EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS

‘OLUTION AND DIVERSITY OF VASCULAR PLANTS

. 2005. Phylogeny of cryptogrannnoid ferns and related taxa based on rbcL sequences Nordic Journal of

‘tp://amerfernsoc.org lists many resources, including pubiications, references, local, national, and internaites, commerjcal fern sites, and fern databases.

LIGNOPHYTIES—WOODY PLANTS

The lignophytes, or woody plants (also called Lignophyta),

are a monophyletic lineage of euphyllous vascular plants that

share the derived features of a vascular cambium, which

gives rise to wood, and a cork cambium, which produces

cork (Figures 5.1, 5.2). Growth of the vascular and cork

cambia is called secondary growth because it initiates after

the vertical extension of stems and roots due to cell expansion

(primary growth). A vascular cambium is a sheath, or hollow

cylinder, of cells that develops within the stems and roots as a

continuous layer, between the xylem and phloem in extant,

eustelic spermatophytes (see later discussion). The cells of

the vascular cambium divide mostly tangentially (parallel to

a tangential plane), resulting initially in two concentric layers

of cells (Figure 5.3A). One of these layers remains as the vas

cular cambium and continues to divide indefinitely; the other

layer eventually differentiates into either secondary xylem

wood, if produced to the inside of the cambium, or secondary

phloem, if produced to the outside (Figure 5.3A,B). Because

129

Ginkgophyta 144

Ginkgoaceae 145

Coniferae—Conifers 145

Pinopsida 148

Pinaceae 148

Cupressopsida 151

Araucariaceae 151

Cupressaceae 151

Podocarpaceae 154

Taxaceae 154

Gnetales 156

Ephedraceae 157

REVIEW QJESTIONS 160

EXERCISES 160

REFERENCES FOR FURTHER STUDY 161

WEB SITES 162

layers of cells are produced both to the inside and outside of

a continuously generated cambium, this type of growth is

termed bifacial. Generally, much more secondary xylem is

produced than secondary phloem. [Note that a secondary

cambium independently evolved in fossil lineages within the

lycophytes (e.g., Lepidodendron) and equisetophytes (e.g.,

Calamites), but this cambium was unifacial, producing sec

ondary xylem (wood) to the inside but no outer secondary

phloem, likely limiting in terms of an adaptive feature.]

Secondary growth results in an increase of the width or girth

of stems and roots (Figures 5.3B, 5.4). This occurs both by

expansion of the new cells generated by the cambium and by

accompanying radial divisions, increasing the number of cells

within a given growth ring. Many woody plants have regular

growth periods, e.g., forming annual rings of wood (Figure 5.4).

A cork cambium is similar to a vascular cambium, only it

differentiates near the periphery of the stem or root axis. The

cork cambium and its derivatives constitute the periderm

(referred to as the outer bark). The outermost layer of the

periderm is cork (Figure 5.3B). Cork cells contain a waxy

I 5EVOLUTION AND DIVERSITY OF

WOODY AND SEED PLANTSLlGNOPHYTESWOODY PLANTS 129

SPERMATOPHYTES—SEED PLANTS 131

Seed Evolution 131

Pollination Droplet 135

Pollen Grains 135

Pollen Tube 136

Ovule and Seed Development 136

Seed Adaptations 139

Eustele 139

DIVERSITY OF WOODYAND SEED PLANTS 139

Archeopteris 139

“Pteridosperms”—”Seed Ferns” 139

Gymnospermae—Gymnosperms 140

Cycadophyta—Cycads 140

Cycadaceae 141

Zamiaceae 142

© 2010 Elsevier Inc. All rights reserved.

doi: 10.101 61B978-0- 12-374380-000005.2

UNIT II EVOLUTION AND DIVERSITY OF PLANTS 131130 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS

t

Ca a

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is.sC) ç

o a) S—o o0 0 a)t a):t S a)

Ct S rjCto 0

— •0-5c .

