as re-frigerants and aerosol propellants – whichdisrupt the delicate balance between ozoneproduction and breakdown in the atmos-

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b-oxidation

P680 See photosynthesis.

P700 See photosynthesis.

pachytene In MEIOSIS, the stage in mid-prophase I that is characterized by thecontraction of paired homologous chro-mosomes. At this point each chromosomeconsists of a pair of chromatids and thetwo associated chromosomes are termed atetrad.

pairing See synapsis.

palea See bract.

paleobotany See paleontology.

Paleocene The oldest epoch of the Ter-tiary, 65–55 million years ago. It was a pe-riod of warm humid climate with largeareas of temperate and subtropical forests,but only limited grasslands, as the grasseswere of very recent origin. During the Pa-leocene there was a great deal of coevolu-tion of plants and animals, the plantsevolving adaptations to animal pollinationand fruit and seed dispersal. Bilateral flow-ers evolved, e.g. those of the pea family(Fabaceae) and orchids (Orchidaceae). Inthe latter part of the Paleocene, large an-giosperm fruits and seeds evolved as fruit-and seed-eating mammals also evolved anddiversified. See also geological time scale.

paleoecology The investigation of pre-historic ecology as revealed by studyingfossils and ancient human artefacts, pollen(see palynology) samples, and the mineraldeposits in which such structures occur.

paleontology The study of extinct or-ganisms, including their fossil remains, and

impressions left by them. Sometimes thesubject is divided into paleobotany, thestudy of fossil plants, paleozoology, thestudy of fossil animals, and PALEOECOLOGY.

Paleozoic The first and oldest era inwhich multicellular life became abundant,about 590–230 million years ago. It is di-vided into six main periods: the Cambrian,Ordovician, Silurian, Devonian, Carbon-iferous, and Permian. Beginning withaquatic invertebrates and algae, the eraended with the invasion of land by treeferns and reptiles. See also geological timescale.

palisade mesophyll See mesophyll.

palmella (pl. palmellae) A stage formedunder certain conditions in various unicel-lular algae in which, after division, thedaughter cells remain within the envelopeof the parent cell and are thus rendered im-mobile. The cells may continue dividinggiving a multicellular mass, which is con-tained in a gelatinous matrix. Palmelloidforms may develop undulipodia and revertto normal mobile cells at any time.

palmitic acid (hexadecanoic acid) A 16-carbon saturated fatty acid occurringwidely in fats and oils of animal and veg-etable origin. It is the commonest saturatedfatty acid in plants, and an intermediate inthe synthesis of oleic and stearic acids. Seealso carboxylic acid; oleic acid.

palynology The study of living and fos-sil pollen. It is mainly used as a means ofobtaining information on the compositionand extent of past floras, and may give re-liable quantitative information on the veg-

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etative cover many thousands of years ago.See also pollen analysis.

panicle See inflorescence.

pantothenic acid (vitamin B5) The pre-cursor of coenzyme A, pantothenic acid is essential for several fundamental reac-tions in metabolism. It is one of the water-soluble B group of VITAMINS, and is synthe-sized by plants and bacteria but not by ver-tebrates, so is an essential vitamin in theirdiet.

paper chromatography A chromato-graphic method by which minute amountsof material can be analyzed. An absorbentpaper strip with a drop of test material atthe bottom is dipped into the solvent(which rises up the paper by capillary ac-tion) and removed when the solvent frontalmost reaches the top of the strip.

papilla (pl. papillae) A short sometimescone-shaped projection from a cell, usuallyan extension of the outer wall of an epi-dermal cell. Papillae are often found onpetals, giving them a velvety appearance.See hair.

pappus (pl. pappi) The ring of hairs,scales, or teeth that makes up the calyx inflowers of the ASTERACEAE. It persists on thefruit and serves to aid the wind dispersal ofseeds. An example is the cypsela of the dan-delion (Taraxacum) in which the pappusremains attached by a long thin stalk andacts as a parachute.

parallel evolution (parallelism) The de-velopment of similar features in closelyrelated organisms as a result of strong se-lection in the same direction. This mayoccur between species of the same genusthat are widely separated geographicallybut which live in similar environments.There are few examples of this phenome-non and some authorities deny its exis-tence. Compare convergent evolution.

paramylum A starchlike polysaccharidecomposed of b(1-3)-linked units of glu-

cose, which is the assimilation product ofeuglenas (Eustigmatophyta).

paraphysis (pl. paraphyses) A sterileunbranched (usually) multicellular hairfound in large numbers between the re-productive organs of certain algae andbryophytes. Club-shaped hyphae in the hy-menial layer of certain basidiomycete fungiare also termed paraphyses.

parasitism An association between twoorganisms in which one, the parasite, ben-efits at the expense of the other, the host.The tolerance of the host varies from beingalmost unaffected to serious illness andoften death. An obligate parasite such aspotato blight (Phytophthora infestans) canlive only in association with a host,whereas a facultative parasite such as thedamping-off fungus (Pythium) can exist inother ways, for example as a saprophyte.

paratonic movements Movements ofplants or plant parts in response to externalstimuli. They may be divided into mechan-ical movements (e.g. the hygroscopicmovements of dead cells) and movementscaused by the stimulation of sensitive cells(e.g. tropisms and nastic movements).Taxes are also paratonic movements.Compare autonomic movements.

parenchyma Tissue made up of livingthin-walled cells that are not differentiatedfor any specific function, but in which im-portant metabolic processes are carriedout. The leaf mesophyll and the stemmedulla and cortex consist of parenchyma.The vascular tissue is also interspersedwith parenchyma; for instance themedullary rays of secondary vascular tis-sue. Parenchyma often forms a basicground tissue in which other tissues areembedded. It is generally considered to bethe tissue from which other types of tissuesevolved.

parthenocarpy The development offruit in unfertilized flowers, resulting inseedless fruits. It may occur naturally, as inthe banana (Musa), which is triploid. It

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may also be induced artificially by the ap-plication of auxin, as in commercialtomato growing.

parthenogenesis Development of un-fertilized ovules to form new individuals. Itoccurs regularly in certain plants, e.g. dan-delion (Taraxacum), in which meiosis isdefective, resulting in a diploid egg. Inhawkweeds (Hieracium) a cell of the nucel-lus takes the place of the megaspore. Seealso apomixis.

partial dominance See incompletedominance.

passage cells Endodermal cells thathave CASPARIAN STRIPS but remain other-wise unthickened after deposits of ligninand cellulose have been laid down else-where in the endodermis. Passage cells arefound opposite the protoxylem and allowtransport of water and solutes from thecortex to the stele.

pathogen Any organism that is capableof causing disease or a toxic response in an-other organism.

PCR See polymerase chain reaction.

peat Partially decomposed plant mater-ial that accumulates in waterlogged anaer-obic conditions in temperate humidclimates, often forming a layer several me-ters deep. Peat varies from a light spongymaterial (Sphagnum moss) to a densebrown humidified material in the lowerlayers.

pectic substances Polysaccharides that,together with hemicelluloses, form the ma-trix of plant cell walls. They serve to ce-ment the cellulose fibers together. Fruitsare a rich source, especially apples (Malus),pears (Pyrus communis), and citrus fruits.See also middle lamella.

pedicel See inflorescence.

peduncle See inflorescence.

pelagic Inhabiting the open upper wa-ters rather than the bed of a sea or ocean.Compare benthic. See also photic zone;plankton.

pellicle (periplast) A thin flexible trans-parent outer protective covering of someunicellular algae that lack a cellulose cellwall, e.g. Euglena. It is made of protein,and maintains the shape of the body.

Pennsylvanian The US name for thelate Carboniferous period in the geologicaltime scale, 320 to 286 million years ago.Rocks that originated during this periodare widespread in the state of Pennsylva-nia, USA.

pentose A SUGAR that has five carbonatoms in its molecules. Examples includeribose, deoxyribose, and arabinose. Pen-toses are synthesized in the Calvin cycle,derived from hexoses in the PENTOSE PHOS-PHATE PATHWAY, and also made by decar-boxylation of nucleoside diphosphatesugars. Polymers of pentoses are calledpentosans.

pentose phosphate pathway (hexosemonophosphate shunt) A pathway ofglucose breakdown in which pentoses areproduced, in addition to reducing power(NADPH) for many synthetic reactions. Itis an alternative to GLYCOLYSIS, and pro-duces sugars containing 3 to 7 carbonatoms. Glucose 6-phosphate is decar-boyxlated and converted to a 5-carbonsugar, with the formation of NADPH. This5-C sugar than enters a sequence of reac-tions, until eventually glucose 6-phosphateis reformed.

PEP See phosphoenolpyruvate.

pepo A succulent fruit resembling aberry. The hard outer ring orginates eitherfrom the epicarp or from noncarpellary tis-sues of the plant. In members of the Cu-curbitaceae, such as Cucurbita species (e.g.cucumber, squash, and pumpkin), the rindis formed from the receptacle, while the

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flesh enclosing the seeds is derived from theovary wall.

peptidase An enzyme that is responsiblefor catalyzing the hydrolysis of certain pep-tide bonds. The peptidases help breakdown peptides into amino acids.

peptide A molecule consisting of a num-ber of amino acid molecules linked to-gether. Peptides can be regarded as formedby a CONDENSATION REACTION in which thecarbonyl group of one amino acid reactswith the amino group of another aminoacid with the elimination of water. Thislink between amino acids is called a pep-tide bond. According to the number ofamino acids linked together they are calleddi-, tri-, oligo-, or polypeptides. In general,peptides have an amino group at one endof the chain and a carboxyl group at theother. They can be produced by the partialhydrolysis of proteins.

peptide bond See peptide.

peptidoglycan The principal compo-nent of the cell walls of most BACTERIA (butnot of Archaea), consisting of polysaccha-

ride molecules cross-linked by short pep-tides chains to form a rigid framework.

perennating organ In biennial or peren-nial plants, a storage organ that enables theplant to survive an adverse season, such aswinter or a dry season (perennation). Mostsuch organs are underground, e.g. rhi-zomes, corms, bulbs, swollen taproots, andtubers. Perennating organs are also used bymany plants in asexual reproduction (veg-etative growth). Buds developing in theaxils of scale leaves on CORMS, BULBS, andstem TUBERS may become detached, putdown ADVENTITIOUS roots, and grow intoindependent plants. Similarly, new shootsarise from buds on RHIZOMES, and the in-tervening segments of rhizome may breakor rot away.

perennation See perennating organ.

perennial A plant that may live for sev-eral years. Perennials may reproduce intheir first growing season or may have to attain a certain age before seed produc-tion commences. Herbaceous perennialsdie back each year and survive until thenext growing season as tubers (e.g. Dahlia),bulbs (e.g. daffodil, Narcissus), rhizomes

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(e.g. Iris), etc. Woody perennials, such astrees and shrubs, persist above groundthroughout the year but may show adapta-tions (e.g. leaf fall) to survive unfavorableseasons.

perforation plates The remains of thecross walls between the vessel elements inthe xylem vessels of angiosperms, the Gne-tophyta, and some ferns, which leave anopening allowing water to move freelythrough the vessel. The cross wall mayhave disintegrated completely so that thevessel is effectively one long cylinder (e.g.oak) or parts of the cross wall may remainas bars across the cavity of the vessel, e.g.alder (Alnus). Compare sieve plate.

perianth The part of the flower that en-circles the stamens and carpels. It usuallyconsists of two whorls of leaflike structuresthat are differentiated into the sepals (calyx)and petals (corolla). In many monocoty-ledons there is no such distinction, and theindividual perianth units are called tepals,e.g. tulip (Tulipa). In some flowers, espe-cially those that are wind pollinated, theperianth is reduced (e.g. grasses) or absent(e.g. willow).

periblem See ground meristem; histogentheory.

pericarp The ovary wall that becomesthe wall of the FRUIT as the fruit develops.Depending on how the pericarp tissuesdifferentiate, the resulting fruit may be dry (papery or leathery) or succulent. Theouter layer is the exocarp, often a toughskin. The middle layer is the mesocarp,whose texture varies with different fruits,e.g. it is juicy in drupes like the plum(Prunus domestica), and hard in almonds(Prunus dulcis). The mesocarp may be pro-tective or aid in dispersal. The endocarp isthe innermost layer and forms the stonycovering of the seed in a drupe, but in otherfruits (e.g. the berry), it is indistinguishablefrom the mesocarp. If the pericarp openswhen ripe, the fruit is said to be DEHISCENT;if it remains closed, it is INDEHISCENT.

periclinal Describing a line of cell divi-sion parallel to the surface of the organ, re-sulting in the formation of periclinal wallsbetween daughter cells and an increase ingirth of the organ. Compare anticlinal.

pericycle The parenchymatous layer ofcells that lies within the endodermis, form-ing the outermost part of the stele. It re-mains meristematic in most roots and givesrise to the lateral roots. In most dicotyle-donous roots showing secondary growth,the pericycle is also involved in the origina-tion of the vascular cambium and the phel-logen.

periderm The secondary tissue thatarises from the activity of the cork cam-bium. It forms the outer layer of the stemand root in plants with secondary growth,having a protective role. It is made up ofthe PHELLOGEN (cork cambium), whichgives rise to the PHELLEM (cork) and PHEL-LODERM (secondary cortex). Periderm mayalso develop at the site of a wound, form-ing a barrier to pathogens.

perigyny The arrangement in flowers inwhich the perianth and androecium are in-serted on the receptacle around the gynoe-cium rather than above or below it. Thereceptacle is extended to a flat or saucer-shaped organ and carries the gynoecium inthe middle and the other floral partsaround the edge. The OVARY is technicallysuperior even though in extreme forms ofperigyny in which the receptacle is cup-shaped, e.g. the wild rose (Rosa sp.), thefloral parts may be inserted at a level abovethe gynoecium. In perigynous flowers theovary is not fused with the receptacle.Compare epigyny; hypogyny.

periplast See pellicle.

perisperm The nutritive tissue in theseeds of many Caryophyllaceae (the carna-tion family). It is a diploid tissue derivedfrom the nucellus rather than the em-bryosac. Such seeds therefore differ fromother angiosperm seeds that have either

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cotyledons or endosperm tissue acting asthe food store.

peristome A single or double ring ofteeth around the opening of the capsule inmosses that is involved in spore dispersal.It is revealed when the operculum falls offor is removed. The teeth absorb moisturefrom the atmosphere, and the coiled fibersof which they are made change length andtwist and bend when subjected to humiditychanges, either opening and closing thecapsule to release the spores or scatteringthem through jerky movements.

perithecium (pl. perithecia) See as-coma.

permanent stain See staining.

permanent wilting point The point atwhich soil has dried to the extent thatplants can no longer remove the remainingwater held on the soil particles, and beginto wilt and will not recover, even if movedto a cool, dark place, unless more water isadded to the soil. At permanent wiltingpoint the water potential of the soil is equalto or lower than the water potential of theplant.

Permian The most recent period of thePaleozoic era, some 280–250 million yearsago. As the climate warmed and driedduring the Permian, the ferns, seed ferns,and lycophytes that dominated Permianmarshes and swamps were joined by theevolving coniferophytes and some othergymnosperms, whose evolution was prob-ably driven by the need to cope with drierenvironments. These groups graduallybegan to replace the older floras domi-nated by nonseed-bearing plants. See alsogeological time scale.

Peronosporales An order of the OOMY-COTA characterized by sexual reproduction(usually) by zoospores and sexual repro-duction by zoospores produced fromoospores. The Peronosporales containssaprobes and many harmful plant para-sites. Most are parasites found in damp ter-

restrial habitats; they include downymildews (Peronospora spp.), late blight of potatoes (Phytophtherainfestans), anddamping-off fungus (Pythium spp.).

peroxidase An enzyme that catalyzesthe oxidation of certain organic moleculesusing hydrogen peroxide as electron accep-tor.

peroxisome See microbody.

petal One of the usually brightly coloredparts of the flower, which together makeup the corolla. The petals are pigmentedand often scented to attract insects. Theyare reduced or absent in most wind-polli-nated flowers. Petals are thought to bemodified leaves, with a much simplified in-ternal structure and vascular system, hav-ing only one vascular bundle. They mayderive from sterile stamens. Comparesepal.

petiole The stalk that attaches the leafblade to the stem. It is similar to the stemexcept that it is asymmetrical in cross-section with the vascular and strengtheningtissues arranged in a V shape rather than acircle. Leaves that lack a petiole are termedsessile.

Pezizales (cup fungi; morels) An orderof ascomycete fungi in which the asci (seeascus) are borne in a HYMENIUM that linesthe surface of a cup-shaped fruiting body(apothecium). The spores are dischargedforcibly into the air. Most are saprobes liv-ing in soil, dung, or rotting wood. A fewspecies are involved in mycorrhizae.

pH A measure of the acidity or alkalinityof a solution on a scale 0–14. A neutral so-lution (such as pure water) has a pH of 7.Acid solutions have a pH below 7; alkalinesolutions have a pH above 7. The lower thepH value, the higher the concentration ofH+. The pH is given by log10 (1/[H+]),where [H+] is the hydrogen ion concentra-tion in moles per liter. Thus one integer onthe pH scale represents a tenfold differencein the concentration of H+.

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Phaeophyta (brown algae) A phylum ofprotoctists comprising mainly marinealgae, notably the macroscopic thalloseseaweeds that inhabit the intertidal zones.They contain the pigments chlorophyll aand c, b-carotene, and the xanthophylls(including fucoxanthin), which give thealgae their characteristic brown color.

phage See bacteriophage.

phagocytosis The taking up of foodparticles by a cell using protrusions of theouter part of the cell, generated by cyto-plasmic flow. These protrusions surroundthe particle, enclosing it in a membrane-bound vesicle and so introducing it into thecell as a food vacuole. Lysosomes then con-gregate around the food vacuole, fusingwith it and releasing hydrolytic enzymes todigest the particle. See endocytosis.

Phallales (stinkhorns) An order of ba-sidiomycetes in which the BASIDIOMA has atwo-layered wall enclosing a slimy, gelati-nous layer. See Basidiomycota.

phanerogam In early classifications,any plant whose reproductive organs areflowers or cones. Compare cryptogam.

phanerophyte A perennial plant withpersistent shoots and buds on uprightstems well above soil level. Phanerophytesinclude herbs, shrubs, trees, and climbingplants. See Raunkiaer’s plant classification.

Phanerozoic See Precambrian.

phase contrast microscopy A form oflight microscopy that amplifies the differ-ence in the phase of light after passingthrough a specimen to give greater contrastin the image. It is based on the fact thatlight is diffracted (changes phase) accord-ing to the refractive index or thickness ofthe material through which it passes. Thephase contrast microscope uses a specialcondenser, which produces a particularpattern of light, usually a hollow cone.This pattern is matched by a groove in aglass plate (the retardation plate) in the

condenser. Light that is not diffractedpasses through the holes in the plate. Lightthat has been diffracted by the specimenpasses through the thicker part of the plate,so is slowed down even further. This tech-nique can be used to study real-time eventssuch as cell division.

phellem (cork) A protective layer of rad-ically arranged cells produced to the out-side of the PHELLOGEN (cork cambium).The cork replaces the epidermis in certainwoody plants, forming an impervious layerbroken only by lenticels. The older corkcells are dead, suberized, and frequentlyonly air-filled, although lignin, fatty acids,and tannins often accumulate. The corkoak, Quercus suber, of Southern Europe,forms a very thick layer and is the mainsource of commercial corks.

phelloderm The inner layer of the PERI-DERM. During secondary growth, themeristematic PHELLOGEN (cork cambium)gives rise to parenchyma tissue, the perid-erm, which forms the secondary cortex.The cells of the phelloderm arise in radialcolumns from the phellogen, and this dis-tinguishes them from the primary cortex.

phellogen (cork cambium; periderm cam-bium) A subepidermal layer of cellsforming a lateral meristem that arises fol-lowing the onset of secondary growth. Thecells of the phellogen (cork cambium) giverise externally to the PHELLEM (cork) andinternally to the PHELLODERM.

phelloid A thin-walled unsuberized corkcell.

phenolics (phenols) Compounds thatcontain a benzene ring or other aromaticring substituted with one or more hydroxylgroups, e.g. phenol (C6H5OH). Phenolicsare similar to alcohols but are more solublein water. Many have sharp, spicy odors. Itis thought that most phenolics are secretedby plants to deter herbivores as many areenzyme inhibitors and disrupt digestion.Plant phenolics include the FLAVONOIDS,LIGNIN, and TANNINS.

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phenolphthalein An acid–alkali indica-tor that is colorless in acids and red in al-kalis. It has a pH range from 8.4–10.0 andis a frequently used indicator for the detec-tion of pH change in acid–alkali titrations.Phenolphthalein, together with borax, isused to test for saccharide derivatives, e.g.glycerol. Such derivatives turn the solutionfrom red to colorless but on boiling the redcolor returns.

phenotype The observable characteris-tics of an organism, which are determinedby the interaction of the GENOTYPE with theenvironment. Many genes present in thegenotype do not show their effects in thephenotype because they are masked bydominant alleles. Genotypically identicalorganisms may have very different pheno-types in different environments, an effectparticularly noticeable in plants grown invarious habitats.

phenylalanine An aromatic AMINO ACID

derived from phosphoenolpyruvate anderythrose 4-phosphate. It is a precursor ofmany aromatic compounds and of alka-loids such as curare and morphine. Phenyl-alanine can be deaminated to cinnamicacid, required in the synthesis of lignin.

phloem Plant vascular tissue in whichfood is transported from areas where it ismade to where it is needed or stored. Itconsists of SIEVE ELEMENTS and COMPANION

CELLS. The PROTOPHLOEM and METAPHLOEM

are primary tissues and derived from theprocambium, while the secondary phloemis formed from the VASCULAR CAMBIUM.There are also fibers (sclerenchyma), andparenchymatous packing tissue in thephloem. See also mass flow.

phloroglucinol A temporary stain thatdyes lignin magenta red in thin sections ofplant tissue. It is usually acidified with hy-drochloric acid before use. See staining.

phosphatide A glycerophospholipid. Seelipid.

phosphodiester bond A covalent bond

linking a sugar and a phosphate group bymeans of an oxygen bridge, for example,the bonds linking sugars and phosphategroups in the backbone of NUCLEIC ACIDS.Phosphodiester bonds are formed by con-densation reactions and may be broken byhydrolysis.

phosphoenolpyruvate (PEP) A 4-car-bon phosphorylated organic acid that isthe main substrate for carbon dioxide fixa-tion in C4 PLANTS, being carboxylated tooxaloacetic acid in a reaction mediated bythe enzyme phosphoenolpyruvate carboxy-lase (PEP carboxylase). It is also an impor-tant intermediate of GLYCOLYSIS, being theimmediate precursor of pyruvic acid. In theGLYOXYLATE CYCLE for the synthesis of glu-cose it is generated from oxaloacetate.

phosphoglyceric acid See glycerate 3-phosphate.

phosphoglyceride (phospholipid) A com-plex lipid similar to acylglycerides (seeacylglycerol), but with a backbone of glyc-erol 3-phosphate rather than glycerol.Phosphoglycerides are major consitituentsof the membranes of cells and organelles,with the exception of chloroplasts, inwhich GLYCOLIPIDS are more important.They have both hydrophilic and hydropho-bic groups, so are reasonably soluble inboth water and lipids.

phospholipid See lipid.

phosphorescence In general usage theterm is applied to the emission of ‘coldlight’ – light produced without a high tem-perature. The light comes from excitedatoms produced directly in the reaction –not from the heat produced. It is thus anexample of chemiluminescence. In biologi-cal systems, the light emitted is often lightthat has been previously absorbed, butwhich is re-emitted at a different wave-length. Phosphorescent seas are due to thebioluminescence of dinoflagllates (Dino-mastigota), in particular Noctiluca species.

phosphorus One of the essential el-

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ements in living organisms, a macronutri-ent needed for plant growth. Phospholipidsare important in cell membrane structure,and phosphates are necessary for the for-mation of the sugar–phosphate backboneof nucleic acids. Phosphates are also neces-sary for the formation of high-energybonds in compounds such as ATP. Phos-phorus has other important roles in livingtissues, being a component of certain coen-zymes. The phosphate ion, PO4

3–, is an im-portant buffer in cell solutions. Plants takeup phosphorus from the soil water as thePO4

3- ion. Phosphorus deficiency results instunted roots, bluish, bronzed, or purpleleaves, and poor germination, seed set, andfruit ripening.

phosphorylase See kinase; phosphory-lation.

phosphorylation The addition of aphosphate group to a compound. Phos-phorylation reactions are catalyzed byphosphorylase enzymes. Phosphorylationis the first step in many metabolic path-ways, the phosphate bond serving as an en-ergy source in a later stage of the pathway.For example, glycolysis starts with thephosphorylation of glucose and fructose1,6-bisphosphate. Phosphorylation is alsoneeded for the activation or deactivation ofsome enzymes. See also oxidative phospho-rylation; photophosphorylation.

phosphotransferase See kinase.

photic zone The surface layer of anocean or lake that is penetrated by sunlightand in which the phytoplankton flourish.Red and yellow wavelengths of light pene-trate to about 50 m while blue and violetlight may reach 200 m. The diatoms,which are the main components of phyto-plankton, may be found down to 80 m.Most of the plankton lives in the top 10 m.Beyond 200 m the water is perpetuallydark. The depth at which the rate of fixa-tion of carbon dioxide by photosynthesis isequal to the rate at which it is lost by res-piration is called the compensation depth.The part of the photic zone above this

depth is termed the euphotic zone, the partbelow the compensation depth is the dys-photic zone. The depth of the euphoticzone depends on the turbidity of the water.See plankton. Compare aphotic zone.

photoautotrophism See autotrophism;phototrophism.

photoheterotrophism See heterotro-phism; phototrophism.

photolysis Chemical breakdown causedby light. In PHOTOSYNTHESIS the process isimportant in providing hydrogen donorsby the splitting of water, as follows:

2H2O ® O2 + 4H+ + 4e–

In the CHLOROPLAST photolysis of water oc-curs in the oxygen- releasing complexlinked to Photosystem II. The protons arereleased into the lumen of the thylakoid.The high-energy electrons go to replaceelectrons displaced from the reaction cen-ter of Photosystem II by the absorption oflight energy. The electrons are the ener-gized by light energy and pass to the electron-transport chain of noncyclic pho-tophosphorylation, which generates ATPand reduced NADP.

photomicrograph See micrograph.

photonasty (photonastic movements) Anastic movement in response to light.Flowers such as the marigold (Calendulaofficinalis) open on exposure to light. Caremust be taken not to confuse photonastywith nyctinasty, in which day–night open-ing/closing of flowers is governed by an in-ternal CIRCADIAN RHYTHM. See nasticmovements.

photo-oxidation The oxidation of asubstance as a result of the absorption oflight energy. Very high light intensity cancause oxidation (bleaching) of photosyn-thetic pigments such as chlorophylls, inac-tivating them.

photoperiodism The response of an or-ganism to changes in day length (photope-riod). In plants, leaf fall and flowering are

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common responses to seasonal changes inday length.

