Insights from Agnes Arber's Concept of the Plant

Annals of Botany 88: 1203-1214, 2001doi:10.1006/anbo.2001.1437, available online at http://www.idealibrary.com on

Character Description in Phylogenetic Analysis: Insights from Agnes Arber's Concept ofthe Plant

BRUCE K. KIRCHOFF*

Department of Biology, P.O. Box 26174, University of North Carolina at Greensboro, Greensboro, NC 27402-6174,USA

Received: 21 September 2000 Returned for revision: 5 November 2000 Accepted: 26 March 2001

Throughout her work Agnes Arber argues for an inclusive, synthetic concept of the vascular plant as 'consisting of aunification of every phase of its existence'. Her view of the leaf as a partial-shoot reflects this unification by relatingthe part (leaf) to the whole (shoot). According to Arber's view of the plant, the part can be fully understood only inthe context of the whole. Morphological character description as it is currently practiced in systematics is in sharpcontrast with this holistic view of plant structure. Systematic characters are removed from their context when they aredescribed. This problem is greatest when characters are expressed verbally. Verbal descriptions convey little of thecontent of the character. A shift from verbal to visual characters allows systematists to capture more information,including some of the context in which the character occurs. By using a photograph, the fringe on a labellum ofAlpinia spp. (Zingiberaceae) can be viewed in the context of the labellum in a way that the word 'fringe' cannotconvey. The use of pictorial characters also allows reliable data storage and retrieval from databases, much as DNAsequences are currently being stored and retrieved. © 2001 Annals of Botany Company

Key words: Agnes Arber, character concept, character state, cladistics, database, holism, partial-shoot theory,phylogeny, phylogenetic systematics, plant morphology, process morphology, typology.

INTRODUCTION

Agnes Arber's (1950) partial-shoot theory of the leaf is oneof the best known alternatives to the classical model of theleaf, based as it is on the division of the plant into themutually exclusive categories of root, stem and leaf (deCandolle, 1827/1841; Eames, 1936 p. 380). Unlike thiscategorical model, the partial-shoot theory breaks down thehard and fast distinction between the stem and leaf. The leafis seen as having some shoot characteristics, and the shootas having some characteristics of leaves. Since mostmorphological characters currently used in systematics arebased on the classical model, it is useful to review Arber's(1950) disagreement with this model to see what can belearned about character definitions from the principlesimplicit in her work.

THE PARTIAL-SHOOT THEORY OF THELEAF

In the first few chapters of her book, The natural philosophyof plant form, Arber (1950) accepts the stem-leaf model ofthe plant implicit in much of the botanical work sinceGoethe (Miller, 1988). According to this model, the plantbody can be divided up into the mutually exclusivecategories of stem and leaf. Although she adopts thismodel as her starting point, Arber soon begins to questionit by pointing out that there is a second, equally ancient wayof viewing vascular plant structure. This second way gives

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primary importance to the shoot, a complex of stem andleaf (Sachs, 1875). In this model the shoot exists in its ownright, apart from its constituent stems and leaves. Theconcepts of stem and leaf refer only to the relationshipbetween parts within the context of the shoot, the largerwhole. By contrasting the stem-leaf and shoot models ofplant structure, Arber (1950) sets up an opposition betweentwo seemingly irreconcilable theories of plant construction,an opposition she immediately works to destroy.

After acknowledging that both models embody truth,Arber (1950) goes on to point out that neither is completelyadequate. Her task is then to find a new, more adequate,model that transcends the limitation of both older models.

'We should aim at including and transcending both in a synthesis,which, while treating the shoot as a primary unit, will yet haveabsorbed into itself such truth as is to be found in the concept ofthe antithesis of stem and leaf.' (Arber, 1950 p. 72)

This new model is her partial-shoot theory of the leaf.Before I present the evidence for this theory, I want to

emphasize that Arber did not believe that the partial-shoottheory solved all of the problems of plant construction. Shedid think that it was better than the existing alternatives,but she recognized its limitations and emphasized that otherscientists would extend or replace it in the future. She sawall models as partial, as embodying some aspects of plantstructure, but never capturing all aspects of the plant. It isimportant to remember this, not because her attitude is sodifferent from that of other botanists, but because thedecline of morphology as a vital science has resulted in theacceptance of rather simplistic models as the final word on

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plant structure. It seems clear that Arber did not realizehow hard it would be to transcend a concept of the plant asconsisting of relatively few fundamental parts (leaf, stem,root, flower, etc.). This view of the plant has persisted to thepresent day and forms the basis for most systematic work.

The evidence for the partial-shoot theory of the leaf canbe divided into two sections, ontogenetic and structural.Arber (1950) begins her summary of the ontogeneticevidence with a review of the early stages of leaf growth.The initial growth of the leaf is terminal, similar to theterminal growth that occurs in the shoot. However, in theleaf, terminal growth is of limited duration. In almost allcases, terminal leaf growth ceases after a short period.Linking this observation to her theory, Arber (1950 p. 79)speculates that the limited growth of the leaf may be due tothe imperfect nature of the leaf as a partial-shoot.

The lamina begins to form during the later stages of leafinitiation. Lamina development in dicotyledons is primarilypleuroplastic: the blade forms from the lateral margins ofthe leaf primordium. Marginal, lamellar meristems form inthe leaf margins and produce the blade. Although thisprocess seems distinctly unlike that which produces theshoot, Arber reminds us that tunica growth is lamellar andthat folds resulting from this growth produce the leafprimordia (Arber, 1950 p. 79). In this way, she finds asimilarity between shoot and leaf growth. Of course, shedoes not deny that there are differences. She is well aware ofthe differences, but attributes them to the observation thatthe shoot is radial while the leaf is dorsiventral.