- 0 5(/D

- Lignophyta (Woody Plants)

Spermatophyta (Seed Plants)

Gymnospermae (Gymnosperms)

Coniferae (Conifers)

Cupressopsida

a)4.,

a)

0-I.

a)oO— (/,

r Gnetales—,

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a)a)

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porose

epimatium +receptacle

iiiiifiiiii 1 ovule/scale

iiiiiiiifiiii leaves simple

t =extincttaxon

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pollen tube—sperm nonmotile (siphonogamy)

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classification and identification of woody plants. Wood ana

tomical features may also be used to study the past, a spe

cialty known as dendrochronolOgy (see Chapter 10).

Another feature of lignophytes is that they possess ances

trally monopodial growth, in which a single main shoot

develops branches from lateral (usually axillary) buds (see

Chapters 4, 9). Although monopodial growth is presumed to

have arisen prior to the monilophyte—lignophyte split, it

enabled woody plants in particular the capability of forming

extensive (sometimes massive) woody branching systems,

permitting them to survive and reproduce more effectively.

SPERMATOPHYTESSEED PLANTS —

The spermatophyta, commonly called spermatophytes or

seed plants, are a monophyletic lineage within the lignophytes

(Figure 5.1). The major evolutionary novelty that unites this

group is the seed. A seed is defined as an embryo, which is an

immature diploid sporophyte developing from the zygote, sur

rounded by nutritive tissue and enveloped by a seed coat

(Figure 5.5). The embryo generally consists of an immature

root called the radicle, a shoot apical meristem called the

epicotyl, and one or more young seed leaves, the cotyledons;

the transition region between root and stem is called the

hypocotyl (Figures 5.5, 5.10). An immature seed, prior to

fertilization, is known as an ovule.

SEED EVOI.,UTION

The evolution of the seed involved several steps. The

exact sequence of these is not certain, and two or more

“steps” in seed evolution may have occurred concomitantly

and be functionally correlated. The probable steps in seed

evolution are as follows (Figure 5.6):

1. HeterospOry. Heterospory is the formation of two types

of haploid spores within two types of sporangia: large,

fewer-numbered megaspores, which develop via meiosis

in the megasporangium, and small, more numerous

microsporeS, the products of meiosis in the microsporafl

gium (Figures 5.6, 5.7). The ancestral condition, in which a

single spore type forms, is called “homosporyY Each

megaspore develops into a female gametophyte that bears

only archegonia; a microspore develops into a male game

tophyte, bearing only antheridia. Although heterospory

was prerequisite to seed evolution, there are fossil plants

that were heterosporous but had not evolved seeds, among

these being species of Archeopteris (Figures 5.1, 5.1 3A;

see later discussion). Note that heterospory has evolved

independently in other, nonseed plants, e.g., in the extant

lycophytes Selaginella and Isoetes and in the water ferns

(Chapter 4).

— s eustele

— — pollen tube—sperm motile (zooidogamy)

— — endosporic, male gametophyte = pollen grain

— — pollination droplet

— — integument with micropyle— — retention of megaspore within megasporangium

— — reduction to 1 megaspore per megasporangium SEED— — endosporic female gametophyte (embryo -

+ nutntive tissue— — heterospory + integuments)— — cork cambium (periderm)— vascular cambium (secondary vascular tissue, md. wood)

FIGURE 5.1 Cladogram of the woody and seed plants. Major apomorphies are indicated beside a thick hash mark. Families in bold aredescribed in detail. Modified from Bowe et al. (2000); Chaw et al. (2000); Frohlich et al. (2000); and Samigullin et al. (1999).

polymer called suberin (similar to cutin) that is quite resist- shrubs or trees with tall overstory canopies (e.g., Figure 5.2),ant to water loss (see Chapter 10). a significant ecological adaptation. Cork produced by the cork

The vascular cambium and cork cambium constituted major cambium functions as a thick layer of cells that protectsevolutionary novelties. Secondary xylem, or wood, functions the delicate vascular cambium and secondary phloem fromin structural support, enabling the plant to grow tall and acquire mechanical damage, predation, and desiccation.massive systems of lateral branches. Thus, the vascular cam- Wood anatomy can be quite complex. The details ofbium was a precursor to the formation of intricately branched cellular structure are important characters used in the

FIGURE 5.2 Composite

giant sequoia, a woody conifer that is the most massive, nonclonal

organism on Earth, and among the tallest of trees.