Plants are classified as short-day plants(SDPs), e.g. Chrysanthemum, or long-dayplants (LDPs), e.g. cucumber (Cucumissativus), according to whether they flowerin response to short or long days. Day-neutral plants, e.g. pea (Pisum sativum),have no photoperiodic requirement. Infact, the critical factor is not the length ofthe day, but the length of the dark period,since flowering of SDPs is inhibited by evena brief flash of red light in the dark period(a PHYTOCHROME response), and an artifi-cial cycle of long days and long nights in-hibits flowering in LDPs. Thus, it is the

interaction between light and dark periodsthat in some way affects flowering throughthe mediation of phytochrome. Phy-tochrome is a protein that can absorb redlight, which converts it to a form (PFR) thatabsorbs far-red light. Absorption of far-redlight causes the phytochrome to revert tothe red light-absorbing form (PR). The PFR

form of phytochrome inhibits flowering inSDPs (PFR slowly disappears during longnights) and promotes flowering in LDPs(PFR remains at high levels in short nights).The light stimulus is perceived by the leavesand in some unknown way transmitted tothe floral apices. See circadian rhythm;

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critical day length; thermoperiodism; ver-nalization.

photophosphorylation (photosyntheticphosphorylation) The conversion ofADP to ATP using light-induced electrontransport. See photosynthesis.

photoreceptor Any light-sensitive organor organelle. The paraflagellar body, aswelling on the undulipodium of certainspecies of Euglena, is an example.

photorespiration (C2 cycle) A light-dependent metabolic process of most greenplants that resembles true (or ‘dark’) respi-ration only in that it uses oxygen andproduces carbon dioxide. Carbon dioxideproduction during photorespiration maybe up to five times greater than in dark res-piration. Photorespiration takes place inPEROXISOMES rather than mitochondria,and it is not coupled to oxidative phospho-rylation. It wastes carbon dioxide and en-ergy, using more ATP than it produces. Itssubstrate is glycolate produced in C3 plantsby the oxidation of ribulose bisphosphateto glycerate 3-phosphate and phosphogly-colic acid. During photorespiration the en-zyme glycolate oxidase catalyzes theoxidation, by molecular oxygen, of gly-colic acid to glyoxylic acid and hydrogenperoxide in the peroxisomes. The glyoxy-late is converted to glycine, and carbondioxide is released when the glycine mol-ecules are converted to serine in the mito-chondria. The term photorespiration isbeing replaced by the C2 cycle (oxidativephotosynthetic carbon cycle), to better re-flect the fact that unlike respiration it usesrather than releases energy.

It is estimated that in C3 plants 50% ofthe potential yield of photosynthesis can belost through photorespiration in warmweather. It is therefore economically im-portant and ways of inhibiting the processare being investigated.

photosynthesis The synthesis of or-ganic compounds using light energy ab-sorbed by chlorophyll. With the exceptionof a small group of bacteria, organisms

photosynthesize from inorganic materials.All green plants photosynthesize, as well asalgae and certain bacteria. In green plants,photosynthesis takes place in chloroplasts,mainly in leaves and stems. Directly or in-directly, photosynthesis is the source ofcarbon and energy for all except chemoau-totrophic organisms. The mechanism iscomplex and involves two sets of stages:light reactions followed by dark reactions.The overall reaction in green plants can besummarized by the equation:

CO2 + 4H2O ® [CH2O] + 3H2O + O2The light reactions of photosynthesis

involve reaction centers in the chloroplasts.Each reaction center lies in a flat sheet of200-400 pigment molecules in the thy-lakoid membrane, the antenna complex,and contains a pair of chlorophyll a mol-ecules. The whole unit is called a photosys-tem. There are two kinds of photosystemsin green plants, Photosystems I and II.Both are usually involved in the light reac-tions of photosynthesis. Light energy ab-sorbed by the pigments of the antennacomplex is passed to the reaction-centerchlorophyll molecules, from which itpasses along an electron-transport chain.Photosystem II contains a type of chloro-phyll a (P680) that shows maximum lightabsorption at a wavelength of 684 nm.When activated by light, a pair of electronsare excited and leave Photosystem II, beingreplaced by electrons from the photolysisof water:

2H2O ® O2 + 4H+ + 4e–

Molecular oxygen is released, and theprotons pass into the lumen of the thy-lakoid. The electrons pass through an ELECTRON-TRANSPORT CHAIN in the thyla-koid membrane, being accepted first byplastoquinone, then by the cytochrome b/fcomplex (a proton pump that pumps H+

into the thylakoid lumen), and on via plas-tocyanin to Photosystem I, which containschlorophyll a (P700) with a maximumlight absorption at 700 nm. Here the elec-trons are boosted by more absorbed lightenergy to an even higher energy level andpass through a second electron-transportchain involving the protein ferredoxin, re-ducing NADP to NADPH + H+ for use in

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Photosynthesis: the Calvin cycle

other reactions, such as various syntheses.This sequence, which involves both photo-systems, is termed noncyclic electron flow.However, it is possible for electron flow toinvolve only Photosystem I, and the ferre-doxin/cytochrome b/f/ plastocyain elec-tron- transport chain, electrons returningto Photosystem I to be reenergized. Thiscyclic electron flow does not produceNADPH. It simply generates a proton gra-dient across the thylakoid membrane. Thisis used to drive ATP synthesis (cyclic pho-tophosphorylation). ATP synthesis associ-ated with the noncyclic electron-transportchain is termed noncyclic photophos-phorylation. The dark reactions of photo-synthesis take place in solution in thechloroplast stroma. Here, ATP andNADPH from the light reactions are usedto reduce carbon dioxide to carbohydrate.Carbon dioxide is ‘fixed’ by combination

with the 5-carbon sugar ribulose bisphos-phate (RuBP), forming two molecules ofphosphoglyceric acid (PGA). This reactionis catalyzed by the enzyme ribulose bispho-sphate carboxylase (rubisco). In a series ofreactions using NADPH and ATP from thelight reactions, PGA is converted to a suc-cession of 3-, 4-, 5-, 6-, and 7- carbon sugarphosphates in a series of reactions collec-tively termed the Calvin cycle. These prod-ucts are then used in the synthesis ofcarbohydrates, fats, proteins, and othercompounds, and RuBP is regenerated. SeeC3 plant; C4 plant.

photosynthetic bacteria A group ofbacteria able to photosynthesize throughpossession of chlorophyll pigments. Theyinclude the Cyanobacteria, green sulfurbacteria (Chlorobia), green nonsulfur bac-teria (Chloroflexa), purple sulfur bacteria,

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Light reaction of photosynthesis

and purple nonsulfur bacteria (both in thephylum Proteobacteria). The photosyn-thetic bacteria fix carbon using the Calvincycle, but use a variety of hydrogensources.

photosynthetic pigments Pigments thatabsorb the light energy required in photo-synthesis. They are located in the chloro-plasts of plants and algae, whereas in mostphotosynthetic bacteria they are located inthylakoid membranes, typically distributedaround the cell periphery. All photosyn-thetic organisms contain chlorophylls andcarotenoids; some also contain phyco-bilins. Chlorophyll a is the primary pig-ment in plants, algae, and cyanobacteria,and bacteriochlorophyll is the pigmentused by many photosynthetic bacteria,since energy absorbed by this is useddirectly to drive the light reactions of pho-tosynthesis. The other pigments (chloro-phylls b, c, and d, and the carotenoids andphycobilins) are accessory pigments thatpass the energy they absorb on to chloro-phyll a. They broaden the spectrum of lightused in photosynthesis. See absorptionspectrum.

photosystem See photosynthesis.

phototaxis (phototactic movement) ATAXIS in response to light. Many motilealgae are positively phototactic, e.g.Volvox. Chlamydomonas cells are alsopositively phototactic, swimming towardlight to increase their photosynthesis rate,but if the light intensity becomes too great,they show negative phototaxis, swimmingaway to protect their pigments. Chloro-plasts also show phototaxis, and take upspecific orientations in relation to the di-rection of the incident light.

phototrophism A type of nutrition inwhich the source of energy for synthesis oforganic requirements is light. Most pho-totrophic organisms are autotrophic (i.e.show photoautotrophism); these comprisethe green plants, Cyanobacteria, and somephotosynthetic bacteria (the purple andgreen sulfur-bacteria).

phototropism (phototropic movement)A directional growth movement of part ofa plant in response to light. The phenome-non is clearly shown by the growth ofshoots and coleoptiles toward light (posi-tive phototropism). The stimulus is per-ceived in the region just behind the shoottip. If light falls on only one side of theapex then auxins produced in the apextend to diffuse toward the shaded side.Thus more auxin diffuses down the stemfrom the shaded side of the tip. This resultsin greater elongation of cells on the shadedside thus causing the stem to bend towardthe light source. Most roots are apho-totropic (light-insensitive) but some (e.g.the adventitious roots of climbers such asivy) grow away from light (negative pho-totropism). See also tropism.

phragmoplast A barrel-shaped bodyappearing in dividing plant cells during lateanaphase and telophase between the twoseparating groups of chromosomes. It con-sists of MICROTUBULES associated with theSPINDLE, together with Golgi apparatus, en-doplasmic reticulum, and ribosomes, andtransports vesicles that coalesce to formthe early cell plate.

phycobilins (phycobiliproteins) A groupof accessory photosynthetic pigmentsfound in Cyanobacteria and Rhodophyta(red algae). They comprise the blue phyco-cyanins, which absorb extra orange andred light, and the red phycoerythrins,which absorb green light, enabling redalgae to grow at depth in the sea.

phycocyanin See phycobilins.

phycoerythrin See phycobilins.

phylloclade See cladode.

phyllode An expanded flattened petiolethat acts as the photosynthetic organ if thelamina is missing or very reduced. Phyl-lodes are seen in various Australian speciesof Acacia. Compare cladode.

phyllotaxis (phyllotaxy) The arrange-

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ment of leaves on a stem. There may beone, two, or several leaves at each node.When there are three or more leaves form-ing a circle around the node the arrange-ment is said to be whorled, e.g. bedstraws(Galium spp.). When leaves arise singly thearrangement may be spiral, e.g. poplar(Populus spp.), or alternate, e.g. hazel(Corylus spp.), and when they arise in pairsthe arrangement is termed opposite, e.g.lilac (Syringa spp.). Opposite phyllotaxismay be either distichous or decussate de-pending on whether the leaf pairs are in thesame plane up the stem or arise alternatelyat right angles to each other.

phylogenetic Relating to the study ofevolutionary relationships between andwithin taxonomic levels, in particular therelationships between groups of organismsbased on their past evolutionary history.

phylogeny The evolutionary history ofgroups of organisms. See phylogenetic.

phylum (pl. phyla) One of the majorgroups into which a kingdom of organismsis classified. Phyla may be divided into sub-phyla. The names of plant phyla end in -phyta, e.g. Anthophyta, Lycophyta. Insome plant classifications (especially olderones) the term ‘division’ is used instead ofphylum.

physiological drought A situation inwhich plants are unable to take up suffi-cient water even though there is water inthe soil: they are effectively in a droughtsituation. It occurs when the concentrationof solutes in the soil water is equal to orhigher than that in the root cells, so wateris unable to enter the root by OSMOSIS.Physiological drought occurs in salt

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marshes and other brackish coastal habi-tats, and in deserts where there are highconcentrations of salts in the upper layersof the soil. It also occurs in cold weatherbecause the permeability of the endoder-mal cells decreases rapidly below 5oC, sothe root resists the entry of water (root re-sistance).

physiological race The existence ofphysiologically distinct but morphologi-cally identical races within a species. Suchphysiological races are important inhost–pathogen studies, particularly inplanning programs to breed for crop resis-tance.

physiology The way in which organ-isms or parts of organisms function. Com-pare morphology.

phytoalexin A chemical produced by aplant that inhibits the growth of a patho-genic fungus. For example, when attackedby the fungus Ceratostomella, the sweetpotato (Ipomoea batatus) concentrates theterpenoid ipomeamarone in its tissues, andthis concentration has been shown to be re-lated to the degree to which it resists the at-tack.

phytochrome A proteinaceous pigmentfound in low concentrations in most plantorgans, particularly meristems and dark-grown seedlings. It exists in two intercon-vertible forms. PR (or P660) has anabsorption peak at 660 nm (red light) andPFR (or P730) at 730 nm (far-red light). Nat-ural white light favors formation of PFR,the physiologically active form, while far-red light encourages reversion to PR if givensoon after the red light. Light intensities re-quired for conversion are very low and itoccurs within seconds. In some species, PFRmay revert to PR in the dark, a process in-hibited by low temperatures. See circadianrhythm; photoperiodism.

phytohormone See hormone.

phytoplankton See plankton.

pico- Symbol: p A prefix denoting onemillion-millionth, or 10–12. For example, 1picogram (pg) = 10–12 gram. See SI units.

pileus (pl. pilei) The cap of the maturemushroom or toadstool (sporophore) incertain basidiomycete fungi (e.g. Agaricus).Its undersurface comprises gills or poreslined with basidia. The upper surface maybe fleshy or leathery, and may retain theflaky remains of the universal veil.

pili (sing. pilus) (fimbriae sing. fimbria)Fine, straight, hairlike protein structuresemerging from the walls of certain bacte-ria. They confer the property of ‘stickiness’whereby bacteria tend to adhere to one an-other. They are hollow tubes and maynumber from one to several hundred.

piliferous layer The region of the rootepidermis that gives rise to the root hairs. Itis located just behind the zone of elonga-tion and is the main absorptive area of theroot.

Pinaceae A family of conifers that bearseparate male and female cones, and inwhich the seeds are borne between the conescales and do not have arils. Most speciesare evergreen trees or shrubs, but thelarches or tamaracks (Larix spp.) are de-ciduous. One of the oldest living trees is abristlecone pine (Pinus aristata), thoughtto be about 4900 years old. The family in-cludes cedars (Cedrus), firs (Abies), hem-locks (Tsuga), and larches and pines(Pinus).

pin mold See Mucorales.

pinna (pl. pinnae) See pinnate.

pinnate Describing a compound leafwith leaflets (pinnae) arranged in tworows, one on each side of the midrib (flow-ering plants, e.g. Vicia) or of a central stalk(ferns, e.g. Dryopteris). The pinnae maythemselves be divided into leaflets, whichare called pinnules.

pinnule See pinnate.

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pinocytosis See endocytosis.

pioneer species A species that colonizesa new physical environment, for exampleland exposed by retreating glaciers, fellingof forest, or bare mud on coasts or estuar-ies. Pioneer species are the first stage inplant succession.

pistil In angiosperms, the seed-contain-ing structure. In an apocarpous gynoeciumit corresponds to the carpel, while in a syn-carpous gynoecium it is made up of two ormore fused carpels.

pit A gap in the secondary cell wall thatenables communication between thickenedcells, e.g. tracheids. According to whetheror not the secondary wall forms a lip overthe pit, pits are described as bordered orsimple, respectively. If a pit occurs singly itis termed a blind pit. Usually pits occur inpairs so that the only barrier separating ad-jacent cells is the middle lamella and the re-spective primary cell walls. The primarycell wall adjacent to the cavity is called thepit membrane.

pitcher plant Any carnivorous plantwith pitcher-shaped leaves. Insects andother small animals fall into the pitchers,which are usually filled with rainwater orplant digestive enzymes, and drown; theplant obtains nitrates and perhaps othernutrients from their decomposing bodies.

pith (medulla) The central region of thestem and, in some herbaceous plants, rootthat is normally composed of parenchyma-tous tissue. It occurs to the inside of thestele.

pitted thickening The most extensiveform of thickening found in XYLEM vesselsand tracheids, where the secondary cellwall is continuous apart from small areascalled PITS. The pits may be arranged in analternate or opposite pattern. If they areelongated horizontally and parallel to eachother, this is described as scalariformthickening.

placenta (pl. placentae) 1. The region oftissue occurring on the inner surface of theovary wall of the carpels of floweringplants where the ovules develop. Thearrangement of ovules within the ovary(see placentation) depends on whetherthere are one or many carpels and whetherthe carpels are free or fused.2. A central swelling on the lower surfaceof fern pinnules linking clusters of sporan-gia with the main frond. See also sorus.

placentation The position of the ovule-bearing placentae in angiosperm seeds. Pla-centation varies according to whether thereare one or many carpels. If there is only oneovule in the ovary, it may be attached atthe base of the ovary wall (basal placenta-tion), or at the apex (apical placentation).If there is more than one ovule, and there isonly one carpel in the ovary, as in pea(Pisum sativum), placentation is along theventral suture and is termed marginal pla-centation. Marginal placentation also oc-curs in apocarpous polycarpellary ovariesof LEGUMES but various types of placenta-tion are seen in syncarpous ovaries. Wherethe carpels are fused to give a unilocularovary, the ovules may be found along thetwo placentae at each line of fusion givingparietal placentation, as in violet (Violaspp.) and passionflower (Passiflora). Ifinstead the ovules are borne on a centralcolumn, placentation is termed central pla-centation; a modification of this is free-central placentation, in which the columndoes not extend to the top of the ovary, as in primrose (Primula vulgaris). Whereother carpels to give a multilocular ovary,the marginal placentae of the carpels mayfuse in the center of the ovary, giving axileplacentation, as in tulip (Tulipa spp.).

plagiotropism See tropism.

plankton A varied collection of aquaticorganisms that drift freely, not being at-tached to any substrate and not possessingany organs for locomotion. The most im-portant components of the plant plankton(phytoplankton) are the diatoms uponwhich the planktonic animals (zooplank-

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ton) feed. Other members of the phyto-plankton include other microscopic algaeand cyanobacteria.The plankton form thebasis of the food chain in the sea. Comparebenthic.

plant An organism that can (usually)make its own food by taking in simple in-organic substances and building these intocomplex molecules by a process termedphotosynthesis. This process uses light en-ergy, absorbed by a green pigment calledchlorophyll, which is found in all plantsbut no animals. There are a few exceptions,in the form of certain parasitic plants.Most plants have cellulose cell walls, andhave starch as a storage polysaccharide,whereas animals have no cell walls, storeglycogen, and do not have plastids orchlorophyll; and fungi usually lack cellu-lose and also do not form plastids. Plantslack motility, with the exception of the ga-metes of many species. One major charac-teristic that distinguishes plants from otherplantlike organisms, such as algae or fungi,is the possession of an embryo that is re-tained and nourished by maternal tissue.Fungi and algae lack embryos and developfrom spores. Plants also differ from fungiin having a regular alternation of diploidand haploid generations. Plants also differfrom animals by generally responding tostimuli very slowly, the response often tak-ing a matter of days and occurring only ifthe stimulus is prolonged. Compare algae;Fungi; Protoctista.

plant hormone See hormone.

plasmagel (ectoplasm) The gel-like re-gion of cytoplasm located in a thin layerjust beneath the plasma membrane, partic-ularly of unicells that move in an ameboidfashion, and cells that exhibit cyclosis.

plasmagene A gene contained in astructure outside the nucleus, e.g. the genesin mitochondria and chloroplasts. See cy-toplasmic inheritance.

plasmalemma (pl. plasmalemmae) Seeplasma membrane.

plasma membrane (cell membrane; plas-malemma) The membrane that sur-rounds all living cells. It ranges from7.5–10 nm in thickness and consists ofphosphoglyceride molecules (about 40%)and proteins (about 60%). The membranebackbone consists of phosphoglyceridemolecules arranged in two rows with theirhydrophilic polar heads facing outwardsand the hydrophobic hydrocarbon tails inthe center. This lipid layer is penetrated byglobular proteins, which in some places ex-tend from one side of the membrane to theother. The whole structure has fluidlikeproperties, as the molecules are able tomove laterally – the so-called fluid-mosaicmodel. The membrane is probably beingconstantly renewed. The plasma mem-brane is selectively permeable, and controlswhich substances can enter and leave thecell.

plasmasol (endoplasm) The sol-likeform of cytoplasm, located inside the PLAS-MAGEL. It is free-flowing and contains thecell organelles.

plasmid An extrachromosomal geneticelement found within bacterial cells thatreplicates independently of the chromoso-mal DNA. Plasmids typically consist of cir-cular double-stranded DNA molecules ofmolecular weight 106–108. They carry avariety of genes, including those for antibi-otic resistance (R factors), toxin produc-tion, and enzyme formation, and may beadvantageous to the cell. Some plasmids,called episomes, can reversibly insert them-selves into the bacterial DNA, replicatingwith it.

plasmodesma (pl. plasmodesmata) Afine strand of cytoplasm that serves to con-nect the protoplasm of adjacent plant cells,allowing material to pass from one cell toits neighbor.

plasmodial slime molds See Myxomy-cota.

Plasmodiophora A phylum of protoc-tists that are obligate endoparasites of

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plants, fungi, and algae. Most plasmodio-phorans cause little damage to the hostplant, but a few cause serious diseases, e.g.Plasmodiophora brassicae causes clubrootof cabbage and related brassicas.

plasmodium (pl. plasmodia) See Myx-omycota.

plasmogamy Fusion of protoplasm,usually referring to the fusion of cytoplasmbut not nuclei. It usually occurs betweengametes or protoplasts. Plasmogamy in theabsence of karyogamy (fusion of nuclei)occurs between fungal mycelia of differentstrains of some ascomycetes and basid-iomycetes to form a heterokaryon.

plasmolysis Loss of water from awalled cell (e.g. of a plant or bacterium)due to osmosis to the point at which theprotoplast shrinks away from the cell wall.The point at which this is about to happenis called incipient plasmolysis. Here the cellwall is not being stretched; i.e. the cell haslost its turgidity or become flaccid (wallpressure is zero). Wilting of herbaceousplants occurs at this point.

plastid An organelle enclosed by twomembranes (the envelope) that is found inplants and certain protoctists (e.g. algae),and develops from a proplastid. Varioustypes exist, but all contain DNA and ribo-somes. Plastids range from 10 m m to lessthan 1.0 m m in diameter. They includechloroplasts, chromoplasts, amyloplasts,etc., and are often interconvertible.

plastid inheritance See plastogene.

plastocyanin An electron carrier inPHOTOSYNTHESIS, involved in the transfer ofelectrons from photosystem II to photosys-tem I.

plastogene A gene present in a self-replicating plastid. Inheritance studies haveshown that plastogenes control leaf colorin some plants, e.g. yellow-leaved Primulasinensis, is an example of plastid inheri-

tance. See cytoplasmic inheritance; plasma-gene.

pleiomorphism The occurrence of dif-ferent morphological stages during the lifeof an organism. Examples are the differentspore forms of the rust fungi. Comparepolymorphism.

Pleistocene The first epoch of the Qua-ternary period, which started with a glacia-tion about two million years ago and endedwith the last glaciation about 10 000 yearsago. Several ICE AGES drove many organ-isms toward the equator while others be-came extinct. For example, tundra coveredtemperate parts of the USA and CentralEurope where today deciduous forests arethe natural vegetation. The boreal forestsextended south almost to northernLouisana and the Mediterranean. Fossilpollen deposits provide evidence for thepast distribution of plants. Some migratingplants found the tops of mountains furthersouth resembled the conditions they hadleft, and survived in isolated areas (refu-gia). When the climate warmed again,these became centers of diversity. The cycleof southward then northward migrationled to the mixing of plants from bothcooler and warmer regions, leading to newpatterns of competition and reoorganiza-tion of plant communities. Modern man(Homo sapiens) evolved during this period,and probably caused further disruption ofplant communuties by his use of fire andthe start of forest clearance. See also geo-logical time scale; pollen analysis.

plerome See histogen theory.

pleuropneumonia-like organisms Seemycoplasmas.

Pliocene The epoch of the Tertiary pe-riod, about 7 to 2 million years ago, whichfollowed the Miocene. In the Pliocene thehominids, such as Australopithecus andHomo, became clearly distinguishablefrom the apes. Grasses became more abun-dant, perhaps because the climate wasbecoming drier. Modern conifers were

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spreading, and leptosporangiate ferns un-derwent adaptive radiation. See also geo-logical time scale.

plumule The shoot apex and first rudi-mentary leaves in the mature embryo andthe seedling. In seedlings showing epigealgermination, the plumule is taken aboveground between the cotyledons. When ger-mination is hypogeal, only the plumuleemerges from the soil; it has a hooked tipto protect it as it pushes up through thesoil. Compare radicle.

pneumatophore (aerophore; breathingroot) A specialized negatively geotropicroot produced by certain aquatic vascularplants (especially trees such as mangroves,e.g. Sonneratia) that protrudes well abovesoil level. The aerial part is covered withLENTICELS through which gases can diffuseto and from the highly developed system of intercellular airspaces. Pneumatophoresare an adaptation to waterlogged, poorlyaerated soils, allowing roots to exchangegases.

Poaceae (Gramineae, grasses) Thegrasses, a family of monocotyledons with9500 species. Most are annual or perennialherbs, although some are woody (but donot have secondary thickening), e.g. bam-boos. Grasslands are the dominant vegeta-tion on the prairies, steppes and savannas,and in times past supported vast herds ofgrazing mammals. Today these areas arealso important are for growing cerealgrasses, such as barley (Hordeum vulgare),maize (Zea mays), millet (Panicum mili-aceum), oats (Avena sativa), rye (Secalecereale), sorghum (Sorghum), and wheat(Triticum). Rice (Oryza sativa) and sugarcane (Saccharum officinarum) are alsograsses.

pod See legume.

Podocarpaceae A family of evergreenconifers. It includes one of the oldest livingorganisms, a huon pine, Dacrydiumfranklinii, from Tasmania, possibly 10 000years old.

podsol (podzol) The type of SOIL foundunder heathland and coniferous forests intemperate and boreal climates where thereis heavy rainfall and long cool winters,such as the coniferous forests of northernNorth America and Eurasia. It is stronglyacid and often deficient in nutrients, espe-cially iron compounds and lime, as a resultof leaching by the heavy rain or snow-meltin spring. Beneath the humus layer lies aleached horizon (A horizon) composedmainly of quartz sand. In the clay-rich Bhorizon below the leached layer iron com-pounds accumulate, staining the layerbrown and forming an impermeable hardpan that prevents drainage. Where thislayer is sufficiently impermeable to main-tain the A horizon in a waterlogged condi-tion, the soil is termed a gley podsol.

point mutation See gene mutation.

polarity A lack of symmetry: a definiteorientation that arises during the differen-tiation of a cell, tissue, or organ. For ex-ample, in the development of the plantembryo from the zygote, one end becomesthe radicle, the other gives rise to cotyle-dons.

polarizing microscope A light MICRO-SCOPE that uses polarized light to illuminatethe specimen. The orientation of moleculesin certain crystalline substances affects thetransmission of polarized light.

polar nuclei The, usually two, haploidnuclei found midway along the EMBRYO

SAC, forming a binucleate central cell. Theymay fuse to form the diploid definitive nu-cleus. The endosperm is formed from thefusion of one or both polar nuclei (occa-sionally several) with one of the male ga-metes from the pollen tube.

pollen The MICROSPORES of seed plants,produced in large numbers in the pollensacs, or stamens. They are formed by meio-sis of somatic pollen mother cells. Pollengrains are rich in mitochondria, Golgi ap-paratus, and endoplasmic reticulum. Inmany gymnosperms the nuclei dividewithin the pollen grain to produce a small

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number of cells (2–40 depending on thegroup) representing the male prothallus. Inangiosperms and some conifers (e.g. Cu-pressus) the pollen grains contain just twonuclei: a GENERATIVE NUCLEUS and a VEGE-TATIVE NUCLEUS (tube nucleus).When apollen grain germinates, a pollen tubepushes its way out through an aperture inthe pollen grain wall, and the nuclei mi-grate into the tube. The generative nucleusdivides to produce two sperm cells consist-ing of only nucleus and cytoplasm, one ofwhich will fertilize the ovum, while theother will fuse with the POLAR NUCLEI toform the endosperm. The vegetative nu-cleus degenerates after fertilization. Pollengrains are highly variable in size and shaperanging from 5m to over 200m in diameterand from circular to tetrahedral or dumb-bell shaped. The wall of the pollen grainconsists of two layers, a highly resistantouter exine, which may be intricatelysculpted and is punctured by apertures,and an inner intine, which sometimes pro-trudes through these apertures. The sizeand shape of the pollen grains and thenumber and orientation of the aperturesand sculpturing is characteristic of particu-lar families, genera, or sometimes species.Pollen grains are adapted according to themethod of pollination, those carried by in-sects often being sticky or barbed, whilewind-pollinated plants generally producesmooth light pollen. See also pollen analy-sis.

pollen analysis A means of obtaining

information on the composition and extentof past floras and environments by exam-ining fossil pollen grains, spores, and simi-lar structures in peat and sedimentarydeposits. The outer wall (exine) of thepollen grain is often diagnostic (see pollen)and very resistant to decay. See also paly-nology.

pollen culture See anther culture.

pollen mother cell (PMC) In an-giosperms and gymnosperms, a somaticcell that gives rise to four haploid pollengrains by meiosis.

pollen sac A chamber in which thepollen is formed in the angiosperms andconifers.

pollen tube A filamentous outgrowth ofthe pollen grain that in most seed plantsgrows out through a pore in the exine andtransports the male gametes to the ovule.Germination of the pollen grain to give thepollen tube usually takes place only whenthe pollen is compatible with the stigma tis-sue. The pollen tube may grow down thestylar canal, or it may secrete enzymes thatdigest the tissues of the style, creating apath for the tube. It usually enters the ovulethrough the micropyle, but in some species,such as beech (Fagus) it may enter throughthe chalaza (chalazogamy).

pollination The transfer of pollen fromthe anther to the stigma in angiosperms,

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Pollen: section of a pollen grain showing the exine

and from the sporangiophores to the mi-cropyle in gymnosperms. If the pollen iscompatible (i.e. of the right type) then thepollen grains germinate, producing apollen tube that grows down the stylecarrying the pollen nuclei to the ovule.Plants may be self-pollinating, e.g. barley(Hordeum vulgare), thus ensuring thatseed will be set, even in the absence ofother members of the same species. How-ever, self-pollination also leads to homozy-gosity and, potentially, less adaptableplants in the long term; thus, in many plantspecies mechanisms exist to prevent it andpromote CROSS-POLLINATION. In some spe-cies of angiosperms stamens and stigmasmature at the same time and pollen mayfall directly onto the stigma of the sameflower, or the stamens and stigmas may ac-tually touch. See also cleistogamy; di-chogamy; homogamy; incompatibility;protandry; protogyny.

pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside-effects of human activities. Pollutioncan affect the atmosphere, rivers, seas, andthe soil.

Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by vehi-cle exhausts. Industrial emissions of sulfurdioxide cause acid rain, and the release intothe atmosphere of chlorofluorocarbons,used in refrigeration, aerosols, etc., leads tothe depletion of ozone in the stratosphere(see acid rain; ozone layer). Carbon diox-ide, produced by burning fuel and bymotor vehicle exhausts, is slowly buildingup in the atmosphere, and is one of the fac-tors leading to an overall increase in thetemperature of the atmosphere (see green-house effect). Vehicle exhausts also containcarbon monoxide, nitrogen oxides, andother hazardous substances, such as fineparticulate dusts. Water pollutants includethose that are biodegradable, such assewage effluent and nitrates leached fromagricultural land, which if allowed to enterwater courses can lead to eutrophication(see eutrophic) and algal blooms. Non-

biodegradable pollutants, such as certainchlorinated hydrocarbon pesticides (e.g.DDT) and heavy metals, such as lead, cop-per, and zinc in some industrial effluents,accumulate in the environment. Heavymetals, even when present at low concen-trations, are toxic to plants, inhibitingwater or nutrient uptake, damaging cellmembranes, and inhibiting enzymes.

polyembryony The occurrence of manyembryos in one ovule. See apomixis;parthenogenesis.

polymer A molecule made up of a seriesof similar units joined together, e.g. starch,which is made up of glucose units.

polymerase An enzyme that regulatesthe synthesis of a polymer. Examples in-clude RNA polymerases and DNA poly-merases.

polymerase chain reaction (PCR) Atechnique for amplifying small samples ofDNA rapidly and conveniently. It is usedwidely in research and forensic science, e.g.to clone genes or to produce a suitablequantity of DNA for genetic fingerprintingfrom the minute amounts present in tracesof blood or other tissue. See also geneticfingerprinting.

polymorphism A distinct form of vari-ation in which significant proportions ofdifferent types of individuals exist within apopulation of a species at the same timeand in the same place, such that the fre-quency of the rarest form cannot be ex-plained on the basis of recurring mutation.The term is generally used where the fre-quency of the least common morph ex-ceeds 1 in 20, less frequent occurrencesbeing usually due to rare mutation or re-combination events. If the differences per-sist over many generations then there is abalanced polymorphism. Compare pleio-morphism.

polypeptide A compound that containsmany amino acids linked together by pep-tide bonds. The three-dimensional shape

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(conformation) of the poplypeptide chainis determined by its amino acid sequence,but can be disrupted by extremes of pH orheat. Proteins are made up of one or morepolypeptides. See peptide; denaturation;conjugated protein.

polyploid The condition in which a cellor organism contains three or more timesthe haploid number of chromosomes. It oc-curs when chromosomes fail to separateduring anaphase I of meiosis, giving rise togametes that are diploid instead of haploid.Fertilization results in triploid or tetraploidindividuals. Polyploidy is far more com-mon in plants than in animals and veryhigh chromosome numbers may be found;for example in octaploids and decaploids(containing eight and ten times the haploidchromosome number). Some 60% ofmonocotyledons and 40% of dicotyledonsare polyploid, as are 90% of ferns, manybryophytes, and some algae. See allopoly-ploidy; autopolyploidy; heterosis.

Polypodiaceae A family of terrestrial orepiphytic leptosporangiate ferns.

polyribosome See ribosome.

polysaccharide A polymer of 10 ormore monosaccharides joined by glyco-sidic links (see glycoside). They containmany repeated units in their molecularstructures and are of high molecularweight. They can be broken down tosmaller polysaccharides (oligosaccharides),DISACCHARIDES, and MONOSACCHARIDES byhydrolysis or by the appropriate enzyme.Important polysaccharides are inulin (hy-drolyzed to fructose), starch (hydrolyzed toglucose), and cellulose (hydrolyzed to glu-cose). Polysaccharides may form branchingchains, or they may be unbranched. Theyhave many roles in plants, as storage mol-ecules (e.g. starch), structural elements incell walls (cellulose), and be components ofgums and mucilages (glucuronic acid). Apolysaccharide containing only one type ofsugar residue is called a homopolysaccha-ride, e.g. starch, which contains only glu-cose units. Heteropolysaccharides contain

two or more different monosaccharides,e.g. hemicelluloses. See also carbohydrates;sugar.

polysome See ribosome.

polysomy See aneuploidy.

polystely Having many steles, as in thestems of some Selaginella species. See alsodistely.

pome A fleshy pseudocarp fruit, charac-teristic of the family Rosaceae, e.g. apple(Malus). The fleshy part of the fruit is de-rived from the greatly enlarged urn-shapedreceptacle, which encloses the entire fruit,and which is fused to the tough carpelwalls. Thus the apple core is the true fruit.

population dynamics The study of thefactors influencing the fluctuations in num-bers in a population or of its gene pool: thefactors affecting birth and mortality rates,immigration and emigration, and repro-ductive potential (for example, seed size,dispersal, and dormancy).

population A group of interbreedingorganisms of the same species (or othergroups within which individuals may ex-change genetic information) occupying aparticular space. A population is continu-ally modified by increases (birth and immi-gration) and losses (death and emigration),and is limited by the effects of environmen-tal factors such as disease. Many plantscannot be divided strictly into populationsas they may reproduce vegetatively, withthe result that one individual can occupy alarge area and parts can detach.

porogamy The usual method of fertil-ization in angiosperms in which the pollentube enters the ovule by the micropyle.Compare chalazogamy.

porphyrins Cyclic organic structurescontaining tetrapyrrole rings (four joinedrings, each consisting of four CH units andone NH unit) that have the important char-acteristic property of forming complexes

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with metal ions. Examples of such metallo-porphyrins are the iron porphyrins (e.g.heme in hemoglobin) and the magnesiumporphyrin, chlorophyll, the photosyntheticpigment in plants. In nature, the majorityof metalloporphyrins are conjugated toproteins to form a number of very impor-tant molecules, such as the cytochromes.

potassium One of the essential elementsin plants. It is absorbed by plant roots asthe potassium ion, K+, and in plants is themost abundant cation in the cell sap. Potas-sium ions are required in high concentra-tions in the cell for efficient proteinsynthesis, and for glycolysis in which theyare an essential cofactor for the enzymepyruvate kinase. Potassium ions balanceanions and organic acids, and are thus im-portant in osmoregulation, the control ofstomatal opening, and leaf movements(through their effects on the turgor of pul-vini). A deficiency of potassium leads to acharacteristic red or purple coloration ofthe leaves, poor root growth, and flowerand fruit formation.

potometer An apparatus for measuringthe rate of water uptake by a cut shoot orwhole plant. This is normally closely re-lated to the rate of water loss by transpira-tion, and the potometer can be used tocompare transpiration rates under differ-ent conditions. The rate of uptake is mea-sured by the progress of an air bubble in acapillary tube along a scale. More mean-ingful comparisons can be made by com-paring water loss from the potometer withthat from an atmometer (an apparatusused to measure the rate of evaporationfrom a porous pot or other nonliving wetsurface), which allows uncontrolled evapo-ration.

PPLO See mycoplasmas.

PR (P660) See phytochrome.

Precambrian The time in the earth’s ge-ological history that precedes the Cam-brian period, i.e. from the origin of theearth, nearly 5 billion years ago, to the

start of the Cambrian, around 570 Ma(million years ago). The term ‘Precam-brian’ is now used mainly descriptively,and has been largely discarded as a geolog-ical term in the light of greater knowledgeof the early evolution of life. Precambriantime is now divided into three eons:Hadean, from the earth’s origin to about3900 Ma; Archaean, 3900–2390 Ma; andProterozoic, 2390–570 Ma (the Cambrianmarks the start of the Phanerozoic eon,which extends to the present day).

The oldest fossils discovered so far areremains of bacterialike organisms, datingfrom about 3500 Ma. There is abundantevidence of flourishing colonies of cyano-bacteria and other bacteria throughout theArchaean and Proterozoic eons. This takesthe form of STROMATOLITES, rock structuresrepresenting the remains of sedimenttrapped or precipitated by bacterial com-munities. Stromatolites are still found in afew special locations today. The ancientrocks also contained spherical carbona-ceous aggregates up to 20 mm diameter,that resemble algae or the cysts of flagel-lates, and delicate carbonaceous filamentsup to 150 m m long. The first appearanceof multicellular animals is in the so-calledEdiacara fauna, in rocks dated to the last100 million years of Precambrian time.Around this time the stromatolites beganto decline, perhaps as they were beingeaten by newly evolved multicellular ani-mals.

pressure potential See turgor pressure;water potential.

prickle A protective multicellular out-growth from the surface of a plant. It is amodified trichome and thus completelyepidermal in origin. Compare spine, thorn.

primary endosperm nucleus See en-dosperm.

primary growth Growth derived solelyfrom meristems present in the embryo, i.e.apical meristems. Such growth generallyincreases the length of plant organs. All thetissues of the young plant are the result of

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primary growth. Compare secondarygrowth.

primary phloem PHLOEM derived fromthe procambium in the young plant. It con-sists of the PROTOPHLOEM (formed first)and METAPHLOEM. In nonwoody plants it isthe only food-conducting tissue, but in ma-ture plants with secondary growth thisfunction is taken over by the secondaryphloem. See secondary growth.

primary structure The sequence ofamino acids in a polypeptide chain. Seealso conformation; secondary structure,tertiary structure; quaternary structure.

primary tissue Plant tissue that is de-rived solely from meristems present in theembryo and their derivatives. See primarygrowth.

primary xylem Xylem derived from theprocambium in the young plant. It consistsof the protoxylem (formed first) andmetaxylem. In nonwoody plants the pri-mary xylem is the only water-conductingtissue, but in mature plants with secondarygrowth this function is mostly taken overby the secondary xylem. See secondarygrowth.

primordium (pl. primordia) A collec-tion of cells that differentiates into a spe-cialized cell, tissue or organ, e.g. the apicalshoot and apical root primordia of the em-bryo. The term is used particularly in rela-tion to a part of the apical meristem thatlater differentiates further.

probability The chance that a givenevent will occur, or that over a series of ob-servations a particular kind of observationwill occur regularly as a given proportionof the total number of observations. Statis-tical probability is usually based on aninfinite number of observations. For exam-ple, in genetics, if a heterozygous plant isselfed, the probability of finding the doublerecessive is 1 in 4, or 25%. The greater thenumber of offspring the better the chancethat this actual percentage will be achieved.

procambium (procambial strand) Thelayer of cells that gives rise to the vasculartissue. Part of the apical meristem, it is dis-cernable just below the apex as a strand offlattened cells which, if traced back alongthe shoot or root, may be seen to give riseto the primary vascular tissues. It is contin-uous with the intrafascicular cambium. Inroots it may also be called plerome. See his-togen theory; primary growth.

procaryote See prokaryote.

producer The first TROPHIC LEVEL in afood chain. Producers are those organismsthat can build up foods from inorganic ma-terials, i.e. green plants, algae, and photo-synthetic and chemosynthetic bacteria.Producers are eaten by herbivores (primaryCONSUMERS).

proembryo A young plant after fertil-ization but before differentiation into em-bryo and suspensor tissue.

profundal The deepwater zone of a lakebeyond a depth of 10 meters. Little lightpenetrates this zone and thus the inhabi-tants are all heterotrophic, depending onthe littoral and sublittoral organisms forbasic food materials. Commonly found in-habitants include bacteria, fungi, mollusks,and insect larvae. Species found in the pro-fundal zone are adapted to withstand lowoxygen concentration, low temperatures,and low pH. Compare littoral; sublittoral;photic zone.

prokaryote (procaryote) An organismwhose genetic material (DNA) is not en-closed by membranes to form a nucleus butlies free in the cytoplasm. Organisms canbe divided into prokaryotes and EUKARY-OTES, the latter having a true nucleus. Inprokaryotes the nuclear material is simplya circular strand of DNA, not complexedwith histone proteins, and cell division is amitotic. This is a fundamental divisionbecause it is associated with other majordifferences. Prokaryotes constitute thekingdom Bacteria (formerly the Prokary-otae). Recent molecular evidence suggests

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the prokaryotes actually comprise two dis-tinct kingdoms, or even domains, the Ar-chaea and the Eubacteria or Bacteria.Eukaryotes comprise all other organisms.Prokaryote cells evolved first and gave riseto eukaryote cells. They are normally muchsmaller (about 1m m in diameter) than eu-karyote cells (about 20m m). Prokaryoteslack organelles such as nucleoli, mitochon-dria, plastids, Golgi apparatus, endoplas-mic reticulum, and Golgi apparatus, anddo not exhibit cyclosis. Their ribosomesare smaller (70S) than those of eukaryotes(80S) – similar in size to the ribosomes ofchloroplasts and mitochondria. This hasled to the theory (endosymbiont theory)that eukaryotes arose as a result of symbi-otic associations between prokaryotes. SeeBacteria; cell; endosymbiont theory.

proline A nonpolar molecule synthe-sized from glutamic acid and broken downto glutamic acid by the same pathway. Thederivative 4-hydroxyproline is an impor-tant component of cell walls. Proline hasbeen found in every protein studied to date.See amino acids.

promoter A specific DNA sequencewithin an OPERON that initiates TRANSCRIP-TION by binding RNA polymerase. Thepromoter is situated between the operatorand the structural genes or genes. It is thestarting point for transcription of the struc-tural genes in the operon, but is not itselftranscribed. In Escherichia coli the RNApolymerase has a protein ‘sigma factor’that recognizes the promoter; in the ab-sence of this factor the enzyme binds to,and begins transcription at, random pointson the DNA strand. In eukaryotic cells,binding of RNA polymerase to the pro-moter involves proteins called transcrip-tion factors.

prophage See lysogeny.

prophase The first stage of cell divisionin meiosis and mitosis. During prophasethe chromosomes become coiled, shorten-ing and thickening and becoming visible,and the nuclear membrane dissolves.

Prophase may be divided into successivestages termed leptotene, zygotene, pachy-tene, diplotene, and diakinesis. In leptotenethe chromosomes, already replicated, startto condense and appear as fine threads, al-though sister chromatids are not yet dis-tinct. The spindle starts to form around theintact nucleus. Zygotene is characterizedby the active and specific pairing (synapsis)of homologous chromosomes leading tothe formation of a haploid number of biva-lents. In pachytene the paired homologouschromosomes contract. At this point eachchromosome consists of a pair of chro-matids and the two associated chromo-somes are termed a tetrad. In diplotene thepairs of chromatids begin to separate fromthe tetrad formed by the association of ho-mologous chromosomes. Chiasmata canoften be seen at this stage. At diakinesis thesister chromatids of homologous pairs ofchromosomes complete their separation,and the chromosomes coil tightly, shorten-ing and thickening. In practice, there is noclear demarcation between these stages:the process is continuous. The events oc-curring during these stages differ in meiosisand mitosis, notably in that bivalents (pairsof homologous chromosomes) are formedin meiosis, whereas homologous chromo-somes remain separate in mitosis. See alsomeiosis; mitosis.

proplastid A self-duplicating undiffer-entiated plastid, about 0.5–1 m m in diam-eter and found in the meristematic regionsof plants. They grow and develop into plas-tids of different types.

prop root An adventitious root, arisingfrom one of the lower nodes of the stemclose to the soil surface, found at the junc-tion of stem and soil that grows down tothe ground and gives additional support tothe stem, as seen in maize (Zea mays).Some palms, mangroves and other tropicaltrees form woody prop roots, often calledstilt roots. See also buttress root.

prosthetic group The nonprotein com-ponent of a conjugated PROTEIN. Thus theheme group in hemoglobin is an example

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of a prosthetic group, as are the coenzymecomponents of a wide range of enzymes.e.g. the heme group is an iron–porphyrincomplex that acts as a prosthetic group inthe cytochromes, important components ofelectron-transport chains in both plantsand animals. See enzyme.

protandry The maturation of the an-thers before the stigma, a very commoncondition seen in many specialized insect-pollinated flowers, especially those polli-nated by bees. It helps to promotecross-pollination and is often combinedwith self-incompatibility. Compare pro-togyny. See also dichogamy.

protease (proteinase) An enzyme thatcatalyzes the hydrolysis of peptide bonds inproteins to produce peptide chains andamino acids. Individual proteases arehighly specific in the type of peptide bondthey hydrolyze.

protein A large, complex moleculemade up of one or more polypeptidechains, i.e. it consists of amino acid mol-ecules joined together by peptide links. Themolecular weight of proteins may varyfrom a few thousand to several million.About 20 amino acids are present in pro-teins. Simple proteins contain only aminoacids. In conjugated proteins, the aminoacids are joined to other groups.

The primary structure of a protein is theparticular sequence of amino acids present;this determines the three-dimensionalshape of the molecule. The secondarystructure is the way in which this chain isarranged; for example, coiled in an alphahelix or held in beta-pleated sheets. Thesecondary structure is held by hydrogenbonds. The tertiary structure of the proteinis the way in which the protein chain isfolded. This may be held by cystine bonds(disulfide bridges) and by weak polar andhydrophobic attractive forces betweenatoms. The quaternary structure is thecombination of more than one polypeptidechain. There are two main groups of pro-teins: globular proteins and fibrous pro-teins. Globular proteins have the more

complex tertiary structure, and are mostlyenzymes. Fibrous proteins usually have astructural or contractile role. Proteins arevital compounds found in all living organ-isms. Their functions range from enzymesand hormones to electron-transfer mol-ecules, contractile elements, and importantgenetic elements such as the histones thatcoat DNA in eukaryotic chromosomes. Bycoding for structural proteins and for theenzymes that catalyze the synthesis ofother proteins and biological molecules,genes control the functioning and develop-ment of all living organisms.

proteinase See protease.

protein sequencing The determinationof the primary structure of proteins, i.e. thetype, number, and sequence of amino acidsin the polypeptide chain. The most widelyused procedure is the Edman degradation.This uses a series of three chemical reac-tions, each taking place at a specific pH, toremove one amino acid at a time from theamino terminus of a protein. The aminoacids are then identified by ion-exchangechromatography. The process is auto-mated in an amino acid analyzer. TheEdman degradation can sequence peptideswith only 5–10 picomoles of protein pre-sent.

protein synthesis The process wherebyproteins are synthesized on the ribosomesof cells. The sequence of bases in MESSEN-GER RNA (mRNA), transcribed from DNA,determines the sequence of amino acids inthe polypeptide chain: each CODON in themRNA specifies a particular amino acid.As the ribosomes move along the mRNA inthe process of translation, each codon is‘read’, and amino acids bound to differenttransfer RNA molecules are brought totheir correct positions along the mRNAmolecule. These amino acids are polymer-ized to form the growing polypeptidechain. See also transfer RNA; translation.

proteolysis The hydrolysis of proteinsinto their amino acids. Enzymes that cat-

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alyze this are proteases or proteolytic en-zymes.

proteomics The compilation and char-acterisation of all the proteins in an organ-ism – its proteome.

proteoplast A colorless plastid (leuko-plast) that stores protein.

Proterozoic See Precambrian.

prothallus (pl. prothalli) A flatteneddisk of cells that forms the free-living hap-loid gametophyte generation of certainferns, e.g. Dryopteris. In homosporousplants, there is only one type of prothalluswith both male and female sex organs. Inheterosporous plants, the microspores giverise to small male prothalli bearing malesex organs (antheridia), and larger femaleprothalli bearing female sex organs(archegonia). The prothallus is greatly re-duced in spermatophytes. See alternationof generations.

Protista In some classifications, a king-dom of simple eukaryotic organisms in-cluding the algae and protozoans. It wasintroduced to overcome the difficulties ofassigning such organisms that may showboth animal and plantlike characteristicsto the kingdoms Animalia or Plantae.Today the grouping is considered to be ar-tificial and many taxonomists support theFIVE KINGDOMS CLASSIFICATION whereby thebacteria and fungi are both assigned to sep-arate kingdoms, while algae and proto-zoans constitute various phyla of thekingdom PROTOCTISTA.

protocooperation See mutualism.

Protoctista A kingdom of simple eu-karyotic organisms that includes the algae,slime molds, funguslike oomycetes, and theorganisms traditionally classified as proto-zoa, such as flagellates, ciliates, and sporo-zoans. The members of this kingdom donot share clear phylogenetic links, but aregrouped together simply because they donot not belong in any of the other four

kingdoms. Most are aerobic, some are ca-pable of photosynthesis, and most possessundulipodia (flagella or cilia) at some stageof their life cycle. Protoctists are typicallymicroscopic single-celled organisms, suchas the amebas, but the group also has largemulticellular members, for example theseaweeds and other large algae, some ofwhich are sometimes classified as plants.Protoctists show a wide range of nutri-tional habits, including photoautotrophs,heterotrophs, phagotrophs and mixo-trophs.

protoderm The outermost layer of theapical meristem: the tissue that developsfrom the tunica initials of the apical meris-tem and gives rise to the epidermis and, insome plants, the root cap. In the root itmay also be called the dermatogen. See his-togen theory.

protogyny The maturation of thestigma before the anthers. It is a mecha-nism that helps prevent self-pollinationand is most common among wind-polli-nated and unspecialized insect-pollinatedflowers. as in figwort. It is less commonthan protandry. Compare protandry. Seealso dichogamy.

protonema (pl. protonemata) The youngbryophyte gametophyte that develops fol-lowing spore germination. The mature ga-metophyte plants develop from buds thatform at several points along the pro-tonema.

proton pump An energy-requiring meta-bolic process that causes protons (hydro-gen ions) to move across a membraneagainst their concentration gradient. Thiscreates a potential difference across themembrane, a source of potential energythat can be used to drive other metabolicprocesses. For example, the inner mem-brane of the mitochondrion is imperme-able to the movement of protons. Duringrespiratory electron transport, protons aredrawn from the internal matrix into thespace outside the membrane, creating a po-tential difference across the membrane. In

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respiration and photosynthesis, the move-ment of protons back across a membranedown a diffusion gradient takes place atsites where it can be coupled to ATP syn-

thesis, using the enzyme ATPase. The en-ergy thus trapped can be transported toother parts of the cell. See chemiosmotictheory; electron-transport chain.

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Pseudocarp: various types of pseudocarp

protophloem The first-formed PRIMARY

PHLOEM, differentiated from the procam-bium in the region just behind the meri-stem.

protoplasm The living contents of acell, comprising the cytoplasm plus nucleo-plasm. The term usually excludes the largevacuoles. See cytoplasm; nucleus.

protoplast The protoplasm and plasmamembrane of a cell after removal of the cellwall, where present. This can be achievedby physical means or by enzymic digestion.

protostele A simple form of stele, unin-terrupted by leaf gaps, consisting of xylemin the center completely surrounded byphloem. Most roots have protosteles. Thestems of some lower plants (e.g. Ly-copodium) also have protosteles.

protoxylem The PRIMARY XYLEM el-ements that are formed from the procam-bium first. In seed plants the protoxylemoccurs at the innermost edge of the vascu-lar bundles in stems, but external to thelater-formed xylem in roots.

protozoa An old name for animal-likePROTOCTISTA that included some photosyn-thetic organisms (e.g. Euglena, Volvox).

proximal Denoting the part of an organthat is nearest the origin or point of attach-ment. Compare distal.

pseudoallele A mutation in a gene thatproduces an effect similar to another muta-tion at a different site in the same genelocus. They are closely linked, so recombi-nations between them are rare. See cis–trans effect.

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Ptyxis: various types of leaf ptysis

pseudocarp (false fruit) A fruit that in-cludes parts of the flower, e.g. the bracts,inflorescence, or receptacle, other thanthose derived from the ovary. See also hip;fruit; pome. See illustration on page 193.

pseudogene A DNA sequence that can-not be transcribed. Although they have noimmediate function, pseudogenes havehigh potential to form new genes by furthermutation as they already have useful se-quences, such as those signaling transcrip-tion. Pseudogenes are genes that havebecome ‘switched off’ during the course ofevolution.

pseudoparenchyma A fungal or algaltissue resembling parenchyma but made upof interwoven hyphae (fungi) or filaments(algae). The stipe of the mushroom and thethallus of red algae (e.g. Porphyra) arepseudoparenchymatous tissues.

pseudoplasmodium See Acrasiomy-cota.

pseudopodium (pl. pseudopodia) 1. Aleafless stalk that bears the capsule inSphagnum and other mosses that lack aseta.

2. In slime molds, a projection of the bodyused for locomotion.3. A temporary fingerlike projection orlobe on the body of an ameboid cell. It isformed by a flowing action of the cyto-plasm and functions in locomotion andfeeding.

Psilophyta (whisk ferns) In the FiveKingdoms classification scheme, a phylumof vascular seedless plants, the whisk ferns. In most modern classifications theyare placed in the class Psilopsida of theFilicinophyta. There are only two livingsubtropical genera (Psilotum and Tmesip-teris).

psychrophilic Describing microorgan-isms that can live at temperatures below20°C. Compare mesophilic; thermophilic.

pteridophyte (Pteridophyta) A generalterm, now largely obsolete, to include anyvascular nonseed- bearing plant. Pteri-dophytes include the club mosses (phy-lum Lycophyta), horsetails (Sphenophyta),ferns (Filicinophyta), and whisk ferns(Psilophyta).

ptyxis The way in which young leaves

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are folded or rolled in the bud. See also ver-nation.

pulvinus (pl. pulvini) 1. A specializedgroup of cells with large intercellularspaces that are located at the bases ofleaves or leaflets in certain plants. They areinvolved in nongrowth nastic movements,bringing these about by rapid changes inturgor through loss of water to the inter-cellular spaces. See nyctinasty.2. A thickened region at the node of a grassstem, which often contains an INTERCALARY

MERISTEM.

punctuated equilibrium A theory ofevolution proposing that there have beenlong periods of geological time, lasting forseveral million years, when there is littleevolutionary change, punctuated by shortperiods of rapid speciation of less than100 000 years. This is in contrast to thetraditional theory (see neo-Darwinism) inwhich it is postulated that species haveevolved gradually throughout geologicaltime.

