The Fundamental Relevance of Morphology and Morphogenesis to Plant Research

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(1)Annals of Botany 80 : 571–582, 1997. REVIEW ARTICLE. The Fundamental Relevance of Morphology and Morphogenesis to Plant Research R O L F S A T T L ER and R O L F R U T I S H A U S ER Biology Department, McGill Uniersity, Montreal, Quebec, Canada H3A 1B1 and Institut fuX r Systematische Botanik und Botanischer Garten, UniersitaX t ZuX rich, Zollikerstrasse 107, CH-8008 ZuX rich, Switzerland Received : 21 August 1996. Accepted : 3 June 1997. Plant morphology, including morphogenesis, remains relevant to practically all disciplines of plant biology such as molecular genetics, physiology, ecology, evolutionary biology and systematics. This relevance derives from the fact that other disciplines refer to or imply morphological concepts, conceptual frameworks of morphology, and morphological theories. Most commonly, morphology is equated with classical morphology and its conceptual framework. According to this, flowering plants and certain other taxa are reduced to the mutually exclusive categories of root, stem (caulome) and leaf (phyllome). This ignores the fact that plant morphology has undergone fundamental conceptual, theoretical and philosophical innovation in recent times. These changes, when recognized, can fundamentally affect research in various disciplines of plant biology. They may even change the questions that are asked and thus may affect the direction of future research. If, for example, plant diversity and evolution are seen as a dynamic continuum, then compound leaves can be seen as intermediate between simple leaves and whole shoots. Recent results in molecular genetics support this view. Phylogenetically, this could mean that compound leaves are the result of developmental hybridization, i.e. partial homeosis. Many other examples are given to illustrate the relevance and potential impact of basic conceptual and theoretical innovations in plant morphology. # 1997 Annals of Botany Company Key words : Plant morphology, plant morphogenesis, molecular plant genetics, developmental genetics, plant physiology, plant ecology, evolutionary plant biology, plant systematics, cladistics, continuum morphology, process morphology, process philosophy, perspectivism, complementarity, homeosis, homeotic mutants, developmental hybridization, developmental mosaics, homology, metamerism.. I N T R O D U C T I ON What is plant morphology ? Literally, the term ‘ morphology ’ is derived from two Greek roots : morphe, which means form and}or structure, and logos, meaning discourse or investigation. Thus, plant morphology is the investigation of plant form and}or structure. This can be interpreted in either a narrow or a broad sense (e.g. Sattler, 1978). In the narrow sense, morphology refers only to external form (see, for example, Bell, 1991). In the broad sense, it comprises form and structure at all organizational levels, i.e. the form and structure of whole plants, organs, tissues, cells, cell organelles, molecules, etc. Thus, morphology sensu lato includes anatomy and even structural biochemistry. Regardless of whether morphology is defined narrowly or broadly, it deals with the change of form during time, during ontogeny and phylogeny. Since morphogenesis is the development of form, especially during ontogeny, morphology comprises morphogenesis. We included ‘ morphogenesis ’ in the title of this paper for two reasons : firstly to make it clear that we do not understand morphology in an extremely narrow and static sense according to which it would refer only to mature form or structure, and secondly to emphasize that the dynamics of form and structure are of special concern to us. The emphasis in this paper will be on plant morphology sensu stricto, i.e. external form as it changes during time. We 0305-7364}97}11057112 $25.00}0. recognize, of course, that external and internal form are intimately related. The purpose of this paper is two-fold : (1) to show that morphological concepts are used or implied in all major disciplines of plant biology such as genetics, molecular biology, physiology, ecology, systematics, evolutionary biology, etc. Therefore, these disciplines are more or less influenced by morphology ; and (2) to show that the disciplines of plant biology are affected to some extent by progress in plant morphology which comprises new empirical data, new concepts and conceptual frameworks, and new ways of thinking. Whereas the relevance of new empirical data is generally recognized, theoretical and philosophical innovations are often overlooked. Many plant biologists believe that the basic conceptual framework and way of thinking of morphology were well established in the 19th and early 20th century. Therefore, it is usually taken for granted, that if reference to morphology is necessary, it must be in terms of classical morphology, i.e. in terms of the categories root, stem, and leaf for most higher plants. Most textbooks and other publications reinforce this belief because they tend to ignore more recent theoretical and philosophical innovations. Thus, the student is misled to think that fundamental progress in morphology has come to an end. However, there have been major theoretical and philosophical innovations during last 30 years or so, some of the most fundamental of which include : (1) the introduction of. bo970474. # 1997 Annals of Botany Company.