AND DIVERSITY OF WOODY AND SEED PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 131

a.)—a.)

a.) a.) a:

a a:a:

C C— -a:

C.) a:a.,) c_)

Ieustele

— —

—pollen tube—sperm motile (zooidogamy)

endosporic, male gametophyte pollen grainpollination droplet

integument with micropyle

retention of megaspore within megasporangjumreduction to 1 megaspore per megasporangjum SEEDendosporic female gametophyte (embryo

+ nutritive tissueheterospory+ integuments)

cork cambjum (periderm)vascular cambjum (secondary vascular tissue, mci. wood)

plants. Major apomorphies are indicated beside a thick hash mark. Families in bold are10); Chaw et al. (2000); Frohljch et al. (2000); and Samigullin et al. (1999).

shrubs or trees with tall overstory canopies (e.g., Figure 5.2),a significant ecological adaptation. Cork produced by the corkcambium functions as a thick layer of cells that protectsthe delicate vascular cambium and secondary phloem frommechanical damage, predation, and desiccation.

Wood anatomy can be quite complex. The details ofcellular structure are important characters used in the

FIGURI 5.2 Composite photograph ofSequoiadendrongiganteum,

giant sequoia, a woody conifer that is the most massive, nonclonal

organism on Earth, and among the tallest of trees.

classification and identification of woody plants. Wood ana

tomical features may also be used to study the past, a spe

cialty known as dendrochronology (see Chapter 10).

Another feature of lignophytes is that they possess ances

trally monopodial growth, in which a single main shoot

develops branches from lateral (usually axillary) buds (see

Chapters 4, 9). Although monopodial growth is presumed to

have arisen prior to the monilophyte—lignophyte split, it

enabled woody plants in particular the capability of forming

extensive (sometimes massive) woody branching systems,

permitting them to survive and reproduce more effectively.

SPERMATOPHYTES—SEED PLANTS

The Spermatophyta, commonly called spermatophytes or

seed plants, are a monophyletic lineage within the lignophytes

(Figure 5.1). The major evolutionary novelty that unites this

group is the seed. A seed is defined as an embryo, which is an

immature diploid sporophyte developing from the zygote, sur

rounded by nutritive tissue and enveloped by a seed coat

(Figure 5.5). The embryo generally consists of an immature

root called the radicle, a shoot apical meristem called the

epicotyl, and one or more young seed leaves, the cotyledons;

the transition region between root and stem is called the

hypocotyl (Figures 5.5, 5.10). An immature seed, prior to

fertilization, is known as an ovule.

SEED EVOLUTION

The evolution of the seed involved several steps. The

exact sequence of these is not certain, and two or more

“steps” in seed evolution may have occurred concomitantly

and be functionally correlated. The probable steps in seed

evolution are as follows (Figure 5.6):

1. Heterospory. Heterospory is the formation of two types

of haploid spores within two types of sporangia: large,

fewer-numbered megaspores, which develop via meiosis

in the megasporangium, and small, more numerous

microspores, the products of meiosis in the microsporan

glum (Figures 5.6, 5.7). The ancestral condition, in which a

single spore type forms, is called “homospory.” Each

megaspore develops into a female gametophyte that bears

only archegonia; a microspore develops into a male game

tophyte, bearing only antheridia. Although heterospory

was prerequisite to seed evolution, there are fossil plants

that were heterosporous but had not evolved seeds, among

these being species of Archeopteris (Figures 5.1, 5.13A;

see later discussion). Note that heterospory has evolved

independently in other, nonseed plants, e.g., in the extant

lycophytes Selaginella and Isoetes and in the water ferns

(Chapter 4).