Punnett square A chequerboard dia-gram used in genetics to illustrate the fre-quencies of various genotypes in theoffspring of a particular cross.

pure line The succession of descendantsof an individual that are identical to eachother and continue to breed true, i.e. theyproduce genetically identical offspring.They are assumed to be homozygous.

purine A simple nitrogenous organicmolecule with a double-ring structure – a6-membered pyrimidine ring fused to a 5-membered imidazole ring. Members of thepurine group include adenine and guanine,which are constituents of the nucleic acids,and certain plant alkaloids, e.g. caffeineand theobromine.

pyramid of biomass A type of ecologi-cal pyramid based on the total amount ofliving material at each trophic level in thecommunity, which is normally measuredby total dry weight or calorific value per

unit area or volume, and shown diagram-matically. The biomass depends on theamount of carbon fixed by green plantsand other producers. The pyramid of bio-mass usually has a more gentle slope thanthe PYRAMID OF NUMBERS because organ-isms at successively higher levels in thepyramid tend to be larger than thosebelow.

pyramid of numbers A type of ecolog-ical pyramid in which the number of indi-vidual organisms at each stage in the foodchain of the ecosystem is depicted dia-grammatically. The producer level formsthe base, and successive levels the tiers. Theshape of the pyramid of numbers dependsupon the community considered; generally,the organism forming the base of a foodchain is numerically very abundant, and

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quadrat A square area of variable sizedepending on the study (standard sizes are0.5 or one meter square to 10 m square ormore) taken at random, within which thecomposition of organisms is noted. Thequadrat sampling technique is mostly usedin plant ecology to study the distributionand abundance of species in plant commu-nities.

qualitative inheritance See discontinu-ous variation.

qualitative variation See discontinu-ous variation.

quantitative inheritance See continu-ous variation.

quantitative variation See continuousvariation.

quaternary structure The pattern inwhich two or more polypeptide chains arelinked together to form a protein molecule.This involves hydrogen bonds, ionicbonds, disulfide bridges and hydrophobicinteractions. See conformation; protein.Compare primary structure; secondarystructure; tertiary structure.

Quaternary (Neogene) The most recentperiod of the Cenozoic era from about twomillion years ago to the present day andcomposed of the Pleistocene and Holoceneepochs. It was a period of cyclical climatechange, including several ice ages in highand mid-latitudes, cycles of varying aridityand humidity in low latitudes, and signifi-cant fluctuations in sea level. This had im-portant implications for plant speciation,as species ranges expanded and contracted,new species radiated out as climate amelio-rated, and retreating ice afforded opportu-nities for colonization of newly exposedland. The Quaternary is also characterizedby the emergence of humans and the startof human modification of the environ-ment. Increased rates of evolution and ex-tinction of mammals (due to both climatechange and human activity) also affectedplant distribution. See also Ice Age.

quiescent center A group of cells in thecenter of the apical meristem in which mi-totic divisions are rare or absent. The cellsmay begin dividing if another part of themeristem is damaged.

quillworts See Isoetales.

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race In classification, a taxon below therank of SPECIES, sometimes being placed be-tween subspecies and variety. The term issometimes used instead of ECOTYPE, forgroups of individuals that have uniformecological preferences or physiological re-quirements.

raceme (racemose inflorescence; indefiniteinflorescence) See inflorescence.

rachis 1. The main stalk on which theleaflets are borne in a compound leaf, or onwhich the pinnae are attached in the fernleaf.2. The central axis of certain inflores-cences, e.g. wheat (Triticum), that bearsthe spikelets.

radial 1. Aligned along the radius of acircle or cross-section.2. See anticlinal.

radial symmetry The arrangement ofparts in an organism in such a way thatcutting in any plane across the diametersplits the structure into similar halves (mir-ror images). Many stems and roots showradial symmetry. The term actinomorphyis generally used to describe radial symme-try in flowers. See also bilateral symmetry.

radicle The embryonic root. It is the firstorgan to emerge from the seed on germina-tion. The radicle is joined to the hypocotyland both tissues are derived from the fouroctants nearest the suspensor. The root capover the tip of the radicle is derived fromthe closest cell of the suspensor. The radi-cle may persist to form a taproot, or it maybe replaced by adventitious roots.

radioactive dating See radiometric dat-ing.

radiometric dating (isotopic dating; ra-dioactive dating) Any method of datingthat uses the decay rates of naturally oc-curring radioactive isotopes to assess theage of a specimen or material. Organicmatter less than 7000 years old can bedated using radiocarbon dating. This usesthe fact that the isotope carbon-14 is foundin the atmosphere and taken in by plantswhen they photosynthesize, and subse-quently assimilated by the animals thatfeed on them. When plants and animalsdie, no more carbon is taken in and the ex-isting 14C decays to the nonradioactive iso-tope carbon-12. If the proportion of 14C to12C in the atmosphere and the decay rate of14C to 12C are both known, as they are,then the sample may be dated by findingthe present proportion of 14C to 12C. Spec-imens of rocks over 7000 years old can be dated by other radioisotope methods,e.g. potassium–argon dating and rubid-ium– strontium dating.

rainforest See forest.

random amplified polymorphic DNA(RAPD) The generation of copies of seg-ments of DNA by means of the polymerasechain reaction, whereby the segments mul-tiplied are those that lie between two iden-tical binding sites for a particular primer,usually about 10 base pairs long. Severaldifferent segments of DNA may fall intothis category, and the amplified DNA canbe sorted using ELECTROPHORESIS. Differentindividuals may differ in the number andlocation of such sites, giving rise to a kindof genetic fingerprint. See genetic finger-

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printing; molecular systematics; poly-merase chain reaction.

Ranunculaceae A highly variable fam-ily of about 2450 species of dicotyledonousplants that includes the buttercups, monks-hoods, anemones, clematis, delphiniums,and hellebores.

raphe 1. In some seeds, a ridge formedby the portion of the funicle that is fusedwith the integument.2. A slit in the valve of motile diatoms,thought to be involved in movement by al-lowing contact between cytoplasm andsubstrate.

raphides Bunches of needlelike crystalsof calcium oxalate found in certain plantcells.

Raunkiaer’s plant classification Aclassification of growth forms based on thepersistence of the shoots and the positionof the resting buds. The system simplifiesassessment of the percentages of differentplant forms in any given type of vegetation,specially in temperate communities. Seealso chamaephyte; cryptophyte; helophyte;hemicryptophyte; phanerophyte; thero-phyte.

Ray, John (1627–1705) English natu-ralist and taxonomist. Ray, from a humblebackground, was appointed a Cambridgefellow in 1648 but lost his fellowship in1662, having refused to take the oath re-quired by the Act of Uniformity. His

friends at Cambridge, especially FrancisWilloughby, helped to finance him in hisfurther studies. His life’s work was an at-tempt to describe all the known flora andfauna. With Willoughby, he traveledthroughout Europe to study a wide rangeof plants and animals. In 1667 he pub-lished a catalog of British plants and in thesame year was elected to the Royal Society.On Willoughby’s death in 1672 he in-herited enough money to continue theirendeavor. His three-volume Historia plant-arum was published between 1686 and1704. He is best known for fixing thespecies as the basic unit in taxonomy andalso for realizing the importance of distin-guishing between monocotyledons and di-cotyledons.

ray floret See inflorescence.

ray fungi See Actinobacteria.

ray initial See initial.

reaction center In PHOTOSYNTHESIS, ahypothetical site at which absorbed lightenergy is used to transport electrons forphotophosphorylation. It contains a mol-ecule of the primary pigment (in plants thisis chlorophyll a, which transfers the elec-trons. Plants have two reaction centers inthe thylakoids of the chloroplast: Photo-systems I and II.

Recent See Holocene.

receptacle 1. The tip of the angiosperm

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flower stalk upon which the floral organsare inserted. If the ovary is on top of the re-ceptacle with the other organs it is said tobe superior but if it is sunk in the center,e.g. in the daffodil (Narcissus), it is said tobe inferior. See also epigyny; hypogyny;perigyny.2. The swollen portion of the lamina(blade) bearing the conceptacles in certainalgae (e.g. Fucus).

recessive An allele that is only expressedin the phenotype when it is in the homozy-gous condition. Compare dominant. Seealso double recessive.

reciprocal cross A cross that testswhether the inheritance of a particularcharacter is affected by the sex of the par-ent. The cross is thus made both ways, i.e.the character under consideration is car-ried by the female in one cross and by themale in the second cross. The procedurecan demonstrate which characters are con-trolled by sex-linked genes.

recombinant DNA 1. DNA producedby recombinant DNA technology.2. DNA formed naturally by recombina-tion, e.g. by crossing over in meiosis or byconjugation in bacteria. See recombina-tion.

recombinant DNA technology Thetechnique by which foreign DNA, whetherfrom another organism or genetically engi-neered, is inserted into another DNA frag-ment or molecule. The product –recombinant DNA – is fundamental tomany aspects of GENETIC ENGINEERING, par-ticularly the introduction of foreign genesto cells or organisms. There are now manytechniques for creating recombinant DNA,depending on the nature of the host cell ororganism receiving the foreign DNA. Par-ticular genes or DNA sequences are cutfrom the parent molecule using specifictype II RESTRICTION ENDONUCLEASES, or areassembled using a messenger RNA tem-plate and the enzyme REVERSE TRANSCRIP-TASE. In GENE CLONING using cultures ofbacterial or eukaryote tissue cells, the for-

eign gene is inserted into a VECTOR, e.g. abacterial plasmid or virus particle, whichthen infects the host cell. Inside the hostcell the recombinant vector replicates andthe foreign gene is expressed. One of themost common vectors used with plants is the Ti (tumor-inducing) plasmid of thebacterium Agrobacterium tumefaciens.This causes crown gall tumors in plants,and its plasmid has been used on a range ofcrop plants.

recombination The regrouping of genesthat regularly occurs during meiosis as a re-sult of the independent assortment of chro-mosomes into new sets, and the exchangeof pieces of chromosomes (crossing over).Recombination results in offspring thatdiffer both phenotypically and genotypi-cally from both parents (recombinants)and is the most important means of pro-ducing variation in sexually reproducingorganisms.

red algae See Rhodophyta.

Red Data Book An updatable set of in-formation on rare, threatened, and endan-gered species worldwide, compiled by theWorld Conservation Monitoring Center,and published at regular intervals by theSpecies Survival Commission of the WorldConservation Union (WCU).

red desert soil A type of coarse SOIL richin salts and lime but poor in humus,formed in hot deserts.

red light Electromagnetic radiation ofwavelength approximately 630 nm. It isthe most effective wavelength for initiatingmany light-dependent reactions in plants,such as the germination of light-sensitiveseeds. See photoperiodism; phytochrome.

red tropical soil A type of SOIL formedunder rainforests and in savannas in areasof alternating wet and dry seasons. It isheavily leached, but nutrients released bydecomposition help to maintain the min-eral balance. The red color is due to oxidesof iron, magnesium, and aluminum, which

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in some such soils form a hard crust (LAT-ERITE) at the soil surface.

reducing sugar A sugar that can reducecertain inorganic ions in solution, such asthe cupric ions in Benedict’s and Fehling’ssolutions. Examples are glucose, lactose,and meltose.

reduction division The first division ofmeiosis, including prophase, metaphase I,and anaphase I. It results in a haploid num-ber of chromosomes gathering at each endof the nuclear spindle. See meiosis.

reflexed Bent backwards.

regeneration The regrowth by an or-ganism of an organ or tissue that has beenlost by injury or other means. It applies tonatural regrowth from living roots or cop-piced stems as well as artificial regrowth inthe laboratory. Many plant propagationtechniques make use of the ability of cut-tings of shoots, roots, or leaves to regener-ate into complete plants.

regulator gene A gene whose productcan promote or prevent the TRANSCRIPTION

of structural genes, which may or may notbe adjacent to other regulator genes, andmay even be on another chromosome. Theregulator may code for a REPRESSOR proteinthat switches off an operator gene (in bac-teria). In eukaryotes it may bind to proteinsin a transcription complex to inhibit its ac-tion, or it may bind to a TRANSCRIPTION fac-tor. See also operon; promoter.

relative humidity The ratio of theamount of water vapor present in a body ofair at a given temperature compared to themaximum quantity of water vapor thatbody can contain at the same temperature,i.e. when saturated at that temperature.Relative humidity affects the diffusion gra-dient for TRANSPIRATION and evaporation,which proceed more slowly with increasein relative humidity.

release factor See translation.

rendzina A type of brown earth soil typ-ical of humid to semi-arid grasslands overcalcareous rocks. The rendzina is rich inlime and has an upper humus-rich horizon.

repetitive DNA DNA that consists ofmultiple repeats of the same nucleotide se-quences (repeated sequences). In someplants, such as lilies, repeats may constituteover 50% of the genome, leading to unex-pectedly large genomes. The repeats mayoccur together, or they may be widely scat-tered. Unlike prokaryotic cells, eukaryoticcells contain appreciable amounts of repet-itive DNA. For example, satellite DNA(tandem DNA) is made up of up to onemillion adjacent repeats of the same shortsequence, usually not more than 10 basepairs long. It occurs in the region of cen-tromeres and telomeres, and is described ashighly repetitive DNA. This DNA oftenhas a nucleotide composition significantlydifferent from the rest of the cell’s DNA,giving it a different density, so it shows upas a separate band (the ‘satellite’) whencentrifuged. Microsatellite DNA, withshort sequences repeated only 10 to 100times, is used in DNA fingerprinting. Somerepetitive DNA is accounted for by multi-ple copies of particular genes, e.g. genes en-coding histones and ribosomal RNA. Seealso junk DNA; transposon.

replica In a general sense an exact copy.In biology specifically used for a thin de-tailed copy of a biological specimen, ob-tained by spraying the surface with a layerof plastic and carbon. Replicas are used inelectron-microscope work.

replication The mechanism by whichexact copies of the genetic material areformed. Replicas of DNA are made whenthe double helix unwinds as a result of he-licase enzyme action and the separatedstrands serve as templates along whichcomplementary nucleotides are assembledthrough the action of the enzyme DNApolymerase. The result is two new mol-ecules of DNA each containing one strandof the original molecule, and the process is termed semiconservative replication.

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Replication of DNA occurs in interphase;by the end of interphase the replicatedDNA and associated histones are in theform of chromosomes each consisting oftwo chromatids joined at the centromere.In RNA viruses an RNA polymerase is in-volved in the replication of the viral RNA.

repressor A protein molecule that pre-vents protein synthesis by binding to theoperator sequence of the gene and prevent-ing transcription. The molecule is pro-duced by a regulatory gene and may acteither on its own or in conjunction with asecond protein called a corepressor. Insome cases another molecule, an inducer,may bind to the repressor, weakening itsbonds with the operator and derepressingthe gene, allowing transcription to pro-ceed. Such a molecule is termed an effector.See operon.

reproduction See asexual reproduction;sexual reproduction.

resin One of a group of substances oc-curring in many trees and shrubs, includingconifers and certain families of floweringplants. They are usually secreted by specialcells into long resin ducts or canals. Some-times they are produced in response to in-jury or infection. They sometimes form asticky covering to buds (e.g. horse chest-nut), reducing transpiration and givingprotection.

resolving power (resolution) The abil-ity of an optical system to form separateimages of closely spaced objects. It dependsupon the perfection of the lens, and is theo-retically limited by the wavelength of light,being greater with decreasing wavelength.See microscope.

respiration The oxidation of organicmolecules to provide energy in plants andanimals. Autotrophic plants respire mol-ecules that they have synthesized by photo-synthesis. Respiration occurs in all cells,whether or not they are capable of photo-synthesis. The energy from respiration isused to attach a high-energy phosphate

group to ADP to form the short-term en-ergy carrier ATP, which can then be usedto power energy-requiring processes withinthe cell. ATP is not transported betweencells, but is made in the cell where it is re-quired.

The chemical reactions of respirationnormally require oxygen from the environ-ment (AEROBIC RESPIRATION). Some organ-isms are able to respire, at least for a shortperiod, without the use of oxygen (ANAER-OBIC RESPIRATION), although this processproduces far less energy than aerobic res-piration. A few bacteria can survive in-definitely in anaerobic conditions. Thecomplex reactions of cell respiration fallinto two stages, GLYCOLYSIS and the KREBS

CYCLE. The first stage, glycolysis, occurswithin the cytoplasm, but the Krebs cycleenzymes are localized within the mitochon-dria of eukaryotes; cells with high rates ofrespiration have many mitochondria. Gly-colysis results in partial oxidation of therespiratory substrate to the 3-carbon com-pound pyruvate. It does not require oxy-gen, so can occur in anaerobic conditions,when the pyruvate is converted to ethanolin plants and most bacteria, or to lacticacid in certain bacteria. The yield of gly-colysis is 2 molecules of ATP plus 2 mol-ecules of the reduced coenzyme NADH2for each molecule of glucose respired. Inthe presence of oxygen, the pyruvate is fur-ther oxidized in the Krebs cycle. In theKrebs cycle, which requires free oxygen,pyruvate is converted into the 2-carbonacetyl group, which becomes attached to acoenzyme forming acetyl coenzyme A. Thisthen enters a cyclic series of reactions dur-ing which carbon dioxide is evolved andhydrogen atoms are transferred to thecoenzymes NAD and FAD. The energy re-leased by the Krebs cycle is transferred viathe reduced coenzymes NADH2 andFADH2 to an ELECTRON-TRANSPORT CHAIN

embedded in the inner mitochondrialmembrane. According to the chemiosmotictheory, electrons flow through the variouscomponents of the chain, while a gradientof hydrogen ions is maintained by an en-ergy-requiring (active) proton pump thatdrives protons into the intermembrane

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space. This gradient drives the formationof ATP. Overall, 38 molecules of ATP aregenerated for each molecule of glucose ox-idized during aerobic respiration, com-pared with only 2 molecules of ATP duringanaerobic respiration. At the end of theelectron-transport chain the electrons reactwith protons and oxygen to give water. Inaerobic respiration, therefore, pyruvate iscompletely oxidized to carbon dioxide andwater.

respiratory chain See electron-trans-port chain.

respiratory quotient (RQ) The ratio ofthe volume of carbon dioxide evolved dur-ing RESPIRATION to the volume of oxygenabsorbed:

RQ = volume CO2/volume O2The complete breakdown of sugars by aer-obic respiration gives an RQ of 1. Thisrises if anaerobic respiration is involved.The respiration of proteins or fats pro-duces less carbon dioxide, so the RQ fallsbelow one.

response A change in an organism or inpart of an organism that is produced as areaction to a stimulus.

restriction endonuclease (restriction en-zyme) A type of enzyme, found mainly inbacteria, that can cleave and fragmentDNA internally (see endonuclease). Theirfunction is to protect the cell against for-eign DNA, such as the DNA of an invadingvirus. Some restriction endonucleasescleave DNA at random, but a particulargroup of enzymes, known as class IIrestriction endonucleases, cleave DNA atspecific sites. The discovery of these en-zymes formed the basis for the develop-ment of genetic engineering, since theyenable the isolation of particular gene se-quences and the DNA fragments are repli-cated by means of base pairing and DNAligases. See also recombinant DNA tech-nology.

restriction fragment length polymor-phism (RFLP) Variation among the

members of a population in the sites atwhich restriction enzymes cleave the DNA,and hence in the size of the resulting DNAfragments. It results from differences be-tween individuals in nucleotide sequencesat the cleavage sites (restriction sites). Thepresence or absence of particular restric-tion sites can be ascertained using DNAprobes in the technique called SOUTHERN

BLOTTING. Restriction sites vary enor-mously, and this variation is exploited inanalyzing and comparing the genomes ofdifferent individuals, e.g. to establish howclosely related they may be. Restrictionsites are also invaluable as genetic markersin chromosome mapping, and can be usedto track particular genes. RFLPs are used inGENETIC FINGERPRINTING. See chromosomemap.

restriction map A map of a segment ofDNA showing the cleavage sites of restric-tion endonucleases and their physical dis-tance apart, usually measured in basepairs. It can be used to reveal variations inrestriction sites between individuals of thesame species or between different species(see restriction fragment length polymor-phism). This variation serves as a key to theorganism’s genes, since the restriction sitescan be used as markers to identify closelylinked genetic loci and allow investigationof deletions, insertions, or other mutations.RFLPs are an essential tool in chromosomemapping.

reticulate thickening The type ofsecondary-wall formation in which an ir-regularly branching mass of lignified de-posits covers the inner wall of the cell. It isusually found in metaxylem tracheids andvessels. Compare annular thickening;scalariform thickening.

reverse transcriptase An enzyme thatcatalyzes the synthesis of DNA from RNA(i.e. the reverse of transcription, in whichmRNA is synthesized from a DNA tem-plate). The enzyme occurs in certain RNAviruses and enables the viral RNA to be‘transcribed’ into DNA, which is then inte-grated into the host DNA and replicates

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with it. It is also used in genetic engineeringto make complementary DNA (cDNA)from an RNA template.

RFLP See restriction fragment lengthpolymorphism.

Rf value In chromatography, a ratiothat expresses the degree to which a solutemoves in a solvent. It is calculated by di-viding the distance moved by the spot ofsolute on the chromatogram column by thedistance traveled by the solvent front. TheRf value is constant for a particular mol-ecule and serves to identify it.

Rhizobium A spherical or rod-shapednitrogen-fixing bacterium that can live ei-ther freely in the soil or symbiotically in theroot nodules of leguminous plants and afew other species, such as alder (Alnus). Seealso nitrogen cycle; nitrogen fixation.

rhizoid A unicellular rootlike structurefound in certain algae and in the gameto-phyte generation of the bryophytes andsome ferns. It serves to anchor the plantand absorb water and nutrients.

rhizome A stem that grows horizontallybelow ground. Rhizomes may be fleshy,e.g. Iris, or wiry, e.g. couch grass (Elytrigiarepens) and may serve as an organ ofperennation or vegetative propagation, oroccasionally both. Compare corm; tuber;stolon. See also rootstock.

Rhodophyta (red algae) A phylum ofprotoctists comprising aquatic, mainly ma-rine, algae characterized by their red color.The color results from photosynthetic pig-ments, phycoerythrin and phycocyanin(see phycobilins). Most species are marine,a few freshwater or terrestrial; their distri-bution ranges from the shore to quite deepwater.

rhytidome See bark.

riboflavin (vitamin B2) A flavin pigmentthat is thought to be involved in the recep-tion of the light stimulus in phototropism,

and in the photooxidation of endogenousauxins. It is one of the water-soluble B-group of VITAMINS. It is found in cerealgrains, peas, and beans. Riboflavin is aconstituent of several enzyme systems(flavoproteins), acting as a coenzyme forhydrogen transfer in the reactions cat-alyzed by these enzymes.

ribonuclease (RNase) Any nuclease en-zyme that cleaves the phosphodiesterbonds between adjacent nucleotides inRNA. Exoribonucleases cleave nucleotidesfrom one or both ends of the RNA mol-ecule, while endoribonucleases cleavebonds within the molecule. RNase isthought to be involved in the incompatibil-ity system that prevents pollen of certaingenetic composition from germinating onthe style of a flower.

ribonucleic acid See RNA.

ribose A monosaccharide, C5H10O5; acomponent of RNA.

ribosomal RNA (rRNA) See ribosome.

ribosome A small organelle that is thesite of protein synthesis in the cell. Ribo-somes are found in large numbers in allcells, free in the cytoplasm or bound to EN-DOPLASMIC RETICULUM. In most species theyare composed of roughly equal amounts ofprotein and ribosomal RNA (rRNA). Theribosome consists of two unequally sizedrounded subunits arranged on top of eachother like a cottage loaf. Eukaryotic cellshave larger ribosomes than prokaryoticcells but the ribosomes in mitochondriaand chloroplasts are about the same size asprokaryotic ribosomes.

The ribosome comprises different typesof rRNA and associated proteins. TherRNA interacts directly with the mRNAduring initiation of translation, and thereare specific sites on the rRNA that interactwith anticodons on the transfer RNAs(tRNA). The proteins have various roles,including keeping the mRNA in a single-stranded state so that the bases are exposedfor TRANSLATION. During translation the

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ribosome moves along the messenger RNA(mRNA), enabling the peptide linkage ofamino acids delivered to the site by trans-fer RNA molecules according to the codein mRNA. Several ribosomes may be ac-tively engaged in protein synthesis alongthe same mRNA molecule, forming apolyribosome, or polysome.

ribozyme Any RNA molecule that actsas an enzyme.

ribulose A 5-carbon sugar that is phos-phorylated to form RIBULOSE BISPHOSPHATE.

ribulose bisphosphate (RUBP) A 5-carbon compound that accepts carbondioxide during PHOTOSYNTHESIS. Each mol-ecule is then converted into two moleculesof 3-carbon phosphoroglyceric acid. Theribulose bisphosphate is regenerated in theCalvin cycle when the carbon dioxide isconverted into carbohydrate.

ribulose bisphosphate carboxylase(RuBP carboxylase; rubisco) An enzymethat catalyzes the carboxylation of ribulosebisphosphate to two molecules of glycerate3-phosphate, the carbon dioxide-fixingfirst step in the Calvin cycle of PHOTOSYN-THESIS. It may be as much as 15% of thetotal protein in the chloroplast. Rubiscocan also act as an oxygenase, breakingdown ribulose bisphosphate in the pres-ence of oxygen to glycerate 3-phosphateand phosphoglycolic aid. This reactionforms the basis of PHOTORESPIRATION. Therelative concentrations of oxygen and car-bon dioxide in the surrounding air deter-mine the balance of photosynthesis andphotorespiration and thus the efficiency ofcarbon fixation. See also C3 plant; C4plant.

ring-porous Describing wood in whichthe largest vessels are in the early wood,giving distinct growth rings, as seen in elm.Compare diffuse-porous. See also annualring.

RNA (ribonucleic acid) A nucleic acidcomprising a single polynucleotide chain

similar in composition to a single strand ofDNA except that the sugar ribose replacesdeoxyribose and the pyrimidine base uracilreplaces thymine. RNA is synthesized inthe nucleus, using DNA as a template, andexists in three main forms (messengerRNA; transfer RNA; ribosomal RNA). Incertain viruses RNA is the genetic material.

RNA polymerase See polymerase.

RNase (ribonuclease) An enzyme thatcatalyzes the hydrolysis of the sugar–phosphate bonds of RNA. There are sev-eral types, each having a specific action.For example ribonuclease T1 degradesRNA to mono- and oligonucleotides termi-nating in a 3¢-guanine nucleotide, whilethose produced by ribonuclease T2 termi-nate in a 3¢-adenine nucleotide.

RNA splicing In eukaryote cells, theprocess whereby noncoding segments (IN-TRONS) of the primary RNA transcript areremoved and the remaining coding seg-ments (EXONS) joined together to form thefunctional mRNA molecule. It is catalyzedby a complex of proteins and RNA mol-ecules (a spliceosome), or sometimes bymRNA itself (see ribozyme). See transcrip-tion.

root The organ that anchors a plant tothe ground and that is responsible for theuptake of water and mineral nutrientsfrom the soil. Roots develop from the radi-cle of the embryo and, according to thenature of branching from the seedling root,a fibrous or tap root system develops.Roots differ from shoots in lacking chloro-phyll and not producing buds or leaves.Roots also differ from shoots in thearrangement of xylem and phloem, havinga solid central strand of vascular tissuerather than a hollow cylinder of conduct-ing tissue.

The growing point of the root is pro-tected by a root cap to withstand the abra-sion that occurs as the root grows throughthe soil. The direction of root growth iscontrolled by both gravity and water sup-ply. The main absorptive region of the root

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is just beyond the zone of elongation be-hind the root tip, where the ROOT HAIRS areformed. Further back along the root, lat-eral roots are formed, which develop fromwithin the vascular tissue and grow outthrough the cortex (endogenous root for-mation). All roots whose derivation caneventually be traced back to the radicle arecalled primary roots. Roots that arise inany other way are termed adventitiousroots, for example those that develop fromthe stem in bulbs and corms. In manyplants, particularly biennials, the root maybecome swollen with carbohydrates andact as an underground food store duringthe winter. Many other root modificationsare seen (see buttress root; contractile root;prop root). The roots of many plant speciesexist in association with fungi as myco-rrhizae or, as in the Fabaceae, with nitro-gen-fixing bacteria to form root nodules(see nitrogen fixation).

root hair An outgrowth, or trichome,from the piliferous layer of the root. Roothairs are projections of single epidermalcells that make contact with the soil for up-take of water and solutes, increasing thesurface area of the root for absorption.They may also have an anchoring role,helping to retain contact between the roottip and the soil under dry conditions whenthe soil tends to shrink away from theroots.

root cap See calyptra.

root nodule See nitrogen fixation.

root pressure The pressure that maybuild up in a plant root system due to theosmotic potential of the root cells, which isthought to help force water upward in thexylem vessels. It is demonstrated by theoozing of water from the surface of a cutstem. Root pressure tends to build up atnight, when the rate of transpiration is low,and is a cause of GUTTATION in somespecies, especially grasses. See osmosis;water potential.

rootstock A vertical, usually short, un-

derground stem or rhizome. It is found inmany angiosperms, e.g. rhubarb (Rheumspp) and strawberry (Fragaria ananassa),and in certain ferns, e.g. Osmunda. It canreproduce vegetatively.