(2) 572. Sattler and Rutishauser—Releance of Morphology. the Lindenmayer languages or algorithms (L-Systems) (e.g. Lindenmayer, 1968, 1978) culminating in Prusinkiewicz’ computer simulations of plant form and development (e.g. Prusinkiewicz and Lindenmayer, 1990) ; (2) the development of fractals, chaos theory and nonlinear dynamics and its application to the study of plant form (e.g. Mandelbrot, 1982 ; Barabe! , 1991 ; Peitgen, Jurgens and Saupe, 1992 ; Kaandorp, 1994 ; Corbit and Garbary, 1995 ; Oldeman and Vester, 1996) ; (3) the elaboration of the metameric and modular concept and the idea of the plant as a metapopulation facilitating the integration of morphology and population biology (White, 1979, 1984) ; (4) the analysis of plant architecture in terms of a continuum of architectural models (Halle! , Oldeman and Tomlinson, 1978 ; Edelin, 1991 ; Oldeman and Vester, 1996) ; (5) the elaboration of perspectivism (general principle of complementarity) according to which contrasting or contradictory models (or conceptual frameworks) are complementary (not antagonistic) to each other (see Rutishauser and Sattler, 1985, 1987, 1989). An example is the complementarity of the classical model (according to which the shoot consists of stem(s) and leaves) and the metameric model that constructs the whole shoot of metamers only (Rutishauser and Sattler, 1985 : Table 1 and below) ; (6) the rediscovery of homeosis which has led to an increased integration of molecular genetics and morphogenesis (see, for example, Meyen, 1973, 1987 ; Cusset, 1982 ; Sattler, 1988, 1994 ; Rutishauser, 1989, 1993 ; Coen, 1991, Meyerowitz, 1995). Homeosis also fundamentally affects the notion of homology (Sattler, 1988, 1994 ; see point (8) below ; (7) the elaboration of a continuum view of plant form (Sattler, 1974) and its confirmation by multivariate analysis (Sattler and Jeune, 1992 ; Cusset, 1994). Philosophically, this view is based on, or compatible with, fuzzy logic and fuzzy thinking (Kosko, 1993) : the world is not only seen in terms of ‘ either-or ’, black or white, this or that, but in terms of ‘ more or less ’ of which the ‘ either-or ’ becomes a special extreme case. Thus, Aristotle’s basic logical law ‘ A or not-A ’ is superseded (see Kosko, 1993 ; for applications in plant development see Rutishauser, 1995) ; (8) the development of the notion of partial homology (Sattler, 1966 ; Meyen, 1973) and the quantification of homology relations of plant form (Jeune and Sattler, 1992 ; Sattler and Jeune, 1992) ; (9) the elaboration of ‘ process morphology ’ according to which form or structure are seen as process (Sattler, 1990, 1992 ; Sattler and Rutishauser, 1990 ; Jeune and Sattler, 1992, 1996 ; Hay and Mabberley, 1994 ; Mabberley and Hay, 1994 ; Mabberley, 1995). This contrasts with the traditional approach in which form or structure are seen as the result of process which implies that there are processes on the one hand and forms or structures on the other. According to process morphology there is only process. Thus, structure is process or, more specifically, a process combination. Similarly, diversity of structures is a diversity of process combinations while morphological evolution is change of process combinations during ontogeny and phylogeny (Sattler, 1990, 1992, 1993). The continuum view of plant form and process morphology has been integrated, thus leading to the notion of a dynamic continuum which has been confirmed by multi-. variate analysis (Jeune and Sattler, 1992). In this dynamic continuum, the process combinations that we call typical roots, shoots, stems, leaves and trichomes are linked by intermediate process combinations that share processes of the typical structures to various degrees. All of the above fundamental innovations (and others that have not been mentioned) can have an impact on research in the various botanical disciplines that rely on morphological concepts. Since it is beyond the scope of a single paper to examine the impact of all major innovations of the past decades, we shall focus on the relevance of the notion of the dynamic continuum, i.e. the continuum view of plant form and process morphology. However, we shall not totally ignore other innovations. Process morphology and other fundamental innovations affect not only the outcome of research in other fields but may even change the questions that are asked and thus influence