Turning from development to mature form, Arber (1950)suggests that the leaf can be seen as consisting of both axialand foliar elements. The axial elements include the petiole,median portion of the leaf base, midrib, non-winged leafapex (where it occurs), and any leaf tendrils that are present.The foliar elements include the stipules, lamina, and lateralwings of any sheathing leaf bases. Continuing thesecomparisons, Arber (1950) refers to the leaflets of acompound leaf and the leaves of a shoot. The primarydifference between these two forms is the presence of axillarybuds on the shoot. The likenesses include their overallsimilarity in form and the repetition of the abscission layerat the base of the petiolule, which is present in Aesculushippocastamim (Arber, 1950 p. 80). Finally, Arber (1950)draws a parallel between stipules (leaf elements) and variousmodified phyllomes such as cotyledons, prophylls andbracteoles (shoot elements). For instance, stipules oftendiffer morphologically from the lamina, just as cotyledonsdiffer from the succeeding leaves of a seedling, or prophyllsfrom the mature leaves of a shoot.

The last line of evidence that I will summarize concernsphyllomes that bear foliar outgrowths. These phyllomesrange from cabbage leaves that bear miniature leaf forms(enations), to the honey-leaves of Ranunculus that bearnectary scales, to the perianth of Narcissus that bears acorona. In all of these cases, the vasculature of the lateralmember bears the same relationship to the parent phyllomeas the vasculature of a leaf bears to its parent shoot: thexylem-poles of the bundles face the parent. The parentphyllomes thus function as axes in that they bear lateralmembers with vasculature oriented as it is in a leaf. Through

comparisons like these, Arber (1950) searches for the'intrinsic relation of parts' (p. 84) that is expressed in thevarious forms of the leaf and shoot. She sees this intrinsicrelation not as a static form, but as a dynamic relationshipbetween the part and the whole. She is not searching for abetter way to describe plant form, but for a better way to seewhat is already visible. She is not interested in just having amethod by which to describe plant form. If she were, shewould be content with the classical leaf-stem theory of plantform. This conceptualization is as good as any other if weonly want to create categories on which to hang our senseimpressions. What Arber is seeking is a way to look into thedynamic movements that form the plant. She is notinterested in static categories. The evidence she presentsshows that these movements cut across traditional morpho-logical categories. Although she speaks of the 'theory' of thepartial-shoot, this is merely a convention that she is forcedto by scientific language and methodology. She is not reallyproposing or testing a theory, certainly not in any modernsense of how theories are tested. Rather, she wants to drawour attention to the interrelation among a number ofphenomena to help us see the plant with fresh eyes. To speakabout the results of this 'seeing', and to place her results inthe context of botanical thought, Arber needs a way tosummarize her work. Only in this sense, as a summary ofempirical observations, can we understand her partial-shoottheory of the leaf. Arber is much closer to the original Greekmeaning of theory (theoria, to behold) than to the currentscientific meaning of this term.

PRINCIPLES IMPLICIT IN ARBER'SANALYSIS

The analytic and holistic principles Arber (1950) employs inexplaining her partial-shoot theory of the leaf are applicablefar beyond the confines of her work. It is worthwhilesummarizing these principles so that they can inform otheraspects of our work as systematists.

One of the primary characteristics of Arber's analysis isher propensity to break down fixed categories. Thispropensity is clearly visible in her partial-shoot theorywhere her intent is to show that leaf and shoot are notmutually exclusive categories. We also find this propensityin her earlier book, Monocotyledons (Arber, 1925). Here,she first describes the root, axis and leaf as independentorgans, but then turns to Saunder's (1922) leaf-skin theoryand discusses the structure of the axis as composed ofdecurrent leaf bases. Although the content of this theorydiffers from that of her later partial-shoot theory, hermethod of analysis is similar.

Arber's ability to look beyond pre-established categorieshelps us see the plant in ways that are not confined to thewell-trodden paths of traditional morphology. The plantbecomes something real in and of itself, real in the sensethat it exists beyond our categorical interpretations of it.While most botanists would agree that the plant exists inthis sense, few put this theory into practice by continuallyreturning to the plant and looking at it again and againwith fresh eyes, as Arber does. To her, the plant is notsomething fixed, something that we have understood and

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now merely have to place within a classification scheme.The plant is something that we are continually in theprocess of understanding. The concepts and categories thatwe apply to the study of morphology help us to understandthe being of the plant as it exists beyond all categories.Thus, our current concepts should not constrain us fromseeing the plant in new ways. At any one time we may workwithin the framework of a prevailing concept of the plant,but with each new conceptualization the plant comes to anew expression in our experience (Rutishauser and Sattler,1985, 1987, 1989). We never have all aspects of the plantbefore us at once, and Arber (1950) never loses sight of thisfact. She is aware that she is continually trying to describesomething that remains out of our reach. She is aware thatthe plant is beyond all categorical interpretations of it. Aplant is not merely a collection of characters. It can never befinally and completely known.

This brings us to the second principle implicit in Arber's(1950) analysis: the realization that knowing a plant is notsimply a matter of conglomerating characters and summar-izing them in a description. On the first page of her book,Arber (1950) makes it clear that she understands thedifference between parts that are merely placed together,and those that have an intrinsic relationship to each other.

'The different branches [of biology] should not, indeed, beregarded as so many fragments which, pieced together, make upa mosaic called biology, but as so many microcosms, each ofwhich, in its own individual way, reflects the macrocosm of thewhole subject.' (Arber, 1950 p. 1)

To Arber, the plant is not a collection of parts, but rather a'unification of every phase of its existence' (Arber, 1950).Hidden within this phrase is the relationship between partand whole in the plant. To understand what she means byit, we need to subject it to the same type of analysis that sheapplies to the plant itself. We need to consider this phrasefrom several points of view to bring out a meaning thatcannot be captured in any one view.

In this phrase, both the word 'unification' and the phrase'every phase of its existence' are significant and must beconsidered separately before we recombine them in thewhole. The phrase 'every phase of its existence' is significantbecause it draws our attention to the characteristics of theplant, without actually enumerating them. Any enumer-ation would necessarily be incomplete, especially givenArber's propensity to think across traditional categories.Saying that a plant consists of the unification of root, stem,leaf and flower is not at all what Arber means. Thesedivisions of the plant are arbitrary and do not capture thefull diversity of ways in which we can conceive of the plant.By using a somewhat vague phrase for a plant's character-istics, Arber is able to suggest an analytic division of theplant into morphological categories, without tying herselfto any one categorization.