Rosaceae A large family of dicotyle-donous trees, shrubs, and herbs with alter-nately arranged (opposite in Rhodotypos),simple, compound, or dissected leaves withstipules, and actinomorphic flowers withparts in fives and stamens in one to severalwhorls of five or more. The ovary is infe-rior or partly inferior or the carpels lie freein a hollow receptacle. The fruit may be ahead of follicles or achenes, as in avens(Geum), achenes on a fleshy receptacle, asin strawberry (Fragaria); a drupe, as incherries, plums, and peaches (Prunus); ahead of drupelets, e.g. blackberry and rasp-berry (Rubus); or a pome, as in apple(Malus). There are about 2825 species.Many species and cultivated hybrids, espe-cially roses and cherry trees, are grown as ornamentals for their showy flowers.Cotoneaster and Spiraea are also populargarden shrubs. Other edible fruits includeloquats (Eriobotrya japonica), pears(Pyrus communis), quinces (Cydonia ob-longa), and strawberries (Fragaria).

rosette plant Any plants whose leavesradiate outward from a short stem at soillevel, e.g. daisy (Bellis perennis). Thisgrowth form helps plants to withstandtrampling or grazing and in exposed habi-tats avoids exposure to strong winds.

royal ferns See Osmundales.

Rubiaceae A large family of dicotyle-donous trees, shrubs, and herbs, includingsome lianas, epiphytes, and ant plants (e.g.Myrmecodia). They have decussatelyarranged simple leaves often bearing elab-orate stipules, and flower parts fused, infours or fives (sometimes including oneextra-large, brightly colored sepal), and4–5 stamens that alternate with the corollalobes. The ovary is usually inferior, withtwo fused carpels. The fruit is a capsule,berry, or schizocarp. There are about

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10 200 species, mostly tropical, but withsome in temperate and arctic regions.Many species have long corolla tubes, anadaptation to pollination by butterflies,moths, or birds. Most temperate speciesare herbs, e.g. bedstraws (Galium spp.),typically with square stems and whorls ofleaves. The family includes a few light tim-ber trees, coffee (Coffea arabica), and Cin-chona, whose bark is a source of quinine.There are some cultivated ornamentals,such as Gardenia jasminoides and Ixora.

rubisco See ribulose bisphosphate car-boxylase.

RuBP carboxylase See ribulose bispho-sphate carboxylase.

runner A branch, formed from an axil-lary bud, that grows horizontally along theground. Runners often have greatly elon-gated inernodes. The axillary buds of therunner may develop into daughter plantswith adventitious roots growing from thenode, as seen in the creeping buttercup.Conversely, only the terminal bud mayform a new plant, as in the strawberry. Anew runner then develops from a branch ofthis daughter plant. If the intervening run-ner breaks or rots away, the daughterplants become independent, so this is alsoa means of vegetative reproduction. Seealso offset; stolon.

rusts Parasitic basidiomycete fungi ofthe order Uredinales. The name derives

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saccharide See sugar.

Saccharomycetales An order of the As-comycota, containing the yeasts, in whichthe mycelium is poorly developed or ab-sent, and the vegetative stage comprisesmainly single cells that reproduce asexuallyby budding or fission. The commerciallyimportant yeasts of the genus Saccha-romyces are used in brewing (S. cerevisiae),wine-making (S. ellipsoideus), and bread-making (s.cervisiae): they feed on sugars,and in anaerobic conditions produceethanol (alcohol) and carbon dioxide.

Sachs, Julius von (1832–97) Germanbotanist. Having obtained his PhD inbotany at the University of Prague in 1856,Sachs occupied a number of posts, culmi-nating in his appointment as professor ofbotany at Würzburg University. He estab-lished that chorophyll is not univerally dis-tributed in plants, but is confined insidedefinite structures that he named ‘chloro-plasts’, and showed that starch is the firstproduct of photosynthesis. He also discov-ered that plants respire in the same way asanimals, taking up oxygen and giving outcarbon dioxide. His work on plant growthmovements led him to invent the KLINOS-TAT.

safranin In microscopy, a permanentred stain used to stain nuclei in plant cells.It also stains lignin and cutin red andchloroplasts pink. It is often used with agreen counterstain such a fast green or ablue counterstain, e.g. hematoxylin. Seestaining.

Salicaceae A family of dicotyledonoustrees, shrubs, and creeping forms includingthe willows (Salix) and poplars (Populus).

S alleles See multiple allelism.

salt gland See hydathode.

saltmarsh Vegetation growing in salineor brackish marshy places, such as river es-tuaries and sheltered, muddy coasts, andconsisting mainly of HALOPHYTES. Saltmarshes can be productive grazing land,and serve as important barriers against in-undation of the hinterland by the sea.

samara A nut or achene whose pericarpis extended to form a wing for wind dis-persal, e.g. ash (Fraxinus excelsior).

sand Mineral particles consisting mainlyof quartz, felspar, and mica, and measur-ing between 2 and 0.05 mm in diameter.Sandy soils contain at least 85% sand andnot more than 10% clay. They are lightand drain well. However, retention of nu-trients and water is poor. Compare silt.

sap 1. The solution of mineral salts andsugars that is found in xylem and phloemvessels and oozes out of cut stems.2. The liquid contents of a plant cell vac-uole.

saponins Any of a class of bitter-tastingGLYCOSIDES that form colloidal solutions inwater and foam when shaken. Saponinsoccur in many plants, such as soapwort(Saponaria offinicalis).

saprobe (saprophyte) An organism thatderives its nourishment by absorbing theproducts or remains of other organisms.Saprobes usually secrete enzymes on theirfood and absorb the soluble breakdownproducts. Many fungi and bacteria aresaprobes, and are important decomposers

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in food chains, returning nutrients to thesoil by putrefaction and decay.

Saprolegniales (water molds) An orderof the OOMYCOTA containing aquatic orsoil-dwelling funguslike microorganisms,e.g. the water mold, Saprolegnia, whichmay be saprobes or parasites. Some formmycelia; others are unicellular.

saprophyte See saprobe.

saprotrophic (saprophytic) The modeof nutrition of a SAPROBE. See decomposer.

sapwood (alburnum) The outer livingxylem cells in a tree trunk, consisting ofsecondary xylem elements, parenchyma,and medullary rays, that are actively in-volved in water transport and food stor-age. Compare heartwood.

satellite DNA See repetitive DNA.

savanna Tropical GRASSLAND with scat-tered trees and/or tall bushes, found in allparts of the tropics where there is a pro-nounced dry season. Some savannas havemany grazing animals that affect the veg-etation structure; these and other savannashave periodic or irregular fires in the dryseason. See climax.

scab Any plant disease that producesdry, raised, scablike lesions due to the for-mation of cork layers. The disease is usu-ally caused by a bacterium, e.g. commonscab of potatoes (Streptomyces scabies), ora fungus, e.g. apple scab (Venturia inae-qualis).

scalariform thickening A type of sec-ondary wall formation consisting of inter-laced helical bands of thickening giving aladderlike formation. It allows for very lit-tle further extension, and is found in tissuessuch as metaxylem tracheids and vessels,which do not elongate after maturation.See xylem. Compare pitted thickening; spi-ral thickening.

scanning electron microscope See mi-croscope.

Schiff’s reagent A reagent used to de-tect the presence of aldehyde and ketonegroups in certain compounds. It is pro-duced by the reduction of fuchsin by sul-furous acid, and is oxidized by aldehydesand ketones, restoring the magenta colorof the fuchsin.

schizocarp A dry fruit, formed fromtwo or more carpels, that divides at matu-rity into one-seeded achenelike segmentstermed mericarps. Such fruits are seen inhollyhock (Alcea).

Schleiden, Matthias Jakob (1804–81)German botanist. Having abandoned a ca-reer in law, Schleiden eventually becameprofessor of botany at the University ofJena in 1839. After years studying the mi-croscropic structure of plants, he proposedhis cell theory Contributions to Phytogen-esis (1838), in which he stated that the var-ious plant structures are all composed ofindividual cells. He also recognized the sig-nificance of the nucleus. Mistakenly, hebelieved that new cells formed by beingbudded off the nuclei. His cell theory wasalso adopted and expanded by SCHWANN,who applied it to animals also.

Schultze, Max Johann Sigismund(1825–74) German zoologist. During histime as professor of anatomy at Bonn Uni-versity, Schultze studied the cells of manydifferent species of animals. This led to thepublication of the famous paper (1861) inwhich he concluded that protoplasm is thephysical basis of life. He described cells asbeing composed of ‘nucleated protoplasm’and maintained that this constituent ofcells was more important than cell walls (ofplants) or membranes. He supported hisconclusion by pointing out that certain em-bryonic cells do not possess membranes.

Schultze’s solution A solution of zincchloride, potassium iodide, and iodineused mainly for testing for cellulose andhemicellulose. Both materials stain a blue

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color with the reagent, that of hemicellu-lose being weaker. See also staining.

Schwann, Theodor (1810–82) Ger-man physiologist. Schwann was educatedat a number of German universities beforeeventually moving to Belgium. He is bestknown for his work on different animal tis-sues, which he suggested were composed ofindividual cells, rather than forming di-rectly from molecules. He thus confirmedthe cell theory of Schleiden and extended itto animals. His earlier work in Germany,attempting to disprove the theory of spon-taneous generation, led him to discoverthat yeast is involved in fermentation. Hewas so discredited for this work that he leftGermany. When Pasteur’s work on fer-mentation was published in the 1850s, hisgood name was restored.

scion A shoot or bud from one plantthat is joined to another plants with roots(the STOCK) by grafting or budding. Thescion supplies only aerial parts to theGRAFT.

sclereid Any SCLERENCHYMA cell exceptthe long fibers. The cell wall has undergonesecondary thickening, and is often ligni-fied. The various forms of sclereid includethe star-shaped astrosclereid, the rod-shaped macrosclereid, and the isodiametricstone cell.

sclerenchyma The main supporting tis-sue in plants, made up of cells with heavilythickened, often lignified, walls and emptylumina. The cells usually have simple un-bordered pits. Unlike collenchyma, it is notvery extensible and is thus not formed inquantity until after the young tissues havefully differentiated. Sclerenchyma is oftenfound associated with vascular tissue andexists as two distinct types of cell: the longFIBER and the shorter SCLEREID. It may de-velop by thickening of the secondary wallsof parenchyma cells, or it may arise directlyfrom meristematic cells.

sclerotium (pl. sclerotia) The restingbody of certain fungi, e.g. ergot (Claviceps

purpurea) and root-rot (Sclerotinia spp),formed from a mass of hyphae. Sclerotiaare often rounded or club-shaped. They donot contain spores, and eventually produceeither a mycelium or fruiting bodies. Seealso stroma.

Scrophulariaceae A family of dicotyle-donous plants, mainly herbs, but includingsome trees, shrubs, climbers, and aquaticswith a wide range of leaf form and flowerstructure. A few are semiparasitic on theroots of angiosperms, especially grasses,such as eyebright (Euphrasia) and louse-wort (Pedicularis). There are about about5100 species. Leaves of the foxglove (Dig-italis purpurea) are the source of drugssuch as digitalin.

scutellum The part of the embryo ofPoaceae (grasses) that lies next to theendosperm. It is thought to be the modifiedcotyledon.

secondary cell wall See apposition.

secondary growth Plant growth de-rived from secondary or lateral meristems,i.e. the vascular and cork cambia. It is usu-ally absent in monocotyledons, but occursin most dicotyledons and conifers, and in afew lower plants. In dicotyledons, the re-sult of secondary growth is termed sec-ondary thickening since there is usually anincrease in width rather than length. Theactivity of the vascular CAMBIUM gives riseto the secondary xylem and secondaryphloem. Wood is mostly secondary xylem.The cork cambium (PHELLOGEN) gives riseto the PERIDERM, a protective layer of tissueon the outside of the stem or root, whichconsists of the cork (PHELLEM), the phel-logen, and the PHELLODERM (secondarycortex). The parts of the plant formed bysecondary growth are called the secondaryplant body. Compare primary growth. Seeannual ring.

secondary phloem See secondary growth.

secondary plant body See secondarygrowth.

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secondary structure In a PROTEIN, thebasic shape of the polypeptide, i.e. the wayit folds. There are two main forms: the a-helix and the b-pleated sheet. The a-helix is an extended spiral with one turnfor every 3.6 amino acids; the turns areheld in position by HYDROGEN BONDS be-tween adjacent C=O and NH groups. Sucha secondary structure allows for flexibility,the extent of which is determined by theamount of further linking of the structureby DISULFIDE BRIDGES. In the b-pleated sheetthe polypeptides are more extended, andadjacent chains of amino acids are ar-ranged antiparallel to each other, linked byhydrogen bonds between the C=O and NHgroups of adjacent chains. b-pleated sheetshave high tensile strength, but are inflexi-ble. Many proteins contain both a-helicesand b-pleated sheets in different parts ofthe molecule. See conformation; primarystructure; quaternary structure; tertiarystructure.

secondary thickening See secondarygrowth.

secondary xylem See secondary growth.

secretion The process of dischargingmaterials from cells. The term is usuallyapplied to materials that have been synthe-sized by the cell.

seed The structure that develops fromthe OVULE following fertilization in an-giosperms or gymnosperms. In floweringplants one or more seeds are containedwithin a fruit developed from the ovarywall. The individual seeds are composed ofan embryo and, in those seeds in whichfood is not stored in the embryo cotyle-dons, a nutritive endosperm tissue. Thisdifference enables seeds to be classified asnonendospermic or endospermic. Thewhole is surrounded by a testa developedfrom the integuments of the ovule. In gym-nosperms the seeds do not develop withina fruit but are shed ‘naked’ from the plant.Following dispersal from the parent plant,seeds may germinate immediately to form aseedling or may remain in a relatively inac-

tive dormant state until conditions are fa-vorable for germination. In annual plants,seeds provide the only mechanism for sur-viving the cold or dry seasons. Seeds maybe formed asexually in certain plants byAPOMIXIS, e.g. in dandelion.

The development of the seed habit,which makes water unnecessary for fertil-ization, is one of the most significant ad-vances in plant evolution. It has enabledgymnosperms and angiosperms to colonizedry terrestrial habitats where lower plantsare unable to establish themselves.

seed ferns Seed-bearing gymnospermsof the extinct order Pteridospermales (Cycadofilicales), represented only by fos-sil forms that flourished in the Carbonifer-ous but became extinct in the Cretaceous.The plant body resembled a fern and didnot produce flowers, the seeds developingfrom megasporangia borne on the marginsof the fronds. Their stems showed somesecondary thickening.

seed plants See spermatophyte.

segregation The separation of the twoalleles of a gene into different gametes,brought about by the separation of ho-mologous chromosomes at anaphase 1 ofmeiosis.

seismonasty (seismonastic movements)A NASTIC MOVEMENT in response to shock.An example is the collapse of leaves of thesensitive plant (Mimosa pudica) whentouched or shaken.

Selaginella See Selaginellales; Lyco-phyta.

Selaginellales An order of the Lyco-phyta (clubmosses) containing the singleliving genus Selaginella, sometimes calledspike-mosses, with about 700 species,found worldwide, but especially in thetropics. They range from erect, sometimestufted, forms to prostrate creeping plantsand climbers.

self-compatibility The capacity to self-

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fertilize. Many flowers are capable of self-fertilization if pollination fails, and a fewspecies do it regularly.

self-incompatibility (self-sterility) Thecondition in whereby male gametes cannotfertilize female gametes from the same in-dividual and give rise to a viable embryo.The pollen tube may be blocked by calloseas it tries to grow down the style, be pre-vented from penetrating the stigma sur-face, abort on reaching the ovule or thepollen grain may fail to germinate. See in-compatibility; heterostyly.

selfish DNA DNA that can movearound within the genome of an organismor insert copies of itself at various siteswithout serving any apparent useful func-tion. The prime examples of selfish DNAare the mobile genetic elements calledtransposons. Some biologists also regardINTRONS as selfish DNA. Its name derivesfrom the hypothesis that selection actswithin the genome, favoring anything thatresults in increased replication of DNA.The apparently surplus DNA does not ap-pear to confer any advantage on the organ-ism, so the DNA is considered to be acting‘selfishly’. Compare junk DNA; repetitiveDNA. See transposon.

self-pollination See pollination.

self-sterility See self-incompatibility.

Seliwanoff’s test A standard test for thepresence of fructose or other ketose sugarsin solution. A few drops of Seliwanoff’sreagent, resorcinol in 50% hydrochloricacid, are heated with the test solution. Ared color or red precipitate indicates fruc-tose.

semiconservative replication See repli-cation.

semipermeable membrane See osmo-sis.

senescence The phase of the agingprocess of an organism or part of an or-

ganism between maturity and naturaldeath. It is usually characterized by a re-duction in capacity for self-maintenanceand repair of cells, and hence deteriora-tion. There is often accumulation of wastemetabolic products, a decrease in dryweight (as substances are withdrawn fromthe affected part), and a rise in the rate ofrespiration (see climacteric).

sensitive plant A plant (Mimosa pu-dica) of the family Fabaceae (pea family)whose leaves are divided into many smallleaflets or pinnae, which progressively col-lapse and fold when the leaf is touched.This is the result of turgor changes in cellsin swellings (see pulvinus) in the petiole atthe bases of the leaflets. See also nasticmovements.

sepal One of the structures situated im-mediately below the petals of a flower.Their collective name is the calyx. They areoften green and hairy and enclose and pro-tect the flower bud. Sometimes they arebrightly colored and attract insects for pol-lination. Sepals contain several vascularbundles, and are thought to be derivedfrom leaves.

septum (pl. septa) A wall, partition, ormembrane separating two cavities. For ex-ample, the capsule of a poppy (Papaver) isdivided by septa, as are the individual com-partments of the mycelia of many fungi.

sere Any plant community in a succes-sion in which each community itself effectschanges in the habitat that determine thenature of the following stage. The succes-sive stages are known as seral stages. Seresresult eventually in a climax community.Stages in a secondary succession, which ap-pear when the biotic components of a pri-mary sere are destroyed, e.g. by fire, arecalled subseres; seres in microhabitats arecalled microseres. The initial (pioneer)community in a succession is termed a pris-ere. See also climax; halosere; hydrosere;succession.

serine An AMINO ACID synthesized from

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glycerate 3-phosphate. Serine is also aproduct of photorespiration and othermetabolic reactions. It is a component ofseveral phosphoglycerides. It is brokendown by removal of the amino group toform pyruvic acid.

sessile Unstalked, for example an acorn,leaf, flower, or other organ that is attachedto the main body of the plant.

seta (pl. setae) The part of the SPOROGO-NIUM that forms the stalk between the footand the capsule in bryophytes, e.g. Funariaand Pellia. It may contain vascular tissue,enabling the sporophyte capsule to drawnutrients from the parent gametophyte.

Sewall Wright effect See genetic drift.

sex chromosomes Chromosomes thatcarry sex-determining genes. They are rarein the plant kingdom, and many plants arehermaphrodite. Sex chromosomes occur insome dioecious plants, e.g. Silene. In otherdioecious plants a single pair of alleles de-termines sex.

sexual reproduction The formation ofnew individuals by fusion of two nuclei orsex cells (GAMETES) to form a diploidZYGOTE. In unicellular organisms whole in-dividuals may unite but in most multicellu-lar organisms only the gametes combine. Inorganisms showing sexuality, the gametesare of two types: male and female. Theymay be produced in special organs (e.g.carpel and anther in angiosperms, archego-nia and antheridia in lower plants andsome algae). The gametes may be derivedfrom the same parent (AUTOGAMY) or fromtwo different parents (ALLOGAMY). Individ-uals producing both male and female ga-metes are termed hermaphrodite if in thesame flower or MONOECIOUS if in separateflowers; those in which male and femalegametes are borne on different individualsare termed DIOECIOUS.

Generally meiosis occurs before gameteformation, resulting in the gametes beinghaploid (having half the normal number ofchromosomes). At fertilization, when the

haploid gametes fuse, the diploid numberof chromosomes is restored. In this waysexual reproduction permits genetic re-combination, which results in greater vari-ety in offspring and so provides amechanism for evolution by natural selec-tion. In organisms showing ALTERNATION

OF GENERATIONS, the gametes may be pro-duced by mitosis in the haploid generation,or by meiosis in the diploid generation andgive rise directly to gametes or gamete-producing cells.

shade plant A plant that can tolerateand thrive in low light intensity. Someshade plants are sensitive to very brightlight and cannot live in open habitats.

shadowing A method of preparation ofmaterial for electron microscopy enablingsurface features to be studied. It can beused for small entire structures, subcellularorganelles, or even large molecules (e.g.DNA). The specimen is supported on aplastic or carbon film on a small grid andsprayed with vaporized metal atoms fromone side while under vacuum. The coatedspecimen appears blacker (more electron-opaque) where metal accumulates, and thelengths and shapes of ‘shadows’ cast (re-gions behind the objects not coated withmetal) give structural information. It isoften used in association with freeze frac-turing. See freeze fracturing; microscope.

shoot The aerial photosynthetic portionof a plant that generally consists of a stemupon which leaves, buds, and flowers areborne.

short-day plant (SDP) A plant that ap-pears to require short days (days with lessthan a critical length of daylight) in orderto flower. In fact, the plants require a min-imum period of darkness (night length).Short-day plants are typically found intemperate latitudes. Examples includespring-flowering plants such as strawberry(Fragaria) and autumn-flowering plantssuch as Chrysanthemum. See critical daylength; photoperiodism. Compare long-day plant; day-neutral plant.

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siblings (sibs) Two or more offspringfrom the same cross. The term sibs is usu-ally used in a narrower sense to meanplants derived by selfing or by crossing be-tween genetically similar parents. The termsibling species is sometimes used to de-scribe species that are extremely similar be-cause of recent common ancestry. Suchspecies may be almost impossible to distin-guish in the field.

sieve cell See sieve element.

sieve element A vascular cell in thePHLOEM, whose function is to transportsugars and other nutrients from the site ofproduction to the site of utilization or stor-age. The term includes both sieve cells andsieve tubes. Sieve cells are long, narrowcells with specialized sieve areas in the pri-mary wall, perforated by several pores, de-rived from pits. The pores are lined withcallose, and allow cytoplasmic strands topass from one sieve cell to the next. Theyare found in gymnosperms and lower vas-cular plants. The sieve tubes of an-giosperms are shorter and wider, and havehighly specialized sieve areas called sieveplates, in some species (e.g. Nicotiana)with several sieve areas, usually on the endwalls of the elements. They are usuallyjoined end to end, allowing continuouspassage of materials up and down theplants. In contrast to xylem components,sieve tube elements have nonlignified cellwalls, and a living enucleate (without a nu-cleus) protoplast. They are usually associ-ated with specialized parenchyma cellscalled COMPANION CELLS, whose proto-plasts are connected with the protoplasmof the sieve tube by plasmodesmata.

sieve plate See sieve element.

sieve tube See sieve element.

silicon A MICRONUTRIENT found in manyanimals and plants, although not essentialfor growth in most plants. It is found inlarge quantities in the cell walls of certainalgae (e.g. desmids, diatoms) and horse-tails, and in smaller amounts in the cell

walls of many higher plants, especiallygrasses, where it can be a useful taxonomicfeature.

silicula A capsular fruit that is formedfrom a bicarpellary ovary. It is flattened,short, and broad and is divided into two orfour loculi (valves) by a false septum,which bears the seeds. Siliculas are typicalof the Brassicaceae, e.g. honesty (Lunariaannua). Compare siliqua.

siliqua (silique) A fruit of some Brassi-caceae, similar to the SILICULA but longerand thinner, for example the wallflowerfruit (Erysimum cheiri).

silt Mineral particles between 0.05 and0.002 mm in diameter. A silt soil is onewith more than 80% silt and less than 12%clay, and has a smooth, soapy texture. Seesoil. Compare sand.

Silurian The period, some 440–405 mil-lion years ago, between the Ordovicianand the Devonian periods of the Paleozoic.It was a relatively warm period in theEarth’s history, during which the first landplants appeared. It is characterized by sim-ple plants such as liverworts; fossils ofplants similar to vascular plants, e.g.Cooksonia, are known from the later partof the period, some of which had simplestrands of tracheids. Cooksonia had a slen-der dichotomously branching stem with noleaves, terminal sporangia and spores im-pregnated with cutin. It resembles thepsilophyte Rhynia from the Rhynie chertof the Devonian period. See also geologicaltime scale.

simple pit See pit.

siphonostele A stele with a central coreof pith internal to the xylem. The phloemmay lie external to the xylem (ectophloic),or both internal and external to the xylem(amphiphloic). See stele.

SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. It has

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seven base units (the meter, kilogram, sec-ond, kelvin, ampere, mole, and candela)and two supplementary units (the radianand steradian). Derived units are formedby multiplication and/or division of baseunits; a number have special names. Stan-dard prefixes are used for multiples andsubmultiples of SI units. See Appendix.

sliding growth A pattern of plantgrowth seen, for example, in many epider-mal cells where, in order to accommodategrowth by adjoining cells, expanding cellwalls of adjacent cells slide over each other.Thus growth is achieved without disrup-tion of neighboring cells, although it doesresult in breakage of plasmodesmata.Compare symplastic growth.

slime bacteria See myxobacteria.

slime molds See Acrasiomycota; Myx-omycota.

smuts Plant diseases caused by basid-iomycete fungi of the order Ustilaginales.Many are important parasites of cerealsand form a mass of sooty black spores inplace of the grain. In other smuts the sporesform an uncovered mass of black powder.Examples of smuts are Tilletia and Usti-lago.

snake’s tongue ferns See Ophioglos-sales.

snurp See spliceosome.

society A minor plant community withina larger community, characterized by aspecific dominant species, for example, aTrillium-dominated society in a commu-nity such as an oak woodland.

sodium An element found in all terres-trial plants, although it is believed not to beessential in most, with the exception ofsome salt-tolerant C4 plants.

sodium bicarbonate indicator A mix-ture of the dyes cresol red and thymol bluein a sodium bicarbonate solution, which

changes from red to orange and yellowwith a slight increase in acidity, and thusindicates change in pH.

soil The accumulation of mineral parti-cles and organic matter that forms a super-ficial layer over large parts of the Earth’ssurface. It provides support and nutrientsfor plants and is inhabited by numerousand various microorganisms and animals.A section down through the soil is termeda soil profile and this can characteristicallybe divided into three main layers or hori-zons. Horizon A, the topsoil, is darker thanthe lower layers due to the accumulation oforganic matter as humus. It is the most fer-tile layer and contains most of the soil pop-ulation and a high proportion of plantroots. Horizon B, the subsoil, contains ma-terials washed down from above and maybe mottled with various colors dependingon the iron compounds present. Horizon Cis relatively unweathered parent materialfrom which the mineral components of theabove layers are derived. The depth andcontent of the horizons are used to classifysoils into various types, e.g. podsols andbrown earths. The texture, structure, andporosity of soil depends largely on the sizesof the mineral particles it contains and onthe amount of organic material present.Soils also vary depending on environmen-tal conditions, notably rainfall. See soilstructure.

soil profile See soil.

soil structure The arrangement of themineral particles in the soil – whether theyare free or bound into aggregates by de-composing organic matter. These unitsmay be further grouped into crumbs,blocks, plates, and so on. The structure af-fects fertility, drainage, aeration, and easeof cultivation. A blocky soil will tend todry out and suffer from leaching of miner-als from the upper horizons. The soil struc-ture is affected by soil texture – the sizes ofthe different mineral particles, which alsoinfluences drainage and aeration, and theretention of water by capillarity. The sizeof the particles depends in part on the par-

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ent rock and how it weathers. The mainsize classes of mineral particles are graveland stones (greater than 2 mm in diame-ter), sand (0.05 to 2 mm in diameter), silt(0.002 to 0.05 mm in diameter), and clay(smaller than 0.002 mm in diameter). Claysoils, which have small particles, tend to bewet and sticky, with poor drainage andaeration. However, they tend to be electro-statically charged and attract humus, form-ing a clay–humus complex that can attractand retain minerals. The humus sticks theclay particles together, forming larger ag-gregates and improving drainage and aera-tion. Clay soils can be further improved byadding lime, which promotes clumping ofparticles. Sandy soils have large particles,and therefore larger spaces between themfor drainage and aeration. They may needaddition of humus or mulches to improvewater and mineral retention and preventleaching.

soil texture See soil structure.