the direction in which research proceeds (Sattler, 1990, 1993). For example, instead of asking ‘ Is it this or that ? ’, one would ask ‘ How is it related to this or that ? ’ (this or that being other processes or process combinations). Thus, a categorical world view is superseded (or at least complemented) by a dynamic relational view. If, in addition, it is recognized that relations are not only external but also internal, this has further consequences that have been elaborated by various process thinkers (e.g. Cobb, 1988 ; Birch, 1990).. GENETICS, MOLECULAR BIOLOGY AND M O R P H O L O GY Genotype and phenotype Traditionally, genetics deals with the relationship of genotype to phenotype, genes to traits, and their inheritance. The phenotype and its traits are often morphological characters such as stem length, flower site, seed form. Thus, morphology and morphological concepts play an important role in genetic analysis. This has been so from the beginnings of genetics to the present time. Mendel elaborated the fundamental principles of genetics using morphological traits of the garden pea. Although modern workers investigate molecular traits in addition to morphological ones, the latter are still widely used because we want to know how morphological traits are related to the genome and how they are inherited. Gottlieb (1986), in an article on ‘ The genetic basis of plant form ’, discussed the relationship between genetics and morphology. In an earlier publication on ‘ Genetics and morphological evolution in plants ’ (Gottlieb, 1984), he emphasized the evolutionary aspect in genotype-phenotype relations which implies inheritance. It is obvious that in many studies of inheritance morphology continues to play an important role. For example, a more recent article by Shaw and Hansen (1993) dealt with ‘ The inheritance of vegetative growth traits in strawberries (Fragaria¬ananassa) grown at low temperatures and their relationship to field productivity ’. Regardless of the special focus and context of the genetic research, morphological concepts form an integral part..

(3) Sattler and Rutishauser—Releance of Morphology Mutants, molecular genetics and deelopmental genetics The molecular elucidation of mutations has become very important. Nonetheless, we remain equally interested in the mutants that are produced by the mutations. These mutants often affect the morphology of plants. Hence, they are described in morphological terms. Examples are mutants in Pisum, the garden pea, such as tl (claicular or tendril-less), st (stipules reduced ), and af (afila) in which leaflets are replaced by tendrils (e.g. Marx, 1987 ; Gould, Cutter and Young, 1994). Mutants, especially homeotic mutants such as those of Arabidopsis and Antirrhinum, play an important role in molecular and developmental genetics (e.g. Coen, 1991 ; Rutishauser, 1993 ; Bowman, 1994 ; Meyerowitz, 1995). Although the emphasis may be on molecular mechanisms, morphology remains relevant. Eventually, we want to explain the development of organisms of which morphogenesis is a central aspect. Releance of continuum and process morphology In genetic and molecular studies, reference to morphology is usually in terms of classical morphology (the root, stem, leaf framework), as if classical morphology were the only conceptual framework that exists. Gottlieb’s (1986) discussion of the genetic basis of plant form is one of the few exceptions where an alternative to classical morphology is considered from a genetic point of view. Gottlieb refers specifically to the concepts of the metamer and plant metamerism. However, when he describes mutants, this description is in terms of classical morphological concepts such as internode, leaf and parts of the leaf, because traditionally geneticists have related genetic changes to units of classical morphology. It would be interesting to know how mutations may affect whole metamers or other non-conventional units such as phytons and phyllomorphs (e.g. Jong and Burtt, 1975 ; Rosenblum and Basile, 1984 ; Rutishauser and Sattler, 1985). Vienne and Gottlieb (1990) investigated proteins in leaves of wild-type and homeotic mutant morphs of pea. In a comparison of 686 proteins in stipules, leaflets and tendrils, they found only seven quantitative differences between leaflet and tendril, and none between leaflet and stipule. These results are clearly described in terms of