Understanding what Arber (1950) means by the word'unification' is more difficult. To Arber, the plant cannot beconceived of as a collection of characters. It is not a'unification' in the sense of separate characters that areplaced together. If plants could be described in this way,then it would be possible to have one correct enumeration

of the characteristics of a plant. Clearly, Arber (1950) doesnot support this view. A plant does not arise from theaggregation of characters. Each plant is an intrinsic unity, aunity that exists prior to our process of analysis. This unitycan be conceptually dismembered into characters, but it cannever be fully represented by any single characterization.

Bortoft's (1996) explanation of Goethe's scientific method(Miller, 1988) helps clarify the difference between 'unifica-tion' in the sense that Arber (1950) uses this term, and theunity that arises from the apposition of separate characters.Bortoft (1996) points out that wholes that are created fromthe aggregation of separate characters are always artificial.The relationship between the parts is external, they are notpart of an intrinsic whole. For instance, relationships amongitems sitting on a desk are not intrinsic to the items, or to thedesk. There is no necessary relationship between a coffee cupand a pen that sits beside it. They are only in proximitybecause someone has placed them together. Their relation-ship is created. Bortoft (1996) calls this type of relationship a'counterfeit whole'. It is created by bringing unrelatedobjects into relationship.

The unity that Arber (1950) speaks about is of a differentsort. It is a unity that is not dependent on human activity inbringing it about. Goethe, who was an important influenceon Arber (Arber, 1946), speaks of this type of unity whenhe says:

'Hence we conceive of the individual animal as a small world,existing for its own sake, by its own means. Every creature is itsown reason to be. All its parts have a direct effect on one another,a relationship to one another, thereby constantly renewing thecircle of life; thus we are justified in considering every animalphysiologically perfect. Viewed from within, no part of the animalis a useless or arbitrary product of the formative impulse (as sooften thought). Externally, some parts may seem useless becausethe inner coherence of animal nature has given them this formwithout regard to outer circumstance.' (Goethe, 1820/1988 p. 121)

When Arber (1950) speaks of the plant as the 'unification ofevery phase of its existence', she is referring to the unity thatexists prior to analysis. Both Goethe (1817/1988) andBortoft (1996) claim that this type of unity is directlyperceptible.

Within the intrinsic unity of the organism there is aspecial relationship between the parts and the whole(Kirchoff, in press). Like any object that is authenticlywhole, the parts are intrinsically related to the whole thatthey create (Bortoft, 1996; Kirchoff, in press). The whole isbuilt up of the parts in such a way that each part bearssomething of the whole within it (Sattler, 2001). Thestructure of each part reflects the whole, and the whole iscreated out of parts that have an intrinsic relationship toeach other and to the whole itself.

Returning to our previous example, a cup sitting on adesk has no intrinsic relationship to the desk. The cup-deskunity is not an authentic whole. The cup can be removedwithout disturbing either the unity of the desk or the cup.The two items only come into relationship because someoneplaced the cup on the desk. The relationship is functional,not intrinsic or structural.

The relationship between a leaf and stem is of a differentsort. Although, in one sense, a shoot is created out of leaves

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and a stem, in another sense a shoot has a reality that goesbeyond the separate categories of leaf and stem. Arber(1950 p. 70) recognizes this when she says 'The possibilityof grafting buds on to an alien stock was one of the pointsthat led botanists long ago to the realization of theindividuality of the bud, and hence of the shoot intowhich it developed'. In any authentic whole, the parts enterinto a mutually supportive relationship that produces thewhole. Neither the parts nor the whole can exist without theother. In this sense, individual characters (leaf and stem) donot exist outside the whole (shoot) to which they belong.The parts create the whole, while the whole gives meaningto the parts. The shoot is created from leaves and stems, butleaf and stem are not mutually exclusive categories. Each isintrinsically related to the other. We see this when we followArber (1950) in her analysis of the leaf as a partial shoot.The leaf has both 'leaf-like' and 'shoot-like' characteristics.

Another way of understanding the difference betweencounterfeit and authentic wholes is to think about how ourability to conceptually dismember the whole differs in thesetwo types. There are relatively few ways in which todismember counterfeit wholes, while authentic wholes canbe conceptualized in many different ways, perhaps ininfinitely many ways. Our cup-desk whole can be separatedinto the parts 'cup' and 'desk', but not in many other ways.At the molecular level we may be able to separate theorganic from inorganic components of the cup-desk, inwhich case we would see a kind of patchwork that looksnothing like a desk or cup. However, with the disappear-ance of these objects we would also move beyond conceptsthat have specific reference to the cup-desk whole. Anyobject can be dismembered into organic and inorganiccomponents. These concepts can be applied to many othersystems besides a cup sitting on a desk. On the other hand,an authentic whole such as a shoot can be conceptualized inmany different ways, all of which have relevance to thewhole (Rutishauser and Sattler, 1985). Arber (1925, 1950)demonstrates this in her discussions of the leaf-skin, partial-shoot, and leaf/stem theories, all of which conceptuallydivide the plant in different ways.

Recognizing the relationship between part and whole inauthentic wholes is important because it helps explain whyArber (1950) continually moves back and forth betweenpart and whole in her analysis of the plant. Doing soenables her to approach the unity of the plant withanalytical tools. We can follow her as systematists bylearning to create characters that retain something of theircontext, characters that retain the relationship between partand whole that Arber tries to include in her theories.

The final principle of Arber's (1950) analysis is thedynamic way in which she views plant structure. By'dynamic' I do not mean that Arber uses developmentalevidence to support her views, but that she sees plantstructure in a continual process of becoming. For Arber,there are no primary morphological categories. All organshave the same potential. They differ only in their ability toactualize this potential.

'. . . in the shoot radiality is actual, or explicit, while dorsiven-trality is potential or implicit; for the leaf, on the other hand, thereverse is true.' (Arber, 1950 p. 87).