Solanaceae A family of dicotyledonousherbs, shrubs, lianas, and small trees. Theyinclude commercial food plants such asaubergine (Solanum melongena), potato(Solanum tuberosum), sweet peppers (Cap-sicum), tomato (Lycopersicon esculen-tum), and Nicotiana (the source oftobacco).

solenostele See stele.

solute potential See water potential.

somatic Describing the cells of an or-ganism other than germ cells. Somatic cellsdivide by mitosis producing daughter cellsidentical to the parent cell.

soredium (pl. soredia) See lichens.

sorus (pl. sori) 1. A reproductive struc-ture found in ferns comprising a collectionof sporangia borne on a cushion of tissuetermed the placenta. The placenta developsover a vein ending on the underside of afoliage leaf or on a specialized fertile leaf.

The sorus is covered by a flap of tissue, theindusium.2. The reproductive area of certain algalthalli, e.g. Laminaria.3. The spore-bearing body of a rust fungus.

Southern blotting A technique fortransferring DNA fragments from an elec-trophoretic gel to a nitrocellulose filter ornylon membrane, where they can be fixedin position and probed using DNA probes.Named after its inventor, E. M. Southern(1938– ), it is widely used in geneticanalysis. The DNA is first digested with re-striction enzymes and the resulting mixtureof fragments separated according to size by electrophoresis on an agarose gel. Thedouble-stranded DNA is then denatured tosingle-stranded DNA using sodium hy-droxide, and a nitrocellulose filter pressedagainst the gel. This transfers, or blots, thesingle-stranded DNA fragments onto thenitrocellulose, where they are permanentlybound by heating. The DNA probe canthen be applied to locate the specific DNAfragment of interest, while preserving theelectrophoretic separation pattern. SeeDNA probe. Compare Western blotting.

SP (suction pressure) See osmosis.

spadix (pl. spadices) See inflorescence.

spathe See inflorescence.

speciation The formation of one ormore new SPECIES from an existing species.Speciation occurs when a population sepa-rates into isolated subpopulations that de-velop distinctive characteristics as a resultof NATURAL SELECTION or random GENETIC

DRIFT, and cannot then reproduce with therest of the population, even if there are nogeographical or other physical reasons toprevent them from doing so. Speciation oc-curs when barriers to interbreeding arise,such as geographical changes (e.g. in rivercourses), habitats becoming uninhabitable(e.g. following urbanization), changes intiming of flowering, or when populationsat the margins of the main populationexperience different selection pressures

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as they colonize different environments.Another cause of rapid speciation in plantsis polyploidy. See also adaptive radiation.

species A taxonomic unit comprisingone or more populations, all the membersof which are able to breed amongst them-selves and produce fertile offspring. Theyare normally isolated reproductively fromall other organisms, i.e. they cannot breedwith any other organisms. Two or more re-lated species unable to breed because ofgeographical separation are called allo-patric species. Related species that are notgeographically isolated, and which couldinterbreed but in practice do not because ofdifferences in behavior, breeding season,etc., are called sympatric species. Somespecies can interbreed with other relatedspecies, but the hybrids are often wholly orpartially sterile. For some species theremay be an almost complete continuum be-tween closely related species, as in certainorchids. This may also happen in disturbedareas or in regions where the range of twoor more species or even genera overlap,producing hybrid swarms. Where speciesform apomictic clones that are very diffi-cult to distinguish from each other, theyare termed aggregate species, e.g. bramble(Rubus fruticosus agg.). In general, speciesare regarded as distinct if they remain re-productively isolated for most of their geo-graphical range and have recognizablydistinct morphological characteristics.Within a species, there may be subgroupswith distinct morphological or (especiallyin microorganisms) physiological charac-teristics; these groups are termed SUBSPECIES

or RACES. Groups of similar species areclassified together in genera.

spectrophotometer An instrument formeasuring the amount of light of differentwavelengths absorbed by a substance. Itgives information about the identity orconcentration of the specimen and can beused to plot absorption spectra. It can alsobe used to investigate how electromagneticradiation interacts with matter, e.g. the ab-sorption of light in photosynthesis.

spermatophyte (seed plant) Any seed-bearing plant. In many older classificationsthey constituted the division (phylum)Spermatophyta, subdivided into the classesAngiospermae and Gymnospermae. Com-pare tracheophyte.

spermatozoid See antherozoid.

S phase See cell cycle.

Sphenophyta A phylum of vascularnonseed-bearing plants that contains oneliving order, the Equisetales, comprisingone genus, Equisetum (horsetails or scour-ing rushes), and three extinct orders, theCalamitales, Sphenophyllales, and Pseudo-borniales. Sphenophytes were particularlyabundant in the Carboniferous periodwhen the genus Calamites formed a largeproportion of the forest vegetation, sometreelike species reaching heights of 30 m.

spherosome A small spherical organelleof plant cells, about 0.5–1.0 m m in diame-ter, bounded by a single membrane andstoring lipid.

spike See inflorescence.

spindle The spindle-shaped structureformed in the cytoplasm during MITOSIS

and MEIOSIS that is responsible for movingthe chromatids and chromosomes to oppo-site poles of the cell. The spindle consists ofa longitudinally orientated system of pro-tein MICROTUBULES whose synthesis startslate in interphase. A special region of thecentromeres of each pair of sister chro-matids, the kinetochore, becomes attachedto one or a bundle of spindle microtubules.During anaphase, the kinetochore itselfacts as the motor, disassembling the at-tached microtubules and hauling the chro-matid toward the spindle pole. Later inanaphase, the unattached interpolar micro-tubules actively slide past each other, elon-gating the entire spindle.

spine A modified leaf reduced to asharply pointed structure. In some xero-phytes, such as cacti, most of the leaves are

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modified into spines, greatly reducing theleaf area for transpiration; the photosyn-thetic function of the leaves is taken overby the green stems. The dense mass ofwhitish spines also helps to reflect incom-ing radiation, further reducing transpira-tion.

spiral thickening (helical thickening)The type of secondary wall formation inwhich a spiral band of lignified deposits isformed on the inner wall of the cell. It isfound in protoxylem and first metaxylemtracheids and vessels. Like annular thick-ening, it allows for continued elongation ofthe xylem. See annular thickening; xylem.Compare reticulate thickening; scalariformthickening.

spirillum A helically shaped bacterium.

spirochetes Long spirally twisted bacte-ria surrounded by a flexible wall. An axialfilament, with a similar structure to a bac-terial flagellum, is spirally wound aboutthe protoplast inside the cell wall. Theyswim actively by flexing the cell. They arefound in mud and water and can withstandlow oxygen concentrations. Many spiro-chetes are pathogens causing yaws,syphilis, and relapsing fever.

Spirogyra A genus of filamentous greenalgae found in freshwater and having acharacteristic spiral chloroplast. Repro-duction is by conjugation: two filamentsbecome aligned and pairs of cells, one fromeach filament, become joined by a conjuga-tion tube. The contents of one cell passthrough the tube and fuse with the contentsof the second cell to form a zygote. SeeGamophyta.

spliceosome A unit in the nucleus thatremoves the noncoding RNA (INTRONS)from the initial RNA transcript and splicesthe RNA molecule back together again. Itconsists of several small ribonucleopro-teins called SNURPS, comprising RNA com-bined with proteins. The RNA moleculesact as RIBOZYMES, catalyzing both the splic-

ing process and the assembly of the spliceo-somes. See transcription.

spongy mesophyll See mesophyll.

spontaneous generation The erroneousbelief that modern living organisms can beformed directly and spontaneously frominorganic material, given the right condi-tions (abiogenesis). This belief was dis-proved by Redi and Pasteur in the 17th and19th centuries.

sporangiophore A structure bearingsporangia (see sporangium). In fungi itmay be a simple hypha or erect branch of ahyphae; in lower vascular plants it may bea leaf (SPOROPHYLL) or a STROBILUS.

sporangium (pl. sporangia) A repro-ductive body in which asexual spores areformed. It may be unicellular, as in simplealgae and fungi, or multicellular, as inplants. The spores are liberated by ruptureof the sporangium wall.

spore A unicellular or multicellularplant reproductive body. Generally theterm is applied to reproductive units pro-duced asexually, such as the spores ofbryophytes and ferns. A prefix is oftenadded, providing information as to the na-ture of the spore, for example coni-diospores arise on a conidium, megasporesand microspores are the larger and smallerspores produced by heterosporous plants,etc. In plants that undergo ALTERNATION OF

GENERATIONS, spores are produced bymeiosis by the sporophyte generation, soare usually haploid; they germinate to pro-duce the haploid gametophyte generation.

spore mother cell A cell that gives riseto four haploid spores by meiosis. In het-erosporous species, a distinction is usuallymade between megaspore mother cells andmicrospore mother cells. Of the four prod-ucts of a megaspore mother cell, three usu-ally abort.

sporocarp A hard multicellular spore-containing structure found in water ferns

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(e.g. Marsilea and Pilularia). The sori be-come enclosed by the growing together offertile fronds, and the spores are not liber-ated until the sporocarp decays and rup-tures.

sporogonium The sporophyte genera-tion in mosses and liverworts. It developsfrom the zygote and comprises the foot,seta, and capsule. The sporogonium is par-asitic on the gametophyte generation.

sporophore The aerial spore-producingbody of certain fungi, e.g. the mushroom ofAgaricus.

sporophyll The sporangium-bearingstructure of vascular plants. In some fernssporangia develop on the normal foliageleaves but in higher plants, the sporophyllsare highly modified leaves. They may begrouped together in a strobilus or, in theangiosperms, located in the flowers. Seemegasporophyll; microsporophyll.

sporophyte The diploid generation inthe life cycle of a plant, which gives riseasexually to haploid spores. It arises by fu-sion of haploid gametes produced by thehaploid generation (GAMETOPHYTE). In vas-cular plants, the sporophyte is the domi-nant generation, while in bryophytes it isparasitic on the gametophyte. See alterna-tion of generations.

staining A procedure that is designed toheighten contrast between different struc-tures. Normally biological material is lack-ing in contrast, protoplasm beingtransparent, and therefore staining is es-sential for an understanding of structure atthe microscopic level. VITAL STAINS are usedto stain and examine living material. Moststains require dead or nonliving material.Staining is done after fixation and eitherduring or after dehydration. Double stain-ing involves the use of two stains; the sec-ond is called the counterstain. Acidic stainshave a colored anion, basic stains have acolored cation. Some stains are neutral.Materials can be described as acidophilicor basophilic depending on whether they

are stained by acidic or basic dyes respec-tively. Basic stains are suitable for nuclei,staining DNA. Stains for light microscopyare colored dyes; those for electron mi-croscopy contain heavy metals, e.g. os-mium tetroxide.

For immediate observation, temporarystains may be used. These stains may fadein time, e.g. ruthenium red. Or the stainmay damage the specimen if left, for exam-ple SCHULTZE’S SOLUTION, which dissolvescell walls and causes swelling. Permanentstains are used for specimens that are to bekept. They do not fade or damage speci-mens. Examples include fast green and thecounterstain safranin. See also negativestaining.

stamen The male reproductive organ inflowering plants consisting of a fine stalk,the filament, bearing the pollen producinganther. It is equivalent to the microsporo-phyll present in the gymnosperms and het-erosporous members of the Filicinophytaand Lycophyta. The collective term for thestamens is the androecium. See also micro-sporophyll.

staminode A sterile stamen. It may berudimentary, consisting of only the fila-ment, as in figwort (Scrophularia), or itmay form a conspicuous part of the flower,as in Iris.

standard deviation In statistics, a mea-sure of the dispersion of a frequency distri-bution: it is the average magnitude ofdeviations from the center of a normalcurve, calculated by squaring all the devia-tions, calculating their mean, then findingthe square root of the mean. This gives avalue s, which is the point of maximumslope either side of the center of the curve.It is more suitable for use than the meandeviation, because it avoids problems ofpositive and negatives values on either sideof the center.

standing crop The total weight of allthe living organisms present in an ecosys-tem at a given moment, usually expressedas dry weight per unit area.

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starch

starch A polysaccharide that occurs ex-clusively in plants. It is an early end prod-uct of photosynthesis, and is storedtemporarily in the chloroplasts. In the darkthis starch is broken down by enzymes tosucrose and transported to other parts ofthe plant. Starch is a storage reservoir forthe plant: it is broken down by enzymes tosimple sugars and then metabolized to sup-ply energy needs. It is present in plant cellsas granules dispersed in the cytoplasm. Theshape of these granules is used in taxon-omy. In most plants it is found mainly inthe root cortex, and in storage organs suchas swollen taproots, corms, etc. Starch isextracted commercially from maize,wheat, barley, rice, potatoes, sorghum,cassava, and other crops. It is also the mainstorage material in many seeds. Starch is apolymer of glucose. It is not a single mol-ecule, but a mixture of amylose (straightchains of glucose subunits) and amy-lopectin (branching chains of subunits).

starch sheath The innermost layer ofcells of the cortex replacing the endodermisin some stems, especially young herba-ceous dicotyledonous stems. Its cells con-tain prominent starch grains. See alsoendodermis.

starch–statolith hypothesis A hypoth-esis concerning the mechanism of gravityperception in plants. See gravitropism.

statocyte A gravity-sensitive plant cellcontaining STATOLITHS. See gravitropism.

statolith One of a number of largestarch grains found in the statocytes, plantcells that are thought to be gravity sensi-tive. They move through the cytoplasm tothe lowermost cell surface. See gravitro-pism.

stearic acid (octadecanoic acid) A satu-rated carboxylic acid, which is widely dis-tributed in nature as the glyceride ester. Itis present in most fats and oils of plant andanimal origin, particularly the so-calledhard fats, i.e. those of higher melting point.Stearic acid is formed from palmitic acid. Itis an intermediate in the synthesis of oleic,linoleic, and linolenic acids.

stele The vascular tissue and associatedparenchyma tissue and (if present) the sur-rounding pericycle and endodermis of astem or root. The arrangement of tissues inthe stele changes from stem to root. In di-cotyledons, for example, the stem has aring of discrete vascular bundles, which al-lows it to resist bending, while the root has

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Stele: types of stele

a central core of vascular tissue, adapting itto resist pulling stresses. Stelar arrange-ments vary considerably in plants from thesimple protostele, in which leaf gaps areabsent, to the complex dictyostele, inwhich there are many closely spaced LEAF

GAPS. Intermediate between these is thesolenostele in which the leaf gaps are morewidely spaced vertically so that only one isseen per cross section of the stem. See poly-stely.

stem A longitudinal axis upon which areborne the leaves, buds, and reproductiveorgans of the plant. The stem is generallyaerial and erect but various modificationsare found, for example underground stemslike RHIZOMES, BULBS, and CORMS, and hor-izontal structures, such as RUNNERS. Thestem serves to conduct water and food ma-terials up and down the plant and, particu-larly in young plants and some succulents,it may serve as a photosynthetic organ. Thestem is generally cylindrical and consists ofregularly arranged conducting (vascular),strengthening, and packing cells, the wholebeing surrounded by a protective epider-mis.

steppe Grassland dominated by drought-

resistant species of perennial grasses, foundin regions of LOESS soil and extreme tem-perature range, often on CHERNOZEM soils,in a zone from Hungary eastwards throughthe Ukraine and southern Russia to Cen-tral Asia and China. There are pronouncedseasons, with hot summers and cold win-ters, with greater temperature extremes

and lower rainfall in the east. The speciescomposition varies with the climate. Largeareas of the steppe are under cultivation forgrain production.

sterigma (pl. sterigmata) A fingerlikeprojection upon which spores are formedin most basidiomycete fungi. Usually foursterigmata, each bearing one basidiospore,are borne on each basidium. The cultivatedmushroom is an exception, with only twosterigmata on each basidium.

steroid Any member of a group ofcompounds having a complex basic ringstructure. Many plant steroids are alcohols(sterols); some are alkaloid-like com-pounds. See also sterol.

sterol A steroid with long aliphatic sidechains (8–10 carbons) and at least one hy-droxyl group. They are lipid-soluble andoften occur in membranes (e.g. cholesteroland ergosterol).

stigma 1. The receptive tip of the carpelof plants. It may be elevated on a stalklikeprotrusion called a style.2. See eyespot.

stilt root An enlarged form of PROP

ROOT, seen in some mangroves and a fewpalms and other trees, that helps supportplants in unstable soils.

stimulus A change in the external or in-ternal environment of an organism thatelicits a response in the organism. Thestimulus does not provide the energy forthe response.

stipe 1. In the more highly differentiatedPhaeophyta (brown algae), such as wracksand kelps, the stalk between the holdfastand the blade.2. The stalk of a mushroom or toadstool,which bears the cap or pileus.

stipule A modified leaf found as an out-growth from the petiole or leaf base.

stock A plant onto which SCIONS (shoots

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Steroid: skeleton of the molecular structure

or buds) of another plant are grafted. Thestock provides the underground parts ofthe plant, and affects its size, vigor, and thetiming of flowering and fruiting. See graft.

stolon An initially erect branch that, dueto its great length, eventually bends overtoward the ground. Where a node touchesthe soil adventitious roots develop and theaxillary bud at that node grows out to forma new plant. See also layering; offset; run-ner.

stoma (pl. stomata) One of a large num-ber of pores in the epidermis of plantsthrough which gaseous exchange occurs.Each stoma is surrounded by two crescent-shaped GUARD CELLS, attached to eachother at their ends, which regulate theopening and closing of the pore by changesin their turgidity. The arrangement of cel-lulose microfibrils in the guard-cell wallscauses the guard cell to increase in lengthrather than width as it takes in water. Thisextra length is accommodated by the cellbulging away from the aperture, so widen-ing the pore. Adjacent to the guard cellsthere are usually modified epidermal cellscalled subsidiary or accessory cells. Thestoma and guard cells are collectivelycalled the stomatal apparatus. In mostplants stomata are located mainly in thelower epidermis of the leaf, where evapo-ration rates are lower. In many xerophytesand in some conifers of cooler climates,where soil water may be difficult to obtainin winter, the guard cells – and hence thestoma – are sunk in pits in the epidermis,often protected by hairs from passing aircurrents. TRANSPIRATION rates are thusgreatly reduced, while allowing gaseousexchange for photosynthesis. In verticalleaves, such as those of grasses, they maybe equally distributed on both surfaces.

stomatal apparatus See stoma.

stomium (pl. stomia) The site at which asporangium or pollen sac ruptures to re-lease the spores or pollen.

stone cell (brachysclereid) See sclereid.

strain Any group of similar or identicalindividuals, such as a CLONE, mating strain,physiological RACE, or PURE LINE.

stratification 1. A seed treatment thatenables seeds that require vernalization togerminate the following spring: the seedsare placed between layers of moist sand orpeat and exposed to low temperatures,usually by leaving them outside throughthe winter.2. The existence of layers of water of dif-ferent density in a lake or other body ofwater. The differences in density may bedue to temperature or to salinity. In manylakes in summer that is a warm low-densitylayer (EPILIMNION) lying above a colderdenser layer (the HYPOLIMNION), the zoneof rapid temperature change between thetwo layers being called the thermocline.

strobilus (pl. strobili) A group of sporo-phylls (spore-bearing leaves) arrangedaround a central axis, found in gym-nosperms, Lycophyta (clubmosses), andSphenophyta (horsetails). In the club-mosses and horsetails only one type of stro-bilus is formed, while in the gymnospermsfemale megastrobili and male microstrobiliboth develop. The strobili of gymnospermsand cycads are also termed cones.

stroma (pl. stromata) 1. The colorlessground matter between the grana lamellaein a CHLOROPLAST. It is the site of the dark(light-independent) reactions of photosyn-thesis.2. A solid mass of fungal hyphae, some-times including host tissue, in which fruit-ing bodies may be produced. An example isthe compact black fruiting body of theergot fungus (Claviceps purpurea).

stromatolite A layered cushionlikemass of carbonate-rich rock formed by theactions of CYANOBACTERIA. Communitiesof these organisms secrete calcium carbon-ate, often building up concentric rings ofdeposits forming cushions up to a meter indiameter. Modern stromatolite-buildingcommunities are confined to salt flats orshallow salty lagoons where invertebrate

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predators cannot survive, but the fossilrecord demonstrates a much more wide-spread distribution in the Precambrianperiod. Some stromatolites date backnearly 4000 million years, making themthe oldest known fossils.

style The stalklike portion of a carpel,joining the ovary and the stigma. The stylemay be elongated in plants relying onwind, insect, or animal pollination so thatthe stigma has a greater chance of cominginto contact with the pollinating agent.

subclimax See climax.

suberin A fatty acid polyester producedin the walls of the endodermis and in bark,where it renders the tissue impermeable towater and resistant to decay. The Caspar-ian strip in roots and some stems containssuberin, lignin, or similar substances. Bypreventing the passage of water throughthe cell walls of the endodermis, the Cas-parian strip forces it to pass through the cy-toplasm, thus allowing selective uptake orexclusion of dissolved substances. Seecutin.

sublittoral 1. (neritic) The marine zoneextending from low tide to a depth ofabout 200 m, usually to the edge of thecontinental shelf. Light penetrates to theseabed, and the water is well oxygenated.Large algae (e.g. kelps) are found in shal-lower waters while certain Rhodophyta(red algae) may be found in deeper water.Compare littoral; benthic.2. The zone in a lake or pond between theLITTORAL and PROFUNDAL zones, extendingfrom the edge of the area occupied byrooted plants to a depth of about six to tenmeters, where the water temperature de-clines. Its depth is limited by the compen-sation level – the depth at which the rate ofphotosynthesis is equalled by the rate ofrespiration, and below which plants can-not live (but some phytoplankton can).

subsere See sere.

subsidiary cell (accessory cell) One of a

number of specialized epidermal cells of aplant that are found adjacent to the guardcells, and may help in opening and closingthe stomata. The number, arrangementand character of subsidiary cells is often ofvalue in taxonomy.

subspecies The taxonomic group belowthe SPECIES level. Crosses can generally bemade between subspecies of a given speciesbut this may be prevented in the wild byvarious isolating mechanisms, e.g. geo-graphical isolation or different floweringtimes. Subspecies usually differ from eachother in morphology, and occupy distincthabitats or regional locations.

substrate 1. The molecule or moleculesupon which an enzyme acts.2. The nonliving material upon which anorganism lives or grows.

succession A progressive series ofchanges in vegetation and animal life of anarea over time from initial colonization tothe final stage, or CLIMAX. The climax is adynamic equilibrium because, although thesuccession can progress no further underthe environmental factors present at thetime, the populations present change, e.g.trees die, creating gaps for other species tocolonize. In addition, the climate is seldomcompletely stable – at best it is cyclical,with variations from year to year. See sere.

succinic acid (butanedioic acid) A di-carboxylic acid formed by fermentation ofsugars. It is an intermediate in the KREBS

CYCLE, and is formed from succinyl CoA ina reaction linked to the formation of GTP.

succulent A fleshy plant. Succulents,such as cacti, store water in large paren-chyma cells in swollen stems and leaves.

sucker An adventitious undergroundshoot that at some stage emerges above thesoil surface and gives rise to a new plant,which initially is nourished by the parentplant until it becomes established.

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Sudan stains

sucrase An enzyme that breaks downsucrose into glucose and fructose.

sucrose A SUGAR that occurs in manyplants. Sucrose is a disaccharide formedfrom a glucose unit and a fructose unit.Sucrose is the main transport sugar inhigher plants. It is formed in the chloro-plasts from fructose 6-phosphate and glu-cose, and transported in the phloem to therest of the plant. It is hydrolyzed to a mix-ture of fructose and glucose by the enzymeinvertase. Since this mixture has a differentoptical rotation (levorotatory) from theoriginal sucrose, the mixture is called in-vert sugar. Invert sugar is found in manyfruits.

suction pressure (SP) See osmosis.

Sudan stains Any of various stainsbased on aniline dyes, and used to colorfats and waxes, and hence cutinized (e.g.cuticle) and suberized (e.g. Casparian strip)tissues. See staining.

sugar (saccharide) One of a class of car-bohydrates that are soluble in water. Sugarmolecules consist of linked carbon atomswith –OH groups attached, and either analdehyde or ketone group. The simplestsugars are the monosaccharides, such asglucose and fructose, which cannot be hy-drolyzed to sugars with fewer carbonatoms. They can exist in a chain form or ina ring formed by reaction of the ketone oraldehyde group with an –OH group on oneof the carbons at the other end of the chain.It is possible to have a six-membered (pyra-nose) ring or a five-membered (furanose)ring. Monosaccharides are classified ac-cording to the number of carbon atoms: apentose has five carbon atoms and a hex-ose six. Monosaccharides with aldehydegroups are aldoses; those with ketonegroups are ketoses. Thus, an aldohexose isa hexose that has an aldehyde group; aketopentose is a pentose with a ketonegroup, etc.

Two or more monosaccharide units canbe linked in disaccharides (e.g. sucrose),

trisaccharides, etc. See also fructose; glu-cose; polysaccharide; sucrose.

sugar acid An acid formed from amonosaccharide by oxidation. Oxidationof the aldehyde group (CHO) of the aldosemonosaccharides to a carboxyl group(COOH) gives an aldonic acid; oxidationof the primary alcohol group (CH2OH) toCOOH yields uronic acid; oxidation ofboth the primary alcohol and carboxylgroups gives an aldaric acid. The uronicacids are biologically important, beingcomponents of many polysaccharides, forexample glucuronic acid (from glucose) is amajor component of gums and cell walls,while galacturonic acid (from galactose)makes up pectin. Ascorbic acid or vitaminC is an important aldonic acid found uni-versally in plant tissues, particularly in cit-rus fruits. Another aldonic acid, gluconicacid, is an intermediate in the pentosephosphate pathway.

sugar alcohol (alditol) An alcohol de-rived from a monosaccharide by reductionof its carbonyl group (CO) so that eachcarbon atom of the sugar has an alcoholgroup (OH). For example, glucose yieldssorbitol, common in fruits, and mannoseyields MANNITOL. GLYCEROL is important inlipid metabolism, and INOSITOL is an inter-mediate in the synthesis of cell wall poly-saccharides.

sugar phosphate A phosphate deriva-tive of a MONOSACCHARIDE.

sulfur An essential element in livingtissues, being contained in the amino acids cysteine and methionine and hence innearly all proteins. Sulfur atoms are alsofound bound with iron in ferredoxin, one of the components of the electron-transport chain in photosynthesis. It is a component of coenzyme A. Plants takeup sulfur from the soil as the sulfate ionSO4

2–. The sulfides released by decay of or-ganic matter are oxidized to sulfur by SUL-FUR BACTERIA of the genera Chromatiumand Chlorobium, and further oxidized tosulfates by bacteria of the genus Thiobacil-

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lus. There is thus a cycling of sulfur in na-ture.

sulfur bacteria Filamentous autotrophicchemosynthetic bacteria that derive energyby oxidizing sulfides to elemental sulfurand build up carbohydrates from carbondioxide. They use sulfides instead of wateras a source of electrons in photosynthesis,releasing sulfur instead of oxygen. An ex-ample is Beggiatoa. They are found mainlyin sulfur-rich muds and springs, includinghydrothermal vents. There are two maingroups: the green sulfur bacteria (Chloro-bia) and the purple sulfur bacteria (certainmembers of the Proteobacteria). A few Ar-chaea, e.g. Sulfolobus, can oxidize elemen-tal sulfur. As well as sulfides, some bacteriaoxidize thiosulfates, polythionates, andsulfites. Sulfur bacteria play an importantrole in the cycling of sulfur in the eco-system.

supergene A collection of closely linkedgenes that determine a particular trait orseries of inter-related traits and behave as asingle unit because crossing over betweenthem is very rare. An example is the S/sgene, which determines pin (s) and thrum(S) style length in certain species of Prim-ula.

superior In plant science, the term isused with reference to the position of theovary in relation to the other parts of theflower. When the ovary is superior, thepetals, sepals, and stamens are inserted atthe base of the ovary where it joins theflowerstalk, as in buttercup (Ranunculus)flowers. In a floral formula a superiorovary is denoted by a line below the carpelnumber. Compare inferior. See also hypo-gyny; perigyny.

suspension culture A method of grow-ing free-living single cells or small clumpsof cells in a liquid medium; the liquidmedium is agitated to keep the cells in sus-pension. Microorganisms or cells of plantcallus tissue may be grown in this way.

suspensor A temporary stalklike row ofcells, found in angiosperms, that differenti-

ates from the proembryo by mitosis andpushes the embryo into the nutritive en-dosperm after fertilization. In certain Lyco-phyta (e.g. Selaginella), a few ferns (e.g.Botrychium), and in gymnosperms itpushes the embryo into the female gameto-phyte tissue. It also serves as a passage fornutrients to pass from the parent tissue tothe embryo.

swamp An area of vegetation domi-nated by trees that develops on ground thatis normally waterlogged or covered bywater all year round, such as the margin ofa lake, a river floodplain, or an area ofwater-retentive clay in an arid region.Swamp vegetation represents the earlystages of a HYDROSERE or HALOSERE. Thepresence of trees distinguishes it from amarsh. Coastal and estuarine swamps inwarm climates are dominated by MAN-GROVES. The swamp vegetation slows theflow of water, and dead plant matter buildsup.

syconium (syconus) A composite inflo-rescence or fruit in which the small indi-vidual flowers are borne on the inside of ahollow receptacle, as in figs (Ficus). Thesedevelop into achenes (the ‘pips’ of thefruit). See pseudocarp.

symbiosis (pl. symbioses) Any close as-sociation between two or more differentorganisms, as seen in parasitism, mutual-ism, and commensalism. Often one or bothorganisms is dependent on the other. Theterm is usually used more narrowly tomean MUTUALISM, but many mutualistic as-sociations may have once been parasitic ormay become so at some stage in their lifecycle, e.g. plants and their mycorrhizalfungi.

sympatric species See species.

symplast The living system of intercon-nected protoplasts extending through aplant body. Cytoplasmic connections be-tween cells are made possible by the plas-modesmata. The symplast pathway is animportant transport route through theplant. Compare apoplast.