— Lignophyta (Woody Plants) — —

Spermatophyta (Seed Plants)

- Gymnospermae (Gymnosperms) —

— Conjferae (Conifers) — —

1 — Cupressopsida

1

1

r Gnetales-,

.

a: a:a.) a.)C.) C.)a: a:

a: a:C.?

aril

Spollen tube—sperm nonmotjie (siphonogamy)

is quite resist

flstituted major‘ood, functionstall and acquirevascular cam

ately branched

2. Endospory. Endospory is the complete development of,

in this case, the female gametophyte within the original

spore wall (Figure 5.6). The ancestral condition, in which

the spore germinates and grows as an external gameto

phyte, is called exospory. The evolution of endosporic

female gametophytes was correlated with that of

endosporic male gametophytes (pollen grains); see later

discussion.

3. Reduction of megaspore number to one. Reduction of

megaspore number occurred in two ways. First, the number

of cells within the megasporangium that undergo meiosis

(each termed a megasporocyte or megaspore mother

cell) was reduced, from several to one (Figure 5.6). This

single diploid megasporocyte gives rise to four haploid

megaspores. Second, of the four haploid megaspores pro

duced by meiosis, three consistently abort, leaving only

one functional megaspore. This single megaspore also

undergoes a great increase in size, correlated with the

megasporangium.

4. Retention of the megaspore. Instead of the megaspore

being released from the sporangium (the ancestral condi

tion, as occurs in all homosporous nonseed plants),

in seed plants it is retained within the megasporangium

(Figure 5.6). This was accompanied by a reduction in

thickness of the megaspore wall.

5. Evolution of the integument & micropyle. The final

event in seed evolution was the envelopment of the

megasporangium by a layer of tissue, called the integu

ment (Figure 5.6). The integument grows from the base of

the megasporangium (which is often called a nucellus

when surrounded by an integument) and envelopes it,

except at the distal end. Fossil evidence suggests that the

integument likely evolved from separate lobes derived

from telomes (ancestral branches) that surrounded the

megasporangium. These “preovules”, i.e., ovules prior to

the evolution of integuments, possessed a rim or ring of

tissue at the apex of the megasporangium, the lagenos

tome, which functioned to funnel pollen grains to a pol

lination chamber. (See, e.g., Stewart and Rothwell 1993

for details.) The epitome of seed evolution occurred with

the evolutionary “fusion” of the telomes to form the

integument, a continuous sheath that completely sur

rounds the nucellus. The integument of all extant seed

plants has a small pore at the distal end called the micro-

pyle. The micropyle replaced the ancestral lagenostome

as the site of entry of pollen grains (or in angiosperms, of

pollen tubes). The micropyle also functions in the

mechanics of pollination droplet formation and resorp

tion (see below). Note that a single integument represents

the ancestral condition of spermatophytes; in angiosperms

a second integument layer evolved later (Chapter 6).

132 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS

I

UNIT II EVOLUTION AND DIVERSITY OF PLANTS 133

I

I FIGURE 5.4 Woody stem cross-section, Pinus. sp. A. One year’s growth. B. Four years’ growth.

vascularcambjum

increased availability of space and resources in the

A

B

FIGURE 5.3 A. Development of the vascular cambium. B. Development of secondary vascular tissue in the stem, illustrated here for aeusteljc stem.

2 xylem

2 phloem

periderrn

cork(epidermis sloughed off to outside)

radicle

fluthtive tissue(female gametophyle

or endosperm)

embryo

cotyledons

FIGURE 5.5 Morphology of a seed. Pinus sp. illustrated here.