This point of view allows Arber to see individual leaves notas static structures, but as incomplete shoots: organs thatembody some, but not all, of the features of the wholeshoot. The features that are not embodied are present inpotential. Their lack is accidental, not essential. Ifconditions change, for instance in cases of teratology,these characteristics may appear. When they do, they showanother aspect of the dynamic nature of the leaf, which cantake many forms.

TAXONOMIC CHARACTERS

Character descriptions

Given the importance of characters to the process ofreconstructing phylogeny, the description of charactersshould be one of the most intensely studied issues insystematics. Unfortunately, until recently, this has not beenthe case. Stevens (1991) and Gift and Stevens (1997) domuch to deconstruct the concept of a character by showingthat there is little validity to character states of quantitative(continuous) characters. Thiele (1993) takes a differentapproach by stressing that cladistic characters are featuresof taxa, and as such cannot overlap. Weston (2000)discusses Sattler's (1992) process morphology in the contextof creating cladistic characters, and Wagner (2001) assem-bles a wide range of papers addressing many aspect of thecreation and use of characters in evolutionary studies.

The modern age of character analysis began whenHennig (1950, 1966) recognized that characters differ intheir usefulness in predicting evolutionary relationships.Hennig understood that to reconstruct phylogeny it is notenough for organisms to be similar. Similarity alone is not aguarantee of evolutionary relationship. For this, a specialtype of similarity is needed. Only organisms that sharederived characters (synapomorphies) also share a commonancestor (Hennig, 1950, 1966). Because of the central roleof Hennig's (1950, 1966) work in reformulating systematics,I will primarily refer to his work in the remainder of thissection.

Although Hennig was undoubtedly the first to formulatethe methods of phylogenetic systematics, it is worth notingthat in the 1950s W. H. Wagner elaborated methods nearlyidentical to Hennig's for use in his plant systematics classesat the University of Michigan, USA (Hardin, 1957;Wagner, 1961). As far as I know, Hardin (1957) was thefirst to publish a phylogenetic analysis using Wagner's'Ground plan-divergence' method (W. H. Wagner, pers.comm.). Still, neither of these authors was able to developtheir work in a way that allowed it to spread throughout thelarger systematic community. Wagner's methods did,however, receive wider recognition through Farris' (1970)formalization of them as computational methods. Thisformalization became the basis for modern numericalcladistics.

From Hennig (1950, 1966) and Wagner (1961) we learnedthat choosing the right characters and determining thederived states are two of the most important steps inphylogenetic analysis. Hennig's full method for reconstruct-ing phylogeny using morphological characters consists

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of: (1) identifying characters and character states; (2)determining the derived character state for each character(Stevens, 1980); and (3) constructing a phylogenetic treebased on some model of character evolution (parsimony isthe most commonly used model). This process is nowwidely thought to give the best estimate of the truephylogeny.

The part of Hennig's (1950, 1966) method that concernsus here is the treatment of characters. Although not originalwith him, Hennig (1950, 1966) gives such a clear descriptionof the process that it is worth review. There is a clearcontrast between the methods of Hennig and Arber.

The unit of systematic analysis for Hennig (1950, 1966) isnot the individual, but the semaphoront. The Greek rootsof this word give us a clue to its meaning. The root 'sema'means mark, sign, or signal, and 'phor' means to carry orbear. A semaphoront is the organism conceived of as a'character bearer'. Hennig (1966) gives a more precisedefinition:

'the individual at a particular point of time, or even better, duringa certain, theoretically infinitely small, period of its life' (Hennig,1966 p. 6)

By considering the taxonomic entity as an organism at oneinstant of its life, Hennig (1966) is able to isolate variableparts of an organism's life history. This can be importantwhen working with holometabolous insects (insects withcomplete metamorphosis), as Hennig did in his work onDiptera.

Like most taxonomists, Hennig (1966) does not give aprecise definition of 'character'. He merely acknowledgesthat semaphoronts possess features that distinguish themfrom other semaphoronts.

'We will call those peculiarities that distinguish a semaphoront (orgroup of semaphoronts) from other semaphoronts "characters".'(Hennig, 1966 p. 7)

This definition is similar to many others proposed bytaxonomists over the years. I have chosen two additionaldefinitions as typical. Other definitions can be found inStuessy (1990).

Davis and Heywood (1973 p. 113) define a character as'any attribute (or descriptive phrase) referring to form,structure or behavior which the taxonomist separates fromthe whole organism for a particular purpose such ascomparison or interpretation'. Stuessy (1990 p. 27) pro-poses the definition 'a feature of an organism that isdivisible into at least two conditions (or states) and thatis used for constructing classifications and associatedactivities'.

Neither of these definitions, nor any other that I haveseen, gives any indication of how to recognize characters, orhow they relate to the form of the organism as a whole. Atfirst glance, Stuessy's (1990) phrase 'divisible into at leasttwo conditions' does seem to give some guidance, but thisimpression quickly evaporates when we realize thatdescribing any attribute of a plant (presence of chlorophyll,for instance) immediately implies the existence of at leasttwo conditions. The presence of chlorophyll implies itsabsence, living implies non-living, etc. All characters haveat least two states. However, not all characters are

informative in resolving taxonomic relationships. Thosethat are homogeneous in the study group (Ingroup) can beexcluded from studies designed to elucidate the relation-ships among taxa in this group. Although these characterssatisfy Stuessy's (1990) definition in that they have twostates (one in the Ingroup, one or more in the Outgroup),they are not useful in phylogenetic analyses.

After discussing semaphoronts and characters, Hennig(1966) moves on to describe how characters can becombined to form the totality of the form, or holomorphy,of the organism.