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syncarpy

symplastic growth A form of plantgrowth in which neighboring cell wallsstay in contact and grow at the same rate.

sympodial Describing the system ofbranching in plants in which the terminalbud of the main stem axis stops growingand growth is taken over by lateral buds.These in turn lose their dominance and lat-eral buds take over their role. Comparemonopodial.

symport A process involving a mem-brane protein that transports two kinds ofsubstance (substrates) across a membrane,one of which is usually an ion. Compareantiport; proton pump.

synapsis (pairing) The association ofhomologous chromosomes during theprophase stage of meiosis that leads to theproduction of a haploid number of biva-lents.

syncarpous See carpel.

syncarpy The condition in which anovary is made up of fused carpels, as in theprimrose (Primula vulgaris). Compareapocarpy.

synchronous culture A culture of cellsin which all the individuals are at approxi-mately the same point in the cell cycle.Cells can be synchronized by a variety ofmeans, e.g. temperature, shock, or drugs.

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tactic movement See taxis.

taiga See forest.

tandem DNA See repetitive DNA.

tangential Orientated at a tangent to acircle, the circle being, for example, thesurface of a plant stem.

tannin One of a mixed group of sub-stances that, as defined by industry, precip-itate the gelatin of animal hides to formleather. Tannins are also used in dyeingand ink manufacture. They are polymersderived either from carbohydrates andphenolic acids by condensation reactions,or from flavonoids. Many plants accumu-late tannins, particularly in leaves, fruits,seed coats, bark, and heartwood. Their as-tringent taste may deter animals from eat-ing the plant and they may discourageinfection. Species that produce largeamounts of tannin include tea (Camelliasinensis) and the bark of oak (Quercus).

tapetum (pl. tapeta) A food-rich layersurrounding the spore mother cells in theanthers of vascular plants. These cells usu-ally disintegrate, liberating food substancesthat are subsequently absorbed by thespore mother cells and the developingspores.

taproot A persistent primary root thatgrows vertically downward. Swollen tap-roots used for food storage are common inmany biennial plants, such as carrot (Dau-cus carota).

Taxaceae A family of evergreen conifer-ous trees and shrubs, the yews, withneedlelike or linear leaves.

taxis (pl. taxes) (tactic movement) Move-ment of an entire cell or organism (i.e.locomotion) in response to an externalstimulus, in which the direction of move-ment is dictated by the direction of thestimulus. Movement toward the stimulus ispositive taxis and away from the stimulusis negative taxis. It is achieved by proto-plasmic streaming, extrusion of cell sub-stances, or by locomotory appendages,such as flagella and undulipodia. See aero-taxis; chemotaxis; phototaxis. Comparenastic movements; tropism.

taxol A diterpene originally isolatedfrom the bark of the Pacific yew (Taxusbrevifolia), that has proved an effectivetreatment for solid tumors, especiallybreast cancer. It blocks tumor cells by sta-bilizing and polymerizing microtubules.

taxon (pl. taxa) A group of any rank intaxonomy. Ranunculaceae (a family) andTriticum (a genus) are examples.

taxonomy The area of systematics thatcovers the principles and procedures ofclassification, specifically the classificationof variation in living organisms. See classi-fication; systematics.

TCA cycle See Krebs cycle.

telophase The final stage in mitosis andmeiosis, in which the separated chromatids(mitosis) or homologous chromosomes ofbivalents (meiosis) collect at the poles ofthe spindle and the nuclei of the daughtercells are formed. During this stage in mito-sis and the second division of meiosis thechromosomes uncoil and disperse, the nu-clear spindle degenerates, nucleoli reap-pear, and a new nuclear membrane forms.

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The cytoplasm may also divide during thisphase.

temperate phage A DNA bacterio-phage that becomes integrated into thebacterial DNA and multiplies with it,rather than replicating independently andcausing lysis of the bacterium. A bacteriumthus infected is said to be lysogenic. Seelysogeny. Compare virulent phage.

temporary stain See staining.

tendril A slender structure found inplants, used for twining and support. Itmay be a modified terminal bud, as in the grapevine (Vitis); a modified lateralbranch, as in the passion flower (Passi-flora); a modified leaf, as in Clematis mon-tana; a modified leaflet, as seen in many ofthe pea family (Fabaceae); or a modified in-florescence, as in Virginia creeper (Par-thenocissus). Tendrils may be branched, orunbranched, and may have terminal adhe-sive disks, as in the Virginia creeper.

terminal bud See bud.

terminalization The movement of chi-asmata to the end of the bivalent arms, aprocess that may occur during lateprophase I of meiosis. The chiasmata canslip off the ends of the bivalents, and thuschiasma frequency may be reduced by ter-minalization.

terpene One of a complex group oflipids based on the hydrocarbon skeletonC5H8 (isoprene). Monoterpenes are builtfrom two C5 residues (C10H16), diterpenesfrom four, etc. The C10 to C20 terpenes arepresent in essential oils, giving the charac-teristic scent of some plants (e.g. mint,lemon). Some terpenoid substances arephysiologically active, e.g. vitamins A, E,and K.

Tertiary The larger and older period ofthe Cenozoic, being composed of the Pale-ocene, Eocene, Oligocene, Miocene, andPliocene epochs (65–2 million years ago).Literally the ‘third age’, it is characterized

by the rapid evolution and expansion ofangiosperms and the emergence of mam-mals. The ferns underwent adaptive radia-tion, and the angiosperms underwent aneven greater explosion of adaptive radia-tion, in which flowers became more spe-cialized. This went hand in hand with theevolution of specialist insect pollinators,especially bees, butterflies, and moths. Theclimate was warm and wet at the start ofthe period, but became colder and drierlater, and this was accompanied by the ex-pansion in range of grasses. Many moderngenera of conifers arose in the Tertiary, butother gymnosperms declined. See also geo-logical time scale.

tertiary structure The mode of foldingof a polypeptide (a-helix or b-pleatedsheet) in a PROTEIN. The shape may be sta-bilized by HYDROGEN BONDS, disulfidebridges, and hydrophobic interactions,whereby hydrophobic parts of thepolypeptide are protected from the sur-rounding aqueous solution by hydrophilicparts. Such folding determines the shape ofthe active site of enzymes. See conforma-tion; primary structure; quaternary struc-ture; secondary structure.

testa The hard dry protective covering ofa seed, formed from the integuments of theovule. After fertilization the layers of theinteguments fuse and become thickenedand pigmented.

test cross See backcross.

tetrad 1. A group of four cells formed asa result of meiosis in a spore mother cell.The four cells may develop into spores, asin the formation of pollen grains.2. In meiosis, the association of four ho-mologous chromatids seen during thepachytene stage of prophase.

tetraploid A cell or organism containingfour times the haploid number of chromo-somes. Tetraploid organisms may arise bythe fusion of two diploid gametes that haveresulted from the nondisjunction of chro-mosomes at meiosis, or by nondisjunction

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of the chromatids during the mitotic divi-sion of a zygote. Many stable tetraploidsarise initially from hybridization. See alsopolyploid; allotetraploid; autotetraploid.

thallus (pl. thalli) A simple plant bodyshowing no differentiation into root, leaf,and stem and lacking a true vascular sys-tem. It may be uni- or multicellular, and isfound in the algae, lichens, bryophytes,and the gametophyte generation of the Fil-icinophyta, Lycophyta, and Sphenophyta.

Theophrastus of Eresus (372–287 BC)Ancient Greek botanist and philosopher.Theophrastus was a pupil of Plato and as-sisted Aristotle at the Lyceum in Athensafter Plato’s death. He became head of theschool when Aristotle died. He is oftenconsidered to be the father of scientificbotany because of his work on classifyingand naming plants, which he recorded inhis prolific writings. His pupils were en-couraged to observe plants growing neartheir homes, which were often far awayfrom Athens, and possibly helped him tomake the connection between plants andthe environment, especially their adapta-tions to soil and climate. His Enquiry intoPlants (nine volumes) is important eventoday. It is evident from his writings thathe had distinguished between monocotyle-dons and dicotyledons and had discoveredthat seeds and fruits were derived fromflowers.

thermocline See stratification.

thermonasty (thermonastic movements)A NASTIC MOVEMENT in response to changein temperature. For example, Crocus flow-ers open rapidly if the temperature in-creases by 5 to 10°C, as the inner side ofthe petals grows faster than the outer side.

thermoperiodism The phenomenonshown by certain plants, e.g. Chrysanthe-mum and tomato (Lycopersicon esculen-tum) in which there is a response to dailyalternations of low and high temperature.Such plants flower earlier and more pro-fusely if subjected to low night and high

day temperatures. See also photoperi-odism; vernalization.

thermophilic Describing microorganismsthat require high temperatures (around60°C) for growth. It is exhibited by certainbacteria that grow in hot springs or com-post and manure. Compare mesophilic;psychrophilic.

therophyte A plant that survives part ofthe year as a seed and completes its lifecycle during the remainder of the year. Seealso Raunkiaer’s plant classification.

thiamine (vitamin B1) One of the water-soluble B-group of VITAMINS. It is a pyrim-idine compound that is synthesized byplants but not by some microorganismsand most vertebrates. It is therefore re-quired in the diet of mammals. Goodsources of thiamine include unrefined ce-real grains.

thigmotropism (haptotropism) A tro-pism in which the stimulus is touch. Thetendrils of climbing plants are thig-motropic. The part touching the substratebends toward the stimulated side, so bring-ing more of the tendril into contact withthe substrate, until it encircles it. See tro-pism.

thin-layer chromatography A chro-matographic method in which a glass plateis covered with a thin layer of inert ab-sorbent material (e.g. cellulose or silica gel)and the materials to be analyzed are spot-ted near the lower edge of the plate. Thebase of the plate is then placed in a solvent,which rises up the plate by capillary action,separating the constituents of the mixtures.The principles involved are similar to thoseof paper chromatography and, like paperchromatography, two-dimensional meth-ods can also be employed. See paper chro-matography.

thorn A stiff sharply pointed woodyprocess that may be found on the stems ofvascular plants. It is a modified branch andis supplied with vascular tissue, e.g.

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hawthorn (Crataegus monogyna). Com-pare prickle; spine.

thorn forest See forest.

threonine An AMINO ACID derived fromaspartic acid. It is broken down to formglycine and acetyl CoA. Isoleucine can besynthesized from threonine.

thylakoid An elongated flattened fluid-filled sac that forms the basic unit of thephotosynthetic membrane system in chloro-plasts and photosynthetic bacteria. Seechloroplast.

thymidine The nucleoside formed whenthymine is linked to D-ribose by a b-glyco-sidic bond. See nucleoside.

thymine A nitrogenous base found inDNA, but not in RNA. It has a PYRIMIDINE

ring structure. It base pairs with adenine inthe complementary strand of DNA. Seebase pairing; thymidine.

tissue In a multicellular organism, agroup of cells that is specialized for a par-ticular function, e.g. palisade mesophyll.Several different tissues may be incorpo-rated into an organ, e.g. a leaf. Compareorgan.

tissue culture The growth of cells, tis-sues, or organs in suitable media in vitro.Such media must normally be sterile, cor-rectly pH balanced, and contain all the nec-essary micro and macronutrients andhormones for growth. Studies of such cul-tures have shed light on physiologicalprocesses that would be difficult to followin the living organism. For example, the cy-tokinins were discovered through work ontobacco pith tissue culture.

tocopherol (vitamin E) A terpenoid-likesubstance found especially in certain seeds,such as cereals, where it probably preventsoxidation of lipids, thus prolonging seedviability.

tonoplast The membrane that sur-

rounds the large central vacuole of plantcells.

torus (pl. tori) A disk-shaped structureformed from lignified primary cell-wallmaterial on the middle lamella of a bor-dered pit. The structure is found mainly inthe conifers. It is thought to act like a valve,sealing the pit when the pressure on thetwo sides is unequal, e.g. when an adjacenttracheid is damaged.

totipotency The ability shown by manyliving cells to form all the types of tissuesthat constitute the mature organism. Thismay be achieved, even if the cells havecompletely differentiated, provided thatthe appropriate balance of nutrients andhormones is given. The best example ofthis phenomenon is the formation of ad-ventitious embryos in carrot tissue cul-tures.

toxin A chemical produced by apathogen (e.g. bacteria, fungi) that causesdamage to a host cell in very low concen-trations. The toxin may be formed withinthe pathogen and released when it dies (en-dotoxin), or secreted through its cell wall(exotoxin). Toxins are often similar to theenzymes of the host and interfere with theappropriate enzyme systems.

trabecula (pl. trabeculae) An elongatedcell wall, cell, or line of cells across a cav-ity. In plants, trabeculae may be found inSelaginella stems, where they suspend thesteles in large air spaces.

trace element See micronutrient.

tracheid An elongated xylem conduct-ing element with oblique end walls. Tra-cheids have heavily lignified walls and theonly connection between adjacent tra-cheids is through paired pits, which aremainly concentrated in the end walls.Many tracheids lack protoplasm at matu-rity and have lignified secondary cell walls.Tracheids form the only xylem conductingtissue of vascular plants other than an-

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giosperms, some Gnetales, and a few fernssuch as bracken. Compare vessel.

tracheophyte (Tracheophyta) Any plantwith a differentiated vascular system; i.e.all plants except the liverworts, mosses,and hornworts.

trama The inner tissue of the gills in ba-sidiomycete fungi that is made up ofloosely packed hyphae.

transamination The transfer of anamino group from an AMINO ACID to an a-keto acid, producing a new a-keto acid anda new amino acid. This is catalyzed by atransaminase enzyme in conjunction withthe coenzyme pyridoxal phosphate. Theamino group becomes attached to thecoenzyme to form pyridoxamine phos-phate, and is then transferred to the a-ketoacid, which is usually pyruvic acid, ox-aloacetic acid, or a-ketoglutaric acid.Transamination is a key step in the biosyn-thesis and breakdown of most amino acids.

transcription The process in living cellswhereby RNA is synthesized according tothe template embodied in the base se-quence of DNA, thereby converting thecell’s genetic information into a coded mes-sage (messenger RNA, mRNA) for the as-sembly of proteins, or into the RNAcomponents required for protein synthesis(ribosomal RNA and transfer RNA). Theterm is also applied to the formation of sin-gle-stranded DNA from an RNA template,as performed by the enzyme reverse tran-scriptase, for example in retrovirus infec-tions. Details of DNA transcription differbetween prokaryote and eukaryote cells,but essentially it involves the followingsteps. Firstly, with the aid of a helicase en-zyme, the double helix of the DNA mol-ecule is unwound in the region of the sitemarking the start of transcription for a par-ticular gene. The enzyme RNA polymerasemoves along one of the DNA strands, thetranscribed strand (or anticoding strand,since the code is carried by the comple-mentary base sequence of the RNA), andnucleotides are assembled to form a com-

plementary RNA molecule. The poly-merase enzyme proceeds until reaching a stop signal, when formation of the RNA strand is terminated. Behind the en-zyme, the DNA double helix re-forms,stripping off the newly synthesized RNAstrand. In eukaryotes, transcription is initi-ated and regulated by a host of proteinscalled transcription factors; in prokaryotesan accessory sigma factor is essential fortranscription. See also translation.

transduction The transfer of part of theDNA of one bacterium to another by atemperate bacteriophage. The process doesoccur naturally but is mainly known as atechnique in RECOMBINANT DNA TECHNOL-OGY, and has been used in mapping thebacterial chromosome.

transect A line or belt designed to studychanges in species composition across aparticular area. The transect samplingtechnique is most often used in plant ecol-ogy to study changes in the composition ofvegetation. See also quadrat.

transferase An enzyme that catalyzesreactions in which entire groups or radicalsare transferred from one molecule to an-other. For example, hexokinase catalyzesthe transfer of a high-energy terminalphosphate group from ATP to glucose togive glucose 6-phosphate and ADP.

transfer cell A specialized type of plantcell in which the cell wall forms protuber-ances into the cell, thus increasing the sur-face area of the wall and plasmamembrane. Transfer cells are active cellscontaining many mitochondria, and areconcerned with short-distance transport ofsolutes. They are common in many situa-tions, for example as gland cells and epi-dermal cells, and in xylem and phloemparenchyma, where they are concernedwith active loading and unloading of ves-sels and sieve tubes.

transfer RNA (tRNA) A type of RNAthat participates in protein synthesis in liv-ing cells. It attaches to a particular amino

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acid and imports this to the site of polypep-tide assembly at the ribosome when the ap-propriate codon on the messenger RNA isreached. Each tRNA molecule consists ofroughly 80 nucleotides; some regions ofthe molecule undergo base pairing andform a double helix, while in others thetwo strands separate to form loops. Whenflattened out the tRNA molecule has acharacteristic ‘cloverleaf’ shape with threeloops; the base of the ‘leaf’ carries theamino acid binding site, and the middleloop contains the ANTICODON, whose basetriplet pairs with the complementarycodon in the mRNA molecule. Hence, be-cause there are 64 possible codons in thegenetic code, of which about 60 or so codefor amino acids, there may be up to 60 orso different tRNAs in a cell, each with adifferent anticodon, although some ofthem will bind the same amino acid. Thecorrect amino acid is attached to a tRNAmolecule by an enzyme called an amino-acyl-tRNA transferase. There are 20 ofthese, one for each type of amino acid. Thisattachment also involves the transfer of ahigh-energy bond from ATP to the aminoacid, which provides the energy for peptidebond formation during TRANSLATION.

transformation A permanent geneticrecombination in a bacterial cell, in whicha DNA fragment is incorporated into theDNA of the cell. This may be demonstratedby growing bacteria in the presence of deadcells, culture filtrates, or extracts of relatedstrains. The bacteria acquire genetic char-acters of these strains.

transgenic Describing organisms, espe-cially eukaryotes, containing foreign ge-netic material. Genetic engineering hascreated a wide range of transgenic animals,plants, and other organisms for both ex-perimental and commercial purposes. Ex-amples include herbicide-resistant cropplants and plants that secrete pharmaceuti-cals. See recombinant DNA.

transition zone The zone in a vascularplant where the root and shoot structuresmerge and where arrangement of the vas-

cular tissue is intermediate between that ofthe root and shoot. See hypocotyl.

translation The process whereby thegenetic code of messenger RNA (mRNA) isdeciphered by the machinery of a cell tomake proteins. Molecules of mRNA are ineffect coded messages based on informa-tion in the cell’s genes and created by theprocess of TRANSCRIPTION. They relay thisinformation to the sites of protein synthe-sis, the RIBOSOMES. In eukaryotes these arelocated in the cytoplasm, so the mRNAmust migrate from the nucleus. The firststage in translation is initiation, in whichthe two subunits of the ribosome assembleand attach to the mRNA molecule near theinitiation codon, which signals the begin-ning of the message. This also involves var-ious proteins called initiation factors, andthe initiator TRANSFER RNA (tRNA), whichalways carries the amino acid N-formylmethionine. The next stage is elongation,in which the peptide chain is built up fromits component amino acids. This involvesthe participation of several proteins calledelongation factors, and tRNA moleculesthat successively occupy two sites on thelarger ribosome subunit in a sequence de-termined by consecutive codons on themRNA. As each pair of tRNAs occupiesthe ribosomal sites, their amino acids arejoined together by a peptide bond. As theribosome moves along the mRNA to thenext codon, the next tRNA enters the firstribosomal site, and so on, leading to elon-gation of the peptide chain. Having deliv-ered its amino acid, the depleted tRNA isreleased from the second ribosomal site,which is then occupied by the tRNA withthe growing chain. This process continuesuntil the ribosome encounters a termina-tion codon. A protein called a release fac-tor binds directly to the stop codon,causing the addition of a water moleculeinstead of an amino acid to the polypeptidechain. The polypeptide chain is thenreleased and the ribosome complex dissoci-ates, marking the termination of transla-tion. Following its release, the polypeptidemay undergo various changes, such as theremoval or addition of chemical groups, or

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even cleavage into two parts. This post-translational modification produces thefully functional protein. Folding of the pro-tein is assisted by a class of moleculescalled chaperones.

translocation 1. The movement of min-eral nutrients, elaborated food materials,and hormones through the plant. In vascu-lar plants, the xylem and phloem serve totranslocate such substances.2. See chromosome mutation.

transpiration The loss of water vaporfrom the surface of a plant. Most is lostthrough stomata when they are open forgaseous exchange. It has been shown thattranspiration through the stomataamounts to 90% of the transpiration thatoccurs from a water surface of the samearea as a the leaf. Typically, about 5% islost directly from epidermal cells throughthe cuticle (cuticular transpiration) and aminute proportion through lenticels. Acontinuous flow of water, the transpirationstream, is thus maintained through theplant from the soil via root hairs, root cor-tex, xylem, and tissues such as leaf meso-phyll served by xylem. Evaporation ofwater from the leaf surface sets up a waterpotential gradient that extends to theroots, helping to draw up water; waterflow is enhanced by the narrow diameter ofthe xylem vessels, which favor capillary ac-tion. ROOT PRESSURE may be a contributingfactor. Water evaporates from wet cellwalls into intercellular spaces and diffusesout through stomata. Transpiration maybe useful in maintaining a flow of solutesthrough the plant and in helping to coolleaves through evaporation, but is oftendetrimental under conditions of watershortage, when wilting may occur. It is fa-vored by low humidity, high temperatures,and moving air. A major control is the de-gree of opening of the stomata. Compareguttation. See antitranspirant; cohesiontheory.

transposon (transposable geneticelement; jumping gene) A segment of anorganism’s DNA that can insert at varioussites in the genome, either by physically

moving from place to place, or by produc-ing a copy that inserts elsewhere. The sim-plest types are called insertion sequences;these comprise about 700–1500 base pairs.More complex ones, called compositetransposons, have a central portion, whichmay contain functional genes, flanked byinsertion sequences. Transposons can af-fect both the genotype and phenotype ofthe organism, e.g. by disrupting gene ex-pression, or causing deletions or inver-sions. They are common in bacteria, wherethey usually contain only one or two genes.In eukaryotes, transposons account formuch of the repetitive DNA in the genome.Compare plasmid. See repetitive DNA.

tree ferns See Filicinophyta.

triacylglyceride (triglyceride) An esterof glycerol in which all the –OH groups areesterified; the acyl groups may be the sameor different. Plant triacylglycerols are usu-ally unsaturated. They are synthesizedfrom glycerol phosphate and fatty acylCoAs, usually in the microsomes. Lipasescatalyze the breakdown of triacylglycerolsto glycerol and free fatty acids. Triacyl-glycerols are important food reserves, espe-cially in seeds such as rape (Brassicanapus), linseed (Linum usitatissimum),castor bean (Ricinus communis) and co-conut (Cocos nucifera). They are commer-cial sources of fats (solid triacylglycerols)and oils (liquid triacylglycerols). See alsocarboxylic acid; glyceride; lipid; micro-some.