AND DIVERSITY OF WOODYAND SEED PLANTS

2. Endospory. Endospory is the complete development of,

in this case, the female gametophyte within the original

spore wall (Figure 5.6). The ancestral condition, in which

the spore germinates and grows as an external gameto

phyte, is called exospory. The evolution of endosporicfemale gametophytes was correlated with that of

endosporic male gametophytes (pollen grains); see later

discussion.3. Reduction of megaspore number to one. Reduction of

megaspore number occurred in two ways. First, the number

of cells within the megasporangium that undergo meiosis

(each termed a megasporocyte or megaspore mother

cell) was reduced, from several to one (Figure 5.6). This

single diploid megasporocyte gives rise to four haploidmegaspores. Second, of the four haploid megaspores pro

duced by meiosis, three consistently abort, leaving only

one functional megaspore. This single megaspore also

undergoes a great increase in size, correlated with the


increased availability of space and resources in themegasporangium.

4. Retention of the megaspore. Instead of the megaspore

being released from the sporangium (the ancestral condi

tion, as occurs in all homosporous nonseed plants),

in seed plants it is retained within the megasporangium(Figure 5.6). This was accompanied by a reduction inthickness of the megaspore wall.

5. Evolution of the integument & micropyle. The final

event in seed evolution was the envelopment of themegasporangium by a layer of tissue, called the integu

ment (Figure 5.6). The integument grows from the base of

the megasporangium (which is often called a nucellus

when surrounded by an integument) and envelopes it,

except at the distal end. Fossil evidence suggests that theintegument likely evolved from separate lobes derived

from telomes (ancestral branches) that surrounded the

megasporangium. These “preovules”, i.e., ovules prior to

the evolution of integuments, possessed a rim or ring of

tissue at the apex of the megasporangium, the lagenos

tome, which functioned to funnel pollen grains to a pol

lination chamber. (See, e.g., Stewart and Rothwell 1993

for details.) The epitome of seed evolution occurred with

the evolutionary “fusion” of the telomes to form the

integument, a continuous sheath that completely sur

rounds the nucellus. The integument of all extant seed

plants has a small pore at the distal end called the micro-

pyle. The micropyle replaced the ancestral lagenostome

as the site of entry of pollen grains (or in angiosperms, of

pollen tubes). The micropyle also functions in the

mechanics of pollination droplet formation and resorp

tion (see below). Note that a single integument represents

the ancestral condition of spermatophytes; in angiosperms

a second integument layer evolved later (Chapter 6).

1

FIGURE 5.4 Woody stem cross-section, Pinus. sp. A. One year’s growth. B. Four years’ growth.

vascularcambium

2’ xylem

vascularcambium 2 phloem

l’phloem

1’ xylem2’ xylem

2’ phloem

periderm

cortex

seed coat

cork(epidermis sloughed off to outside)

radicle

nutritive tissue(female gametophyte

or endosperm)

embryo

epicotyl

cotyledons

ambium. B. Development of second vascular tissue in the stem, illustrated here for a FIGURE 5.5 Morphology of a seed. Pinus sp. illustrated here.

134 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 135

POLLINATION DROPLET

One possible evolutionary novelty associated with seed evo

lution is the pollination droplet. This is a droplet of liquid

that is secreted by the young ovule through the micropyle

(Figures 5.1OA, 5.171). This droplet is mostly water plus

some sugars or amino acids and is formed by the breakdown

of cells at the distal end of the megasporangium (nucellus).

The cavity formed by this breakdown of cells is called the

pollination chamber (Figure 5. bA). The pollination drop

let functions in transporting pollen grains through the micro-

pyle. This occurs by resorption of the droplet, which “pulls”

pollen grains that have contacted the droplet into the pollina

tion chamber. It is unknown whether a pollination dropletwas present in the earliest seed plants. However, the presenceof a pollination droplet in many nonflowering seed plants

suggests that its occurrence may be apomorphic for at least

the extant seed plant lineages. Note that the ovules of

angiosperms lack pollination droplets or pollination cham

bers, as flowering plants have evolved a different mechanism

of pollen grain transfer (see Chapter 6).