'We will call the totality of all these characters simply the totalform (or the holomorphy) of the semaphoront, which thus is to beregarded as a multidimensional construct.' (Hennig, 1966 p. 7)

Although he speaks of the holomorphy as a 'multi-dimensional construct', it is unlikely that Hennig mistookthis construct for the organism itself. In this sense he isclose to Arber. For both, the organism in its full realityremains unknown. Still there is a difference between the twoapproaches. For Arber, the organism continually disclosesitself through our various theories. While none of thesetheories is completely adequate to describe the organism,each captures part of the whole. Hennig (1966), on theother hand, is not interested in the organism that existsbehind our theories. He focuses his efforts on creating a'general reference classification' not on understanding theorganism. In fact, a large portion of the first part of hisbook is devoted to showing that a phylogenetic classifi-cation is better suited for this purpose than any otherclassification scheme (Hennig, 1966). Hennig's methods ofcharacter analysis make sense in light of this goal. They arefocused on giving us tools (synapomorphies) with which wecan discover evolutionary relationships. A method thatdismembers the individual into semaphoronts, dismembersthe semaphoront into characters, and then combines thecharacters to form the holomorphy can never provide anadequate description of organismal form. Arber's workshows this clearly, but Hennig has focused our attention onanother goal. The task now is to find a way of applyingArber's insights to the process of creating characters for usein phylogenetic systematics.

The context of character descriptions

Character description is always done in a specific contextby specific investigators. All that is absolutely necessary tocreate a character is variability in the Ingroup and theability to categorize this variability into characters andcharacter states. The ability to partition the variabilitydepends on the taxonomic context in which the charactersare to be defined. This context includes the taxa under study(the Ingroup), the morphological theory under which thecharacters are created, and the training of the investigator,which influences his choice of characters and his descriptionof these characters in specific ways.

The selection of an Ingroup determines which charactersare chosen by partitioning morphological variability inways that are unique to the taxa under study. For instance,a phylogenetic study of the Cannaceae will contain few

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characters dealing with inflorescence branching, while astudy of the Marantaceae will produce several. There isrelatively little variability in the degree of branching in theCannaceae compared with the Marantaceae (Andersson,1976; Sell and Cremers, 1994).

The morphological theory under which the characters aredefined is a major part of the conceptual context ofcharacter description. As currently used, morphologicalcharacters are abstractions, conceptually separated from theplant as a whole. They are a view of the plant through aparticular conceptual lens. This is one reason why differentscientists studying the same taxon describe differentcharacters. The whole can be broken up in many differentways.

As discussed above, the most commonly used morpho-logical theory divides the plant into leaf, stem and root.This method of conceptualizing plant structure is wellestablished, but as Rutishauser and Sattler (1985, 1987,1989) demonstrate, these divisions are only one way ofviewing the plant. The choice of this model focuses ourattention on specific characters and character combinationsto the exclusion of others. If, instead, we adopt Arber's(1950) partial-shoot theory we will look for characters thatcross the boundaries of leaf and stem. Using this theory asour basis we might define characters that emphasize theaxial nature of the leaf or the foliar nature of the shoot.

As an example we can consider the shoots of theZingiberales, which are supported by sheathing leaf basesthat provide much of the stem's rigidity. The degree towhich the shoot receives support from sheathing leaf basesvaries throughout the order. Familiarity with Arber'spartial-shoot theory might lead us to define charactersthat describe the extent to which the shoot is supported inthis way. For example, except at flowering, the stem of theMusaceae is completely composed of overlapping leafsheaths; the stems of most members of the Strelitziaceaeare woody and arborescent and receive no support from theleaves; the Lowiaceae have short stems that receive little orno support from the leaf sheaths; while those of theZingiberaceae, Cannaceae, Heliconiaceae and Marantaceaeare intermediate in structure in ways that have never beenfully described. Characters based on shoot structure in theMusaceae and Strelitziaceae have been used in phylogeneticanalyses, but the intermediate structure of the shoot in theother families has not (Dahlgren and Rasmussen 1983;Kirchoff, 1988; Kress 1990, 1995). One possible explanationfor this is that our conceptual dissection of the plant intoleaf, stem and root impedes our identifying characteristicsof these shoots that are strongly supported by, but notsolely composed of, leaf sheaths.

In addition to the taxonomic and conceptual contexts,the social context of the scientist also plays an importantrole in how he describes characters. A scientist's traininginfluences how he will approach his work at least as muchas does his choice of taxa. At the grossest level, scientistsacculturated as molecular biologists (which includes manymolecular systematists) are less likely to include extensivemorphological analyses, or to appreciate the relevance oftheoretical morphology in creating characters. On the otherhand, morphologists are less likely to include genetic

analyses in their studies. Even within morphologicalanalyses, the choice of characters is influenced by theapproaches the scientist was exposed to as a student. Forinstance, if his or her main influence was from Eames (1936)and his followers, the characters he or she chooses willprobably be based on the concept of the plant as divisibleinto leaf, stem, root and flower. Exposure to the work ofArber (1950) and Rutishauser and Sattler (1985, 1987,1989) will probably lead to less traditionally definedcharacters (Weston, 2000). Of course, all of these short-comings can be ameliorated by selecting co-workers whobring complementary skills to the analysis. My point is onlythat each of us brings his or her training with them to anytask. This training is part of our social context because in itwe are exposed not only to specific theories, but to whatKeller (1985) calls an 'aesthetic' of science. We use ourscientific aesthetic to tell us what is important, what can beignored, and to establish the preferred methods for carryingout our day-to-day scientific work. The vehicles forconveying this aesthetic are our social interactions withour teachers and colleagues.

Alternatives to traditional character descriptions

Process morphology. Sattler's (1992) and Jeune andSattle's (1992) use of process morphology is an alternativeto the conceptual division of the plant into mutuallyexclusive, static categories (leaf, stem, root, etc.) (Weston,2000). Adopting this framework allows the description ofcharacters that cut across traditional boundaries betweenorgan classes. Describing characters based on processesinstead of structural categories will yield a completelydifferent set of characters and may influence the final formof the phylogeny. A brief summary of the principles ofprocess morphology will make this clear.