classical morphology according to which stipules, leaflets and tendrils are parts of the leaf which as a whole structure is fundamentally different from the stem and the root. In what sense is continuum and process morphology relevant to the perception of these results and their interpretation ? It would not change the results, but would place them into a different perspective which would lead to different questions and consequently to a different direction of subsequent research. From the continuum point of view, the stem is not fundamentally different from the leaf because stems and leaves are linked through intermediate forms such as tendrils (Sattler and Jeune, 1992). Tendrils share some developmental processes such as radial growth with typical stems (e.g. Gould and Cutter, 1986 ; Meicenheimer et al., 1983 ; Cote! et al., 1992 ; Gould et al., 1994). Therefore, it would be. 573. relevant to ask how the proteins of tendrils (such as those of peas) relate to those of stems. Leaves share developmental processes with stems. In Chisocheton and Guarea (Meliaceae) there are compound leaves that show indeterminate growth combined with seasonal growth increments due to an apical meristem at the leaf tip (Fisher and Rutishauser, 1990 ; Miesch and Barnola, 1993). Thus, such ‘ leaves ’ share developmental processes with distichous shoots. Each flush of young leaves at the shoot tip is temporarily correlated with the flush of young pinnae at the end of compound ‘ leaves ’. Even more typical compound leaves share processes with shoots (Sattler and Rutishauser, 1992 ; Lacroix and Sattler, 1994 ; Rutishauser and Sattler, 1997 ; see also Jackson, 1996). Not only traditional morphologists, but also molecular biologists have difficulties defining the term ‘ leaf ’ in dicotyledonous plants (Tsukaya, 1995). If they are to understand the leaf, they must analyse leaf-specific mutations. However, the leaf axis (petiole and rachis) and the shoot axis (stem) may show a certain degree of developmental similarity. For example, some mutants of Arabidopsis exhibit the same defect in the petiole and in the stem (Tsukaya, 1995). According to Tsukaya et al. (1993), the acaulis 2 (acl2) mutant has a defect in the elongation of the inflorescence axis, the flower stalks (pedicels) and the leaf petiole, but leaf blades are of normal size. Therefore, Tsukaya (1995) concluded that the petiole should be considered an axial organ. This view corroborates ideas proposed by Rutishauser and Sattler (1985, 1997) : the axial organs of leaves are petioles and rachides, whereas the axial organs of shoots are stems, including peduncles and pedicels within inflorescences. All these axial organs have some developmental processes in common. New findings in molecular and developmental biology may support the view that certain developmental traits in leaves and stems (shoots), are related to a set of endogenous factors that are identical in both leaves and stems (shoots). As pointed out already, morphology also plays an important role in molecular research on floral homeotic mutants of Arabidopsis and other taxa (e.g. Coen, 1991 ; Coen and Carpenter, 1993 ; Meyerowitz, 1995). In most, if not all, of this work, reference is made only to classical morphology, i.e. the mutants are described in classical terms. Coen and Carpenter (1993) state this explicitly. According to classical morphology, a flower is a modified monaxial, determinate shoot whose appendages such as sepals, petals, stamens and carpels are homologous to leaves (e.g. Goethe, 1790 ; Weberling, 1989 ; Coen and Carpenter, 1993). Besides this common view, there are other interpretations and descriptions of flowers (e.g. Croizat, 1960 ; 1962 ; Melville, 1962 ; 1963 ; Sattler, 1965, 1988 ; Macdonald and Sattler, 1973 ; Meeuse, 1986, 1990 ; Leroy, 1993 ; Lo$ nnig, 1994 ; Burger, 1996). Even Goethe, who is often considered the founder of classical plant morphology, entertained alternative views (see, for example, Rutishauser and Sattler, 1985 : Table 1). For example, he acknowledged ‘ the fertility which is latent in the leaf ’ [Arber’s (1946) English translation of Goethe’s (1790) essay] which implies that the leaf, including floral appendages, has the potential to form branch(es) (Dickinson, 1978). Others, such as Croizat and.

(4) 574. Sattler and Rutishauser—Releance of Morphology. Meeuse, diverged further from the classical flower concept