POLLEN GRAINSConcomitant with the evolution of the seed was the evolution of

pollen grains (Figure 5.8). A pollen grain is, technically, an

immature, endosporic male gametophyte. Endospory in pollen

grain evolution was similar to the same process in seed evolu

tion, involving the development of the male gametophyte within

the original spore wall. Pollen grains of seed plants are extremely

reduced male gametophytes, consisting of only a few cells.

They are termed “immature” male gametophytes because, at the

time of their release, they have not fully differentiated.

After being released from the microsporangium, pollen must

be transported to the micropyle of the ovule (or, in angio

sperms, to the stigmatic tissue of the carpel; see Chapter 6) in

order to ultimately effect fertilization. Wind dispersal, in com

bination with an ovule pollination droplet (see later discus

sion), was probably the ancestral means of pollen transport.

After being transported to the ovule (or stigmatic tissue), the

male gametophyte completes development by undergoing

additional mitotic divisions and differentiation. The male

gametophyte grows an exosporic pollen tube, which functions

antheridia

/gametophyte

male gametophyte

microsporangium

(n)

(n) SpOraflgium

1. Heterospory megasporangium

Sporophyte Body,.._-‘ (2n)

mitosis, growth, & differentiation mitosis, growth, & differentiation

7Embryo

(2n) Microsporangium Megasporangium(2n) (2n)

/ \ \mitosis, growth, & differentiation mitosis, growth, & d(fferentianon

‘\Zygote SPOROPHYTE GENERATION Microsporocyte Megasporocyte

(2n) (2N) (2n) (2n)

t——fertilization rneiosis —— meioszs ——

/‘ Jn‘jN I) GAMETOPHYTE GENERATION) (N)

Microspores Megaspores..)

(n) (n) j Conifers (mci. Gnetales) ©

Egg Sperm 1. (sperm nonflagellate in © ( (n) (n)

k and Angiosperms)female gametophyte { Archegonium Antheridium 1

(lost in the Angiosperms (reduced to absent in& some Gnetales) (n) (n) 3’ extant seed plants)

archegonia

/wall

female gametophyte(contained in megaspore)

2. Endospory

megasporangiurn

archegonia

0

1/mitosis, growth, & differentiation

mitosis, growth, & derentiaiionGametophyte

(n)Male

Female Gametophyte(n)

megasporangium

3. Reduction to 1 megaspore

megaspore

FIGURE 5.7 Life cycle of heterosporous seed plants.

gametophyte

megasporangium

4. Retention of megaspore

FIGURE 5.6 Ovule and seed evolution in the spermatophytes (hypothetical, for purpose of illustration).

5. Evolution of Integument & Micropyle

as a haustorial organ, obtaining nutrition by absorption from thesurrounding sporophytic tissue (Figure 5.9; see Pollen Tabe).

POLLEN TUBEThe male gametophytes of all extant seed plants form a pollentube (Figure 5.9) soon after the pollen grains make contactwith the megasporangial (nucellar) tissue of the ovule. Inextant seed plants the ancestral type of pollen type (found incycads and ginkgophytes) was haustorial, in which the malegametophyte feeds (like a parasite) off the tissues of thenucellus. Motile sperm is delivered from this male gametophyte into a fertilization chamber, where the sperm swims tothe archegonium containing the egg, a process known aszooidogamy (zoom, animal + gamos, marriage). In the conifers (including Gnetales), pollen tubes are also haustorial, butdeliver nonmotile sperm cells to the archegonium or egg, aprocess known as siphonogamy (siphono, tube + gamos,

marriage). A type of siphonogamy evolved independently inthe angiosperms. In angiosperms, however, the pollen tubesgrow through stylar tissue prior to delivering the sperm to theegg of a female gametophyte (see Chapter 6).