Process morphology is an attempt to break down the fixedcategories of plant construction that have influenced muchof plant morphology since de Candolle (1827/1841). Theconcepts of process morphology follow on (though are notdependent on) Sattler's (1966) earlier work on partialhomology, in which he suggests that strict one-to-onehomology is inadequate to deal with the full range ofplant form. Sattler (1966, 1996) sees strict homology asforcing variation into mutually exclusive morphologicalcategories. One remedy to this situation is to consider theplant dynamically rather than statically. To do this, Jeuneand Sattler (1992) identify a number of process-categoriesthat describe plant structure. For instance, they use thetwo pairs of processes growth/decay and differentiation/dedifferentiation to analyse the structural dynamics ofplants. Within the growth category, they define thesubcategories (1) growth rate, (2) growth duration, and (3)growth distribution (Jeune and Sattler, 1992; Sattler, 1992).The structures themselves disappear in this analysis. Whatremains are 'process combinations' that can be placed intoat least partial equivalence with traditional morphologicalcategories (Jeune and Sattler, 1992). These 'process com-binations' are dynamic systems that exist through theirconstituent processes, but which are not endowed with anessential nature (Sattler, 1993).

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The processes Jeune and Sattler (1992) use to create theircombinations cut across traditional morphological organdescriptions (Jeune and Sattler, 1992; Sattler, 1993). Asshown above, their growth characters are defined withoutreference to any specific organ or organ system. To takeanother example, a single character state of their character'branching' occurs in both stems and compound leaves(Jeune and Sattler, 1992). In both organs the 'branches'form acropetally. In stems the 'branches' are shoots. Incompound leaves they are phyllomes. Thus, the characterstates of the character 'branching' are not confined to asingle organ category. The lack of correspondence betweenprocess-characters and organs means that there will be littleoverlap between process-characters and traditional char-acters.

The move from traditional characters to process-characters is similar to Arber's (1950) move fromtraditional morphological description to her partial-shoottheory. Both transitions break down static categories andreplace them with dynamic alternatives. The effect ofprocess-characters on the form of a phytogeny remains tobe investigated.

Other holistic approaches. Underlying Arber's (1950)partial-shoot theory of the leaf is her consistent attempt tothink holistically. The shift from defining characters basedon theories of plant construction to a more holisticapproach involves a change in the way we conceive of theplant (Bortoft, 1996; Kirchoff, in press). For Arber (1950)this conception is expressed in her partial-shoot theory, butit is clear that her methods are applicable far beyond thistheory. The potential to move beyond this theory is inherentin her method from the start. In essence, her methodinvolves a shift in the way we 'see' plants. Her worksuggests the need to shift from seeing the plant as composedof parts, to seeing the plant as a whole that can bedecomposed into parts. Bortoft (1996) has characterizedthis change in conceptual framework as a shift from seeingdisconnected parts (isolated characters) to seeing the partsintrinsically related to the whole. An illustration from artwill clarify how we can make this conceptual shift.

In Ando Hiroshige's View of Ml. Akira, Kakegawa(Fig. 1 A), the artist uses several compositional principles tocreate the composition. The main action of the picture takesplace in the space created by two curving lines, one formedby the kite string and the other by the curve of the bridge.These two movements are echoed and reinforced by similarlines throughout the picture. The curve of the kite string isechoed in the string of the balloon, the slopes of themountain and several other subsidiary curves (Fig. 1 A, redlines). The arch of the bridge is reflected in the backs of thebowing figures, the upraised arm of the child, the backs ofthe peasants planting rice, and the bent stalks of weeds(Fig. 1A, blue lines). The composition is stabilized by thevertical and horizontal lines, some of which are broken(bridge support and official's walking stick) (Fig. 1A, whitelines). In addition to reinforcing the vertical stability of thecomposition, the mountain provides a strong stop to oureye's movement across the page.

All of these compositional principles are built up by anumber of seemingly independent parts that cooperate tocreate the work of art (Fig. 1A, lines). The composition isconstructed through the harmonious interaction of theseparts, much as the form of an organism is built up throughthe interaction of its individual characteristics. Eachelement of the composition both plays a role in creatingthe whole, and has its own specific form because of the roleit plays. The vertical lines stabilize the picture only inrelationship to the strong implied movements of the centralcurves. In the same way, the curved backs of the peasantsprovide a counter to the strong left to right movement ofthe official crossing the bridge, not just by their centralposition on the page, but through the similarity of theirforms. As in all great works of art, the parts reflect thewhole, which they help create. Our goal is to describesystematic characters that reflect the integrity of the plantmuch as compositional element reflects the composition asa whole.

A simple step in this direction can be taken by movingfrom verbal descriptions of characters, to pictorial ones.Typically, morphological characters originate from studiesof plant structure, and are then converted to verbaldescriptions for use in phylogenetic analyses. In describingcharacters we make decisions on similarities betweenobservations to place them into classes (character states),which are then grouped into characters (Stevens, 2000). Inthis process, we drastically reduce the amount of informa-tion available to future investigators. For instance, thecharacter 'labellum fringed' captures only a small amountof the information available from observing the labellum ofAlpinia spp. (Zingiberaceae) (Fig. IB and C). Several of thedifferences that are not captured are (1) the presence orabsence of a split at the tip of the labellum; (2) the degree towhich the margin of the labellum is recurved; and (3) thesymmetry of the labellum. Of course we can include thesecharacters, and others, in our analysis, but even includingthem does not do justice to the structure of the labellum.The character 'labellum symmetry' does not take intoaccount the handedness of the symmetry. The largerportion can be either on the left or right side. Althoughthese problems can be partially dealt with by creating finerand finer verbal descriptions, verbal descriptions will neverfully capture the structure of the labellum. They will nevercapture it even at the level of detail that is present in aphotograph (Fig. IB and C). The use of visual characters(photographs or drawings) obviates the need for complexverbal descriptions. 'A picture is worth a thousand words'was never more true than in systematics. The use ofpictorial characters unifies features that would otherwise beseparated into distinct characters. This allows us tosummarize complex information in a form that is easy toassimilate and understand.