Triassic The oldest period of the Meso-zoic era, 250–215 million years ago. Dur-ing the Triassic, the climate graduallychanged from arid to more temperate.Conditions for preservation in the drierperiod were poor, so there are few fossils.There was a diversification of the gym-nosperms, and fossil groups such as theBennettitales and Caytoniales evolved dur-ing this period. See also geological timescale.

tribe A taxonomic rank consisting of agroup of closely related genera. A numberof tribes comprise a subfamily. The tribe is

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introduced only in classifications of verylarge families such as the grasses (Poaceae).

tricarboxylic acid cycle See Krebscycle.

trichocyst See Dinomastigota.

trichome See hair.

triglyceride See triacylglyceride.

triplet A sequence of three nucleotidebases on a DNA or messenger RNA mol-ecule. See genetic code.

triploid A cell, tissue or organism con-taining three times the haploid number ofchromosomes. In flowering plants the en-dosperm tissue is usually triploid, resultingfrom the fusion of one of the pollen nucleiwith the two polar nuclei. Triploids areusually sterile.

trisomy See aneuploidy.

trophic level In complex natural com-munities, organisms whose food is ob-tained from plants by the same number ofsteps are said to belong to the same trophicor energy level. The first and lowest trophiclevel contains the producers, green plantsthat convert solar energy to food by photo-synthesis. Herbivores occupy the secondtrophic level and are primary consumers:they eat the members of the first trophiclevel. At the third level carnivores eat theherbivores (the secondary consumer level),and at the fourth level secondary carni-vores eat the primary carnivores (the ter-tiary consumer level). These are generalcategories, as many organisms feed on sev-eral trophic levels, for example omnivoreseat both plants and animals. Decomposersor transformers occupy a separate trophiclevel, which consists of organisms such asfungi and bacteria, which break downdead organic matter into nutrients usableby the producers. See food chain.

tropism (tropic movement) A direc-tional growth movement of part of a plant

in response to an external stimulus. Tro-pisms are named according to the stimulus.The organ is said to exhibit a positive ornegative tropic response, depending onwhether it grows toward or away from thestimulus respectively, e.g. shoots are posi-tively phototropic but negatively gravit-ropic. Growth straight toward or awayfrom the stimulus (0°and 180°orientationrespectively) is called orthotropism. Pri-mary roots and shoots are orthotropic tolight and gravity. By contrast, growth atany other angle to the direction of the stim-ulus as by branches or lateral roots is calledplagiotropism. The mechanism involved inthe latter is poorly understood. In diatro-pism the plant part is orientated at rightangles to the direction of the stimulus. Forexample, rhizomes grow horizontally in re-sponse to gravity (diagravitropism), andleaves often grow at right angles to the in-cident light. Since the receptor for the stim-ulus is often separate from the region ofgrowth, tropic movements are often medi-ated by hormones. See phototropism; grav-itropism. Compare nastic movements;taxis.

tryptophan See amino acids.

tuber A swollen underground stem orroot that contains stored food and acts asan organ of perennation and vegetativepropagation. Stem tubers, e.g. potato(Solanum tuberosum) develop at the end ofunderground stems by swelling of nodesand internodes. There is an increase in pithtissue to form a round tuber that bearsbuds in the axils of greatly reduced scaleleaves. The stem connecting the tuber tothe parent plant then severs. Root tubersmay develop in the same way from adven-titious roots, as in Dahlia. A root tuber canbe distinguished for a stem tuber by the ab-sence of buds or ‘eyes’. See perennatingorgan.

tubulin See microtubule.

tundra A region or type of vegetation athigh latitudes and high altitude. Tundra iswidespread in the Arctic north of the tree

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lines of North America and Eurasia, butoccurs in only scattered localities onAntarctic islands, where it consists mainlyof lichens and mosses.

tunica–corpus theory A theory of api-cal organization and development that dis-tinguishes two separate tissue zones, thetunica and the corpus, in the apex of aflowering plant. The tunica is made up ofone or more peripheral layers in which celldivision is mostly anticlinal. The corpus isthe inner area of tissue in which cellarrangement and division is irregular. Theepidermis originates in the tunica regionand the other stem tissues arise from eitherthe tunica or the corpus, depending on thespecies. Compare histogen theory.

turgor The state, in a plant or prokary-ote cell, in which the protoplast is exertinga pressure on the cell wall owing to the in-take of water by osmosis. Being slightlyelastic, the cell wall bulges, but is rigidenough to prevent water entering to thepoint of bursting. The cell is then said to beturgid. Turgidity is the main means of sup-port of herbaceous plants and other unlig-nified tissues. See osmosis; plasmolysis;turgor pressure.

turgor pressure (pressure potential) Thehydrostatic pressure exerted by the con-tents of a cell against the cell wall. It resultsfrom the uptake of water into the cell byosmosis. It may be alternatively expressed

as pressure potential – the hydrostatic pres-sure to which water in a liquid phase issubjected.

turion 1. A swollen detached winter budthat contains stored food and is protectedby an outer layer of leaf scales and mu-cilage. It is an organ of perennation or veg-etative propagation, and is characteristic ofvarious water plants (e.g. Sagittaria). Turi-ons are usually released from the plant andlie dormant on the bed of the pond or riveruntil conditions become favorable forgrowth.2. Any vegetative shoot or sucker.

tylose A bladderlike ingrowth from aparenchyma cell into an adjacent tracheidor vessel through a paired pit. Tyloses areoften found in injured tissue, older wood,and below an abscission layer, and cancompletely block the conducting vessel.They often become filled with tannins,resins or pigments, giving the heartwoodits dark color, and helping to preserve andstrengthen it. Some of these pigments areused commercially as dyes, e.g. hema-toxylin.

type 1. The material used to define aspecies. It is usually a dried specimenstored in a herbarium but may also be adrawing. Type specimens of microorgan-isms are often living cultures.2. The representative species of a genus, therepresentative genus of a family, etc. For

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ubiquinone See coenzyme Q.

ultracentrifuge A high-speed cen-trifuge, operating at up to a million revolu-tions per second that is used to sedimentprotein and nucleic acid molecules. Ultra-centrifuges operate under refrigeration in avacuum chamber and forces 50 milliontimes that of gravity may be reached. Therate of sedimentation depends on the mol-ecular weight of the molecule and thus theultracentrifuge can be used to separate amixture of large molecules, such as largecolloidal proteins, and estimate particlesizes.

ultramicrotome See microtome.

ultrastructure (fine structure) The de-tailed structure of biological material as itcan be revealed, for example, by electronmicroscopy, but not by light microscopy.

umbel See inflorescence.

undulipodium (pl. undulipodia) Awhiplike organelle that protrudes from aeukaryotic cell and is used chiefly for loco-

motion (e.g. motile gametes) or feeding(e.g. ciliate protoctists). Undulipodia in-clude all eukaryotic cilia and flagella,which share the same essential structure,and differ markedly from bacterial flagella(see flagellum). They are complex struc-tures, containing over 500 different pro-teins. The shaft comprises a cylindricalarray of nine doublet microtubules sur-rounding a central core of two single mi-crotubules. The outer wall of the shaft is anextension of the cell membrane. Its struc-ture is similar to that of a centriole. Strictly,the term flagella should be used only in re-lation to bacteria. Long undulipodia (over10 m m), formerly called flagella, occursingly or in pairs and produce successivewaves of bending that are propagated tothe tip of the shaft, e.g. Chlamydomonas.

unicellular Describing organisms thatexist as a single cell. Such a state is charac-teristic of most protoctists and bacteriaand is also found in many fungi. Comparemulticellular; acellular.

unisexual Describing organisms thathave either male or female sex organs, butnot both. Unisexual plants are said to bedioecious. Compare hermaphrodite; mo-noecious.

universal indicator See indicator.

uracil A nitrogenous base that is foundin RNA but not in DNA. During TRANSCRIP-TION, the thymine of the DNA template issubstituted by a uracil in the RNA copy.Uracil has a PYRIMIDINE ring structure, andis derived from sugars and amino acids.

uridine The NUCLEOSIDE formed whenuracil is linked to D-ribose by a b-glyco-sidic bond.

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vacuole A fluid-filled organelle of vari-able size found in the cytoplasm and sepa-rated from it by a single membrane, theTONOPLAST. Many mature plant cells havea single large central vacuole that confinesthe cytoplasm to a thin peripheral layer.Newly formed plant cells often have anumber of small vacuoles derived from theendoplasmic reticulum. As they fill withsap, they enlarge and coalesce to form asingle vacuole. The vacuole is filled withcell sap, which contains substances in solu-tion, e.g. sugars, salts, and organic acids,often in high concentrations resulting in ahigh osmotic pressure. Water thereforemoves into the vacuole by osmosis makingthe cell turgid. Vacuoles may also containstorage products such as starch grains, oildroplets, crystals, and waste substancesthat are byproducts of the cell’s metabo-lism.

valine An AMINO ACID synthesized frompyruvate. It is broken down to yield suc-cinyl CoA.

variation The extent to which the char-acteristics of the individuals of a speciescan vary. Variation can be caused by envi-ronmental and genetic factors. Environ-mental variation (phenotypic plasticity)results in differences in the appearance ofindividuals of a species because of differ-ences in nutrition, disease, light intensity,etc. Genetic variation is caused by recom-bination and occasionally by MUTATION.These differences may be favored or dis-criminated against by NATURAL SELECTION.See discontinuous variation; continuousvariation; recombination.

variegation The occurrence of patchesof different colors on leaves or flowers. Itmay be caused by infection with a patho-

gen, mineral deficiency, or genetic or phys-iological differences between patches ofcells. It occurs naturally in some species,such as the dumb canes (Dieffenbachia).

variety The taxonomic group below thesubspecies level. The term is often looselyused to describe breeds of livestock or var-ious cultivated forms of agricultural andhorticultural species. Varieties are mor-phological variants, which may differ incolor or growth habit. See also cultivar.

vascular bundle A strand of conductingtissue found in vascular plants. In plantsshowing secondary thickening (e.g. manydicotyledons) the vascular bundles join to-gether to form a ring and contain meris-tematic cambium tissue between the xylemand phloem, but this is generally lacking inplants without secondary thickening (e.g.most monocotyledons). See also vascularplants.

vascular cambium The ring of CAM-BIUM formed by the activity of the inter-fascicular cambium and the intrafascicularcambium that gives rise to secondaryxylem and secondary phloem. It is a lateralmeristem and contains fusiform initialsgiving rise to secondary xylem and phloemand ray initials giving rise to medullaryrays. See also secondary growth.

vascular plants Plants containing dif-ferentiated cells forming conducting tissue(vascular tissue, also known as fasciculartissue), which comprises the xylem andphloem. Vascular tissue transports waterand nutrients through the plant and alsoprovides strength and support. It consistsprincipally of XYLEM and PHLOEM (water-and food-conducting tissues respectively),and also contains strengthening tissue

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(SCLERENCHYMA) and packing tissue(PARENCHYMA). The arrangement of vascu-lar tissue (the vascular system) in the stemis very varied, giving a number of differenttypes of STELE. Primary vascular tissue,which is found in all vascular plants, isformed from the procambium. Secondaryvascular tissue, found only in plants withsecondary thickening, develops from thevascular cambium. The vascular cambiumextends to form a complete ring of meri-stematic tissue around the stem, the sepa-rate vascular bundles being linked byinterfascicular cambium. Vascular plantsare able to achieve considerable verticalgrowth both upwards and into the soil.They have thus been able to colonize thedrier habitats that are inaccessible to thenonvascular bryophytes. The vascularplants may be grouped into a single divi-sion, the Tracheophyta.

vascular system (vascular tissue) Seevascular plants.

vascular tissue See vascular plants.

vector 1. An agent that carries a disease-causing organism to a healthy plant oranimal, causing the latter to become in-fected;. In its widest sense, the term in-cludes agents such as wind, rain, etc., but itis usually restricted to vectors that are ani-mals or microorganisms. Insects such asaphids carry viruses from plant to plant asthey feed, and mites and nematodes canalso carry disease. Humans, including gar-deners and scientists, may carry fungal andbacterial spores on their clothing or skin.2. (cloning vector) An agent used as a vehi-cle for introducing foreign DNA, for ex-ample a new gene, into host cells. Severaltypes of vector are used in GENE CLONING,notably bacterial plasmids and bacterio-phages. The segment of DNA is firstspliced into the DNA of the vector, such asa plasmid, then the vector is transferred tothe host cell (e.g. the bacterium, E. coli),where it replicates along with the host cell.The result is a clone of cells, all of whichcontain the foreign gene, which is ex-pressed by the host cell machinery. A com-

mon vector used to transfer cloned DNA toplant cells is the bacterium Agrobacteriumtumefasciens. The foreign DNA is splicedinto a plasmid, which is taken up by thebacterium. The bacterium then infects aplant cell, and the plasmid DNA may be-come integrated into the plant chromo-some.

vegetative cell See vegetative nucleus.

vegetative growth (vegetative propaga-tion; vegetative reproduction) Growth ofany parts of the plant other than flowers.This involves both upwards growth andoutwards. Sometimes outward growth in-volves STOLONS or RHIZOMES or the bud-ding of BULBS or CORMS. These parts mayget detached from the parent and becomeindependent, although they are identicalgenetically. Some plants have specializedorgans such as GEMMAE or BULBILS for suchspread. See also budding; cutting; graft;layering; perennating organ.

vegetative nucleus (tube nucleus; vegeta-tive cell) One of the two or three nuclei ina young pollen grain that are formed afterdivision of the haploid nucleus. After thepollen grain germinates on the surface ofthe stigma, the vegetative nucleus migratesto the tip of the pollen tube. It is thought toregulate the growth and development ofthe pollen tube. The tube nucleus disinte-grates as the pollen tube penetrates the nu-cellus.

vegetative parts All parts of a plant ex-cept for those involved in sexual reproduc-tion.

vein One of the vascular bundles in aleaf. The pattern in which the veins arearranged in a leaf is called the VENATION.

velamen A layer surrounding the aerialroots of epiphytic plants (e.g. orchids)which, due to the spongy nature of thecells, is able to soak up surface water. It ismade up of several layers of dead emptycells situated on the epidermis. The cellsare spirally thickened, and are translucent

vascular system

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to allow light through to the photosyn-thetic tissue beneath.

velum See annulus.

venation The distribution of veins (vas-cular strands) in a leaf. Dicotyledons usu-ally show a netlike arrangement whereasmonocotyledons generally show a paralleldistribution of veins. Venation is a charac-ter sometimes used in taxonomy.

venter The swollen base of an archego-nium that contains the egg cell (oosphere).

ventral 1. In thallose plants, such assome liverworts, describing the lower sur-face closest to the substrate.2. Designating the upper or adaxial surfaceof the lateral organs of plants, such asleaves. The term is not in common use inthis context. Compare dorsal.

vernalization The cold treatment ofungerminated or partially germinatedseeds. Certain plants will germinate andother flower only if exposed to low tem-peratures (1–2% C) at an early period ofgrowth, i.e. they have a chilling require-ment. Thus winter varieties of cereals willonly flower in summer if sown the previousautumn. Spring-sown winter varieties re-main vegetative throughout the season un-less they have been vernalized.

vernation The arrangement of leaves inrelation to each other in the bud, or thearrangement of perianth segments in aflower bud. See illustration overleaf. Seealso ptyxis.

versatile Describing an anther that is at-tached to the tip of the filament abouthalfway along its length in such a way thatit can turn freely in the wind, thus aidingpollen dispersal. Compare basifixed; dorsi-fixed.

vesicle A small vacuole of variable ori-gin and shape. It may contain secretoryproducts, e.g. a vesicle budded off from theGolgi apparatus, or particles, e.g. endocy-

239

vesicle

Venation

totic vesicle. The membrane envelope iso-lates the contents from the surrounding cy-toplasm.

vessel A form of xylem conducting tis-sue composed of vertically arranged vesselelements (the individual cells that make upa xylem vessel) joined end to end, generallybroader than TRACHEIDS. The end walls ofthe vessel elements have broken down toleave perforation plates. These are usuallyhorizontal rather than the slanting endwalls of tracheids. Vessel elements have lig-nified secondary cell walls and at maturitylack a living protoplast. Vessels are foundin most angiosperms, the gymnosperm

order Gnetales, and a few other plants suchas bracken (Pteridium aquilinum).

vestigial organ An organ that is func-tionless and generally reduced in size butbears some resemblance to the correspond-ing fully functioning organs found in re-lated organisms. Examples include thescale leaves of parasitic flowering plants orthe reduced stamens in females of dio-ecious species.

virion The extracellular inert phase of avirus. A virion consists of a protein coatsurrounding one or more strands of DNAor RNA. Virions may be polyhedral orhelical and vary greatly in size.

viroid A tiny infectious agent found inplants that is similar to a virus but lacks acapsid, consisting simply of a circle ofRNA, 300–400 nucleotides long. Viroidsreplicate within the plant cell and causecharacteristic disease symptoms; examplesare the potato spindle tuber viroid and thehop stunt viroid.

virulence The relative pathogenicity ofan organism: its ability to cause disease.Virulence may vary between strains of thesame organism.

virulent phage A bacteriophage that in-fects a bacterial cell and immediately repli-cates, causing lysis of the host cell.Compare temperate phage.

virus An extremely small infectiousagent that causes a variety of diseases inplants, such as tobacco mosaic disease.Viruses can only reproduce in living tis-sues; outside the living cell they exist as in-active particles consisting of a core of DNAor RNA surrounded by a protein coat (cap-sid). Most plant viruses are single-strandedRNA viruses. The inert extracellular formof the virus, termed a VIRION, penetratesthe host membrane and liberates the viralnucleic acid into the cell. Usually, the nu-cleic acid is translated by the host cell ribo-somes to produce enzymes necessary forthe reproduction of the virus and the for-

vessel

240

Vernation: types of vernation

mation of daughter virions. The virions arereleased by lysis of the host cell. Otherviruses remain dormant in the host cell be-fore reproduction and lysis, their nucleicacid becoming integrated with that of thehost. Viruses are transmitted by VECTORS

such as aphids and nematodes. Some pro-duce visible symptoms, such as leaf mo-saics or deformed growth. Others have lessobvious effects, but can significantly re-duce crop yields. A virus that infects a bac-terium is termed a BACTERIOPHAGE (phage).

vital stains Nontoxic coloring materialsthat can be used in dilute concentrations tostain living material without damaging it.See also staining.

vitamins Organic chemical compoundsthat are essential in small quantities for me-

tabolism but are not synthesized by ani-mals, which therefore need to obtain themfrom plant or microorganisms. The vita-mins have no energy value; most of themseem to act as coenzymes for essentialchemical changes in the body, each one in-fluencing a number of vital processes.Vitamins A, D, E, and K are fat-soluble vi-tamins, while vitamins B and C are water-soluble vitamins.

vivipary The germination of seeds orspores that are still attached to the parentplant. An example is mangroves, in whichthe seeds germinate into good-sizedseedlings before they are released from theplant, enabling them to plunge far enoughinto the mud to establish themselves.

Volvocales An order of the Chloro-

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Volvocales

Wallace, Alfred Russel (1823–1913)British naturalist. After a basic educationand a short career as a surveyor, Wallacemet the entomologist Henry Bates and to-gether they embarked on a scientific expe-dition as naturalists to the Amazon.Wallace described this expedition in ANarrative of Travels on the Amazon andthe River Negro (1853). His second expe-dition, to the Malay Archipelago, tookeight years during which he collected over125 000 specimens. He noticed the markeddifferences between the Australian andAsian flora and fauna and drew a line onthe map separating the regions, still re-ferred to as Wallace’s line. His observa-tions of the marsupials in Australia led himto conclude that Australian animals weremore primitive than Asian ones. He be-lieved that these animals had been cut offfrom the Asian animals when the land massof Australia had broken away from Asia.This resulted in less competition for theAustralian animals than those on the largerEurasian land mass. These conclusions ledhim to formulate a theory of evolution bynatural selection, which he communicatedto DARWIN; as a result the two men pre-sented joint papers at the famous LinnaeanSociety meeting of 1858. Unlike Darwin,Wallace never believed that the theory ofevolution could be applied to humans.Wallace continued to gather evidence forthe theory and published his results in Ge-ographical Distribution of Animals (1876)and Island Life (1880).

wall pressure See turgor pressure.

water culture See hydroponics.

water ferns See Marsileaceae.

water molds See Saprolegniales.

water pore See aquaporin.

water potential Symbol: y A measureof the energy available in an aqueous solu-tion to cause water to move across a selec-tively permeable membrane duringosmosis. Water potential may be defined asthe chemical potential of water in a biolog-ical system compared to the chemicalpotential of pure water at the same tem-perature and pressure. It is measured inkilopascals (kPa), and pure water has thevalue 0 kPa; solutions with increasing con-centrations of solute have more negativevalues of y, since the solute molecules in-terfere with the water molecules. This ef-fect is termed the solute potential (orosmotic potential), denoted by ys; it is mea-sured in kPa and always has a negativevalue, with increasing concentrations ofsolute having increasingly negative valuesof ys. In turgid plant cells there is also apressure exerted by the walls of the cell;this is called the pressure potential; it is de-fined by yp, and has a positive value (al-though in xylem cells it may be negativedue to water movement in the transpira-tion stream). Water potential is alsoaffected by capillary forces, especially inthe narrow tubes of the xylem, and in theinterstices of cell walls, and by imbibitionalforces. These forces are particularly impor-tant in cell walls and intercellular spaces,increasing in wilting plants.

wax One of a group of water-insolublesubstances with a very high molecularweight; they are esters of long-chain alco-hols with fatty acids. Waxes form protec-tive coverings to leaves, stems, fruits, seeds,

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W

animal fur, and the cuticles of insects, serv-ing principally as waterproofing.

WCMC See World Conservation Moni-toring Centre.

WCU See World Conservation Union.

Western blotting A technique analo-gous to SOUTHERN BLOTTING used to sepa-rate and identify proteins instead of nucleicacids. The protein mixture is separated byelectrophoresis and blotted onto a nitrocel-lulose filter. Antibodies specific to the pro-tein of interest are applied and bind to theirtarget proteins. A second antibody, specificto the first antibody, is then applied. Thiscarries a radioactive label, so enabling it tobe located by autoradiography wherever itbinds to the antibody–protein complex.

wild-type The most commonly foundform of a given gene in wild populations.Wild-type alleles, often designated +, areusually dominant and produce the ‘nor-mal’ phenotype.

wilt A plant disease characterized bywilting, usually due to pathogenic fungi orbacteria, or to parasites. It is often a rootdisease that eventually prevents the planttaking up water. Other wilts occur whenthe vascular tissues become blocked, e.g.Dutch elm disease, caused by the fungusCeratocystis ulmi, which causes the plantto block off its own infected tissues.

wilting The loss of turgor due to lack ofwater. The tissues become limp and leavesand flowers droop. Wilting may be due tothe rate of transpiration exceeding the rateat which the roots can take up water in thesoil during drought periods, or to infection(see wilt). The permanent wilting point isreached when the plant has wilted to apoint where it will not recover, even ifmore water is added, because the cells haveplasmolyzed. See turgor.

wood The hard fibrous structure foundin woody perennials such as trees andshrubs. It is normally formed from the sec-ondary xylem and thus found in plants that

show secondary thickening, namely theconifers, Gnetum and dicotyledons. Waterand nutrients are only transported in theoutermost youngest wood, termed the SAP-WOOD. The nonfunctional compactedwood of previous seasons’ growth is calledthe HEARTWOOD and it is this that is impor-tant commercially.

woody perennial See perennial.

World Conservation Monitoring Cen-tre (WCMC) An information service setup by the World Conservation Union, theWorldwide Fund for Nature and theUnited Nations Environment Program tocompile information on conservation andthe sustainable use of the world’s naturalresources. The WCMC collects data onthreatened and endangered species andhabitats, national parks and nature re-serves, international agreements, and con-servation and environment programs. SeeRed Data Book.

World Conservation Union (WCU)Formerly the International Union for theConservation of Nature (IUCN), foundedin 1948, an organization that brings to-gether states, government agencies, and arange of nongovernmental organizations topromote conservation and the sustainableuse of natural resources. Over 10 000 ex-pert volunteers collect information on particular species and biodiversity con-servation projects, providing a base of ex-pertise that enables the WCU to advisecountries on national conservation strate-gies.

Worldwide Fund (for Nature) (WWF)The largest independent conservation or-ganization in the world, comprising aglobal network of national organizations,associates, and program offices. It providesconservation services based on global pol-icy, fieldwork, and scientific information.Its goal is to prevent genetic, species, andecosystem diversity, to ensure the sustain-able use of resources, and to promote ac-tions to reduce pollution.

WWF See Worldwide Fund (for Nature).

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WWF

xanthophyll One of a class of yellow toorange pigments derived from carotene,the commonest being lutein. In mostplants, xanthophylls function as accessorypigments in photosynthesis: they absorblight at wavelengths where chlorophyll haslow absorption and pass the absorbed en-ergy to chlorophyll. In the Phaeophyta(brown algae) the xanthophylls fucoxan-thin and peridinin act as the primary light-absorbing pigments. See carotenoids;photosynthetic pigments.

xeromorphic Structurally adapted towithstand dry conditions. See xerophyte.

xerophyte Any plant adapted to grow-ing in dry conditions or in a physiologicallydry habitat, such as an acid bog or a saltmarsh, or an exposed, very windy situa-tion, by storing available water, reducingwater loss, or possessing deep root sys-tems. Succulents, such as cacti and agaves(family Agavaceae), have thick fleshy stemsor leaves that store water. Features associ-ated with reducing water loss include:shedding or dieback of leaves, e.g. ocotillo(Fouquieria splendens); waxy leaf coatingscoupled with closure or plugging of stom-ata, e.g. Kalanchoe; sunken or protectedstomata e.g. marram grass (Ammophilaarenaria); folding, rolling or repositioningof leaves to reduce sunlight absorption, e.g.marram grass; a dense covering of white re-flective spines, e.g. cacti; and the develop-ment of a dense hairy leaf covering, e.g.Espeletia. Compare hydrophyte; meso-phyte. See also halophyte.

xylem The water-conducting tissue invascular plants. It consists of dead hollowcells (the TRACHEIDS and VESSELS), whichare the conducting elements. It also con-

tains additional supporting tissue in theform of fibers and sclereids and some livingparenchyma. The proto- and metaxylem,which together constitute the primaryxylem, are formed from the procambiumof the apical meristems, while the sec-ondary xylem is differentiated from thecambium or lateral meristem. See sec-ondary growth.

yeasts See Saccharomycetales.

zinc A micronutrient that is needed forthe activity of the enzyme alcohol dehydro-genase, which catalyzes the conversion ofacetaldehyde to ethanol in anaerobic respi-ration. See micronutrient.

zoosporangium (pl. zoosporangia) Asporangium that produces ZOOSPORES.

zoospore An asexual motile spore pro-duced by a zoosporangium. It has one ormore undulipodia. These motile sporesmay encyst in adverse conditions or may bethe means by which a fungus or parasiticprotoctist penetrates a new host.

zwitterion An ion with both a positiveand a negative charge. Amino acids canform zwitterions: the amino group has theform –NH3

+ and the acid group is ionizedas –COO–.

zygomorphy See bilateral symmetry.

zygomycete Any fungus belonging tothe phylum Zygomycota.

Zygomycota A phylum of fungi whosemembers have haploid, nonseptate hyphae(i.e. hyphae that lack cross walls). Theircell walls contain chitin or chitosan. They

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XYZ

do not produce motile spores at any stageof the life cycle. Zygomycetes are mostlysaprophytic, absorbing nutrients from de-caying vegetation and other organic mat-ter; the bread molds Rhizopus and Mucorare examples.

zygospore A resistant sexual sporeformed when a zygote develops a thick wall. Zygospores are characteristic of theZYGOMYCOTA, especially the Mucorales

(which include such fungi as Mucor andRhizopus) and the GAMOPHYTA (conjugat-ing green algae, e.g. Spirogyra and Zyg-nema).

zygote The diploid cell resulting fromthe fusion of two haploid gametes. A zy-gote usually undergoes mitosis immedi-ately. See sexual reproduction.

zygotene See prophase.

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zygotene

APPENDIXES

248

Appendix I

BASE AND DIMENSIONLESS SI UNITSPhysical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr*supplementary units

DERIVED SI UNITS WITH SPECIAL NAMESPhysical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Welectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITSSubmultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- m 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

SI Units

The following all have useful information:

American Society of Plant Biologists www.aspb.org/education/

Botanical Society of America www.botany.org

European Initiative for Biotechnology www.rdg.ac.uk/EIBE/Education

Internet Directory for Botany www.botany.net/IDB/info.html

Missouri Botanical Garden www.mobot.org/

National Museum of Natural History nmnhwww.si.edu/departments/botany.html

New York Botanical Garden www.nybg.org

Photosynthesis and the Web photoscience.la.asu.edu/photosyn/photoweb/default.html

Royal Botanic Gardens, Kew www.rbgkew.org.uk/

University of California Botanical Garden www.mip.berkeley.edu/garden/

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Appendix IIWebpages

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. MolecularBiology of the Cell. 4th ed., New York: Garland Science, 2002

Ayala, Francisco J., Fitch, Walter M., & Clegg, Michael T. (eds). Variation andEvolution in Plants and Microorganisms. Washington: National Academy Press,2000

Bell, Arnold D. Plant Form: An Illustrated Guide to Flowering Plant Morphology.Oxford, U.K.: Oxford University Press, 1991

Bell, Peter R. & Hemsley, Alan R. Green Plants: Their Origin and Diversity. 2nd ed.Cambridge, U.K.: Cambridge University Press, 2000

Campbell, Neil A. & Reece, Jane B. Biology. 6th ed. New York: Benjamin Cummings,2002

Hopkins, W. G. Introduction to Plant Physiology. 2nd ed. New York: John Wiley andSons, 1995

Lack, A. J. & Evans, D. E. Instant Notes: Plant Biology. New York: Springer-Verlag,2001

Mabberley, D. J. The Plant-Book: A portable dictionary of the vascular plants. 2nd ed.Cambridge, U.K.: Cambridge University Press, 1997

Margulis, L. & Schwartz, K. V. Five Kingdoms: An Illustrated Guide to the Phyla ofLife on Earth. 3rd ed. New York: W. H. Freeman, 1998

Niklas, Karl J. The Evolutionary Biology of Plants. Chicago: University of ChicagoPress, 1997

Purves, William K., Sadava, David, Orians, Gordon H., & Heller, H. Craig. Life: TheScience of Biology. 6th ed. New York: W. H. Freeman, 2001

Raven, P. H., Evert, R. F. & Eichorn, S. E. Biology of Plants. 6th ed. New York: W.H.Freeman, 1998

Ridge, Irene (ed). Plants. New York: Oxford University Press, 2002

Stern, K. R. Introductory Plant Biology. 8th ed. New York: Sinauer Associates, 1998

Westhoff, P., Jeske, H., Jurgens, G., Kloppstech, K., & Link, G. Molecular PlantDevelopment - from Gene to Plant. New York: Oxford University Press, 1998

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