OVULE AND SEED DEVELOPMENTAfter pollination, the megasporocyte develops within themegasporangium of the ovule (Figures 5.1OA, 5.11A). Themegasporocyte is a single cell that undergoes meiosis, producinga tetrad of four haploid megaspores, which in most extant seedplants are arranged in a straight line, or linearly (Figure 5.IOA).The three megaspores that are distal (away from the ovule base)abort; only the proximal megaspore (near the ovule base) continues to develop. In the pollination chamber, the resorbedpollen grains (Figures 5. iDA, 5.1 1A) develop into mature malegametophytes and form pollen tubes, which grow into the tissueof the megasporangium (Figures 5. iDA, 5.1 1B). In gymnosperms

these male gametophytes may live in the megasporangial tissue

for some time, generally several months to a year.

The functional megaspore greatly expands, accompanied

by numerous mitotic divisions, to form the endosporic

female gametophyte (Figures 5.1OA, 5.11B,C). In the seeds

of gymnosperms, archegonia differentiate at the apex of the

female gametophyte (Figure 5.11C,D). As in the nonseed

land plants, each archegonium has a large egg cell and a

short line of neck cells (plus typically a ventral canal cell or

nucleus). Eventually, the male gametophytes either release

motile sperm cells (in cycads and Ginkgo) into a cavity

between the megasporangium and female gametophyte

(known as the archegonial chamber; Figure 5.1 DA), or the

pollen tube of the male gametophyte delivers sperm cells

directly into the archegonial neck (in conifers). (Note that

micropyle

the ovules of some Gnetales and all angiosperms lack arche

gonia.) The end result is that a sperm cell from the male

gametophyte fertilizes the egg of the female gametophyte. A

long period of time (perhaps a year or more) may ensue

between pollination, which is delivery of the pollen grains

to the ovu)e, and fertilization, actual union of sperm and

egg. Note: This is not true for the flowering plants, in which

fertilization generally occurs very soon after pollination (see

Chapter 6).The resulting diploid zygote, once formed, undergoes

considerable mitotic divisions and differentiation, eventually

maturing into the embryo, the immature sporophyte (Figures

5.1DB, ShE). The tissue of the female gametophyte contin

ues to surround the embryo (Figure 5.11E) and serves as

nutritive tissue for the embryo upon seed germination (except

136 CHAPTER 5 EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS UNIT II EVOLUTION AND DIVERSITY OF PLANTS 137

A

FIGURE 5.8 Pollen grains—immature male gametophytes of seed plants. A. Zamia sp., a cycad. B. Ginkgo biloba. C. Pinus sp., a conifer.

SICL

pollen pollengrains

pollination droplet

- integumentpollinationchamber

integument (2n)

megasporangium(nucellus)

(2n)

mitosiS and

archegonial chamber

archegonium(with egg)

differentiation

A

functionalmegaspore

(n)

(2n)

micropyle

femalegametophyte

(n)

egg)

seed coat’

mitosis and

micropyle

megaSporangiUm(nucellus)

(2n)

B

femalegametophyte

(n)

embryo(new 2n)

megasporangium

pollen tube(haustorial)

germination &

radicle

hypocotyl

epicotyl(shoot apex)

cotyledons

femalegametophyte

(n)

megaspOrangiUm(degenerate)

differentiation

FIGURE 5.10 A. Ovule development in the nonflowering spermatophytes. B. Seed development.

pollen grain(immature endosporic

male gametophyte)

mature malegametophytes, each

with pollen tube

sperm

motilesperm cell

FIGURE 5.9 Male gametophyte morphology and development in the nonflowering spermatophytes; Cycas sp., illustrated. (Reproducedand modified from Swamy, B. G. L. 1948. American Journal of Botany 35: 77—88, by permission.)

1

138 CHAPTER 5 EVOLUTiON AND DIVERSITY OF WOODYAND SEED PLANTS

1, phloem

1xylern

in the flowering plants; see Chapter 6). The megasporangium

(nucellus) eventually degenerates. The integument matures

into a peripheral seed coat, which may differentiate into

various hard andlor fleshy layers.