An example of the pictorial description of characters canbe found in Thiele and Ladiges' (1996) cladistic analysis ofBcmksia (Proteaceae). I will use their analysis of cotyledonshape as an example of this process. Thiele and Ladiges(1996) published outlines of the cotyledons of all species inthe genus Banksia. They presented their two-state character,based on these shapes, by drawing a line to separate the

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shapes into groups. This partitioned diagram represents thecharacter. Their description of the two states as flabellateand spathulate refers to this diagram. Although I do notknow how they proceeded in practice, the terms could havebeen applied after the division, as a way of drawingattention to the different shapes in the two groups. Thismethod would be consistent with a pictorial approach tocreating characters. It is possible to use terms to drawattention to a partition that has been made on othergrounds. Shape terms do not have to be used as criteria forseparating the shapes into groups. Ribeiro et al. (1999) alsoused a pictorial approach, but this time to create a flora, thekeys of which are mainly based on pictures of plantcharacteristics.

An additional advantage to the use of pictorial charactersis that they are amenable to storage and retrieval fromdatabases. Although verbal characters can be stored, theinformation they contain cannot easily be retrieved. Therepresentation of organismal features in verbal descriptionsdoes not provide sufficient information for subsequentinvestigators to recreate these features. This is because theprocess of creating verbal characters emphasizes one featureof the organism over another. The features that areemphasized will vary from study to study, with the resultthat comparisons across studies will be almost impossible.For instance, in describing a labellum we can emphasize theoutline of the organ and can create the character 'labellumshape' with states 'obdeltoid' (Fig. IB) and 'obdeltoid/irregular' (Fig. 1C). In labelling the shapes in this mannerwe emphasize the shape of the labellum of Alpinia zemmbet(Fig. IB) and give less attention to that of Alpinia henryi(Fig. 1C). The larger attachment site of the latter and theway that its sides slope steeply toward the irregular fringesuggest the term 'cordate/irregular' instead of 'obdeltoid/irregular' for this species (Fig. 1C). Giving emphasis to theattachment site and steeply sloping sides results in thecharacter states 'cordate' (Fig. IB) and 'cordate/irregular'(Fig. 1C). The choice of which pairs of terms to apply isarbitrary. Both pairs describe an aspect of labellum shape.Neither captures the shape as well as do the photographs.Someone who only had access to the verbal descriptionsfrom a database of characters would have a very difficulttime determining the shape of the labellums, let aloneincorporating the labellum of a third species into thischaracter. Stevens (1991) and Gift and Stevens (1997) havediscussed similar problems in relation to quantitativecharacters. The situation is similar to that which wouldoccur were there no standardization for the representationof the identity of the bases in DNA. Each laboratory wouldhave to develop its own scheme, which would have to be

decoded before the sequences could be used or evaluated byother investigators.

Quantifying organ shape does not help solve thisproblem. Although it appears accurate, morphometricquantification depends upon qualitative assessments ofshapes, which are subjective. For instance, to measure thedegree to which the petal is reflexed in Heliconia spp.(Fig. ID and E) we must establish the procedures fordefining the angle we will measure. This can be done basedon landmarks (Fig. ID and E white lines) (Bookstein, 1998),or based on any of several measures of the apparent angle(Fig. ID and E, green lines). The use of landmarks allowscomparable measurements to be made easily across species,but the apparent angle may be biologically more relevant asit is related to the opening that pollinators perceive whenapproaching the flower (Fig. ID and E, green lines). Bothmethods will give precise answers, but neither will beaccurate in the sense that neither will yield the 'true' angle ofsepal reflexion. Though not completely free of these prob-lems, photographs are a much more accurate way of repre-senting not only this angle, but all aspects of flower shape.

A related approach to character descriptions builds onthe work of the European typological school of plantmorphology (Troll, 1964; Weberling, 1989; among manyothers). Through comparisons of the form and position ofthe various organs, the scientists of this school havedescribed inflorescence structure in many groups of plants.These descriptions and their summary in diagrams arerepresentations of the morphology of the inflorescence as awhole (Fig. 2A-E). Although positional information isgiven primacy in these studies, other types of informationcan also be included. Presence or absence of organs andaspects of organ form and development can be included indiagrams only slightly more complex than those publishedin typological studies (Fig. 2F). For instance, internodelength can be shown by varying the length of the linesconnecting the phyllomes or by using dashed lines(Fig. 2F-H), and the presence of organ rudiments can beshown by using broken lines to draw the organ (Fig. 2F).Flower symmetry and more complex aspects of inflores-cence morphology can be illustrated with other conventions(Fig. 2G and H).

An additional benefit of adding information to a singlediagram is that it helps clarify the morphological relation-ships between the parts. For instance, in the Cannaceae, theflowers of a cincinnus always have the same symmetry, butwithin a florescence (i.e. between cincinni) the flowers maybe mirror images (Fig. 2G). In the Marantaceae flowersoccur in pairs and have mirror image symmetry (Fig. 2H).The relationship between the two arrangements of flowers isclearly seen in diagrams that include floral symmetry

F I G . 1. Ando Hiroshige's View of Ml. Akira, Kakegawa from the series Fifty-Three Stations of the Tokaido Road (1833-34). Wood block print.Bar = 2 cm. Original size 16-3 cm by 22-1 cm. The composition is built up through the use of repetitive forms and movements, some of which arestructured from several unconnected elements (e.g. bridge pillar and official's walking stick, white lines). Compositional principles are illustratedby the coloured lines. B, Labellum of Alpinia zerumbet (Zingiberaceae). Bar = 1 cm. C, Labellum of Alpinia henryi (Zingiberaceae). Bar = 1 cm.D, E, Flowers of Heliconia spp. The degree of reflexion of the free sepal can be measured in at least two ways either by determining the anglebetween landmarks such as the tip of the petals and the tip of the free sepal (white angle) or by situating the vertex at the point where the free sepalmeets the remaining corolla members and measuring the angle between the tangents to the sepal and other members (green angle). D, Heliconia

metallica, lateral view. Bar = 1 cm. E, Heliconia lingulata lateral view. Bar = 1 cm.