SEED ADAPTATIONS

The adaptive significance of the seed is unquestioned. First,

seeds provide protection, mostly by means of the seed coat,

from mechanical damage, desiccation, and often predation.

Second, seeds function as the dispersal unit of sexual repro

duction. In many plants the seed has become specially modi

fied for dispersal. For example, a fleshy outer seed coat layer

may function to aid in animal dispersal. In fact, in some plants

the seeds are eaten by animals, the outer fleshy layer is

digested, and the remainder of the seed (including the embryo

protected by an inner, hard seed coat layer) passes harmlessly

through the gut of the animal, ready to germinate with a built

in supply of fertilizer. In other plants, differentiation of the

seed coat into one or more wings functions in seed dispersal

by wind. Third, the seed coat may function in dormancy

mechanisms that ensure germination of the seed only under

ideal conditions of temperature, sunlight, or moisture. Fourth,

upon germination, the nutritive tissue surrounding the embryo

provides energy for the young seedling, aiding in successful

establishment.Interestingly, in seed plants the female gametophyte (which

develops within the megaspore) remains attached to and

nutritionally dependent upon the sporophyte. This is exactly

the reverse condition as is found in the liverworts, homworts,

and mosses (Chapter 3).

EUSTELEIn addition to the seed, an apomorphy for spermatophytes is

the eustele (Figure 5.12). A eustele is a primary stem vascu

lature (“primary” meaning prior to any secondary growth)

that consists of a single ring of discrete vascular bundles.

Each vascular bundle contains an internal strand of xylem

and an external strand of phloem that are radially oriented,

i.e., positioned along a radius (Figure 5.12).

The protoxylem of the vascular bundles of a eustele is

endarch in position, i.e., toward the center of the stem. This is

distinct from the exarch protoxylem of the lycophytes and the

mesarch protoxylem of most monilophytes (Chapter 4) and of

some fossil relatives that diverged prior to the seed plants.

DIVERSITY OF WOODYAND SEED PLANTS

ARCHEOPTEPJS

A well-known lignophyte that lacked seeds was the fossil plant

Archeopteris (not to be confused with the very famous fossil,

reptilian bird Archeopteryx). Archeopteris was a large tree, with

wood like a conifer but leaves like a fern (Figure 5.13A,B).

Sporangia, producing spores, were born on fertile branch

systems. Some species of Archeopteris were heterosporous.

“PTERIDOSPERMS”—”SEED FERNS”

The “pteridosperms,” or “seed ferns:’ are almost certainly

a paraphyletic group of fossil plants that had femlike foliage, yet

bore seeds. Medullosa is a well-known example of a seed fern

integument

I’,

Iintegument

femalegametophyte

Vgametophyte

.

rchegonia

/


4female

gametophyte

cortex

a7pith

/--megasporocyte

A

r.

U-.

-

B C

11female

gametophyte

FIGURE 5.12 Eustele. A. Diagram of eustele. Note single ring of vascular bundles, with xylem inside, phloem outside. B. Helianthus stem

cross-section, an example of a eustele. C. Close-up of vascular bundle, showing xylem, phloem, and associated fibers.

vascularbundle

embryo

/

nuêleus

-

. ‘\sIrile••• ‘

‘.:;. ... • cells

-1

EFIGURE 5.11 Ovule and seed development, illustrated by Pinus sp. A. Young ovule, longitudinal-section, at time of pollination. Pollengrains are pulled into micropyle by resorption of pollination droplet. Meiosis of the megasporocyte has yet to occur. B. Post-pollination,showing development of the female gametophyte and haustorial pollen tube growth of the male gametophytes within tissue of megasporangium(nucellus). C. Mature ovule, showing two functional archegonia within female gametophyte. D. Close-up of archegonia, each containinga large egg cell with a surrounding layer of sterile cells and apical neck. E. Seed longitudinal-section, seed coat removed, showing embryoand surrounding nutritive layer of female gametophytic tissue.

EVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS

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