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G •F I G . 2. A-E, Inflorescence structure in the Zingiberales showing positional relationships between the organs (after Kunze, 1996). Flowers arerepresented by circles. A, Zingiberaceae, Costaceae, Musaceae, Heliconiaceae, Strelitziaceae (Phenakospermum). B, Strelitziaceae (Ravenala,Stretitzia). C, Cannaceae, Zingiberaceae (Alpinia spp.). D, Marantaceae. E, Lowiaceae. F, Modified diagram of inflorescence structure in theCannaceae. Short internodes are represented by dashed ( ) lines. Organs that are initiated but do not complete development are drawn withbroken ( - • • • - ) lines. G, Diagram of inflorescence structure in the Cannaceae. Internode length and flower development are represented as in F.Shaded circles indicate flower symmetry. Flowers on opposite sides of the main axis are represented as having mirror image symmetry. H, Diagramof inflorescence structure in the Marantaceae. In this family the inflorescence is represented by a pair of flowers connected by very short internodes

(dashed lines). Flower symmetry is represented as in G.

(Fig. 2G and H). Insights gleaned from these diagramssupport Kunze's (1984) interpretation of the flower pair ofthe Marantaceae as equivalent to a reduced florescence ofthe Cannaceae. It only became apparent that observationsof flower symmetry were relevant to this issue when I addedsymmetry to Kunze's (1984) diagrams of inflorescencemorphology.

I see two possibilities for the use of photographs ordiagrams like these in phylogenetic analyses. The first is toinclude as much information as possible in each diagram,and to use the whole diagram as the character state for a

taxon. This approach has the advantage of placing differentaspects of the inflorescence in their context. For instance,the relationship between the position of the flowers and thelength of the subtending internode can easily be seen inthese diagrams (Fig. 2F). However, using only a fewelaborate characters has the disadvantage of reducing thetotal number of characters in the analysis. This could causea loss of resolution in the resulting phylogenetic tree, butmight also increase the reliability of the resolution thatremains because of the broadened context for establishinghomologies (Weston, pers. comm.). Weighting the complex

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characters could help resolve this problem in studies thatuse both complex and traditional characters. Thetraditional characters would provide the resolution, whilethe complex characters would increase the accuracy.

A second possibility is to create a number of diagrams,each focused on a specific aspect of organ structure. Eachset of diagrams would express an aspect of the organ'sstructure but in a larger context than is usually possiblewhen using verbal characters. For instance, one set ofdiagrams could be used to express symmetry relationshipsbetween flowers (Fig. 2G and H), one to express positionalrelationships (Fig. 2A-E), another to represent sizerelationships (foliage leaves vs. bracts, etc.), and anotherto show internode length (Fig. 2F), etc. The advantage ofthis approach is that it allows the creation of a number ofcharacters while at the same time retaining some infor-mation from the whole inflorescence. Each character wouldbe placed in at least part of its context.

The use of visual methods of representing charactersraises two problems that need to be addressed before thesemethods can be used in phylogenetic analyses: standardiz-ation of the images and the taxonomic level at which thecharacters are described. The problem of standardization isone that will have to be dealt with no matter what methodsof character description are used. To a certain extent, theproblems of using verbal characters can be seen as a problemof standardization. There is no standard way in which shapeterms can be applied. The application of the terms 'acute','obtuse' and 'rounded' to leaf form cannot be standardized.It will always involve judgements on the part of theinvestigator. Considered against this background, the useof photographs is easy to standardize. All that is needed is arepeatable method of photographing a particular part of aplant. The procedures are so basic that they hardly need tobe specified. The major plane of the organ should be parallelto the film plane. An accurate scale should be included ineach photograph. The image should, as much as possible, fillthe frame, etc. It is even possible to begin to standardizecolour by specifying the brand (and consequently the colourcharacteristics) of the film to be used and the colourtemperature of the illumination. If the photographs are to bestored in a database it will also be necessary to standardizethe type of scanner to be used and the colour spacedefinition (RGB, CMYK, LAB, etc.) in which the imagesare stored. This is because scanner type and quality influencethe colour rendition of the scans, and colour spacedefinitions do not completely overlap. Converting fromone colour space to another can change colours. Theseproblems are real, and not minor. Still, we can begin todefine reasonable standards for photographic characters.

The problem of the proper taxonomic level at which todescribe characters has already been solved by molecularsystematics. DNA sequences are usually only reported for asingle specimen representing a species level taxon. When astudy involves taxa of higher rank, several species areincluded from each taxon. For instance, when Caddick et al.(2000) carried out their phylogenetic analysis of theDioscoreales they included sequences from 182 species.They did not attempt to combine any of this data and createcharacters that typify genera or families, as is commonly

done for morphological characters. Although Caddick et al.(2000) collected some of the data themselves, much of it wasavailable from the literature, and is stored in electronicdatabases. Contrasting with this surfeit of molecular data,their morphological data set consists of only six charactersfor 31 species. They collected the greater part of these datathemselves. Their morphological analysis would probablyhave included many more characters if databases ofmorphological characters had been available.

Describing morphological characters at the species levelbegins to address one of the problems of using morpho-logical characters in phylogenetic analyses, the problem ofvariability. Any taxon at a rank higher than species willshow more variability than can be conveniently representedin a single term, photograph or diagram. Diagrams ofinflorescence structure prepared at the family level serve asconvenient introductions to inflorescence structure, but theydo not adequately represent the variability that is present.Variability in inflorescence structure of the Marantaceae isso great that no single diagram can do it justice (Sell andCremers, 1994). Rather than try to reduce this variation toa single type, it will be more beneficial to standardizemethods of data collection and report inflorescencestructure on a species by species basis. If the species ispolymorphic, a set of diagrams can be used, much asAndersson (1985) uses several diagrams to describe floralvariation in Heliconia. If this process can be combined withthe creation of an on-line database of visually basedcharacters, we will have solved one of the major problems insharing morphological data between investigators.

ACKNOWLEDGMENTS

I thank the many people who helped me develop and refinethe ideas presented here including Andrea Shapiro, PeterWeston, and my art history professors at the University ofMichigan. I also thank Rolf Sattler, Rolf Rutishauser,Reginae Classen-Bockhoff and Paula Rudall for commentsthat improved the manuscript. Portions of this researchwere supported by a grant from the Future Value Fund ofthe Rudolf Steiner Foundation. I retain all responsibilityfor the views expressed here.

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Insights from Agnes Arber's Concept of the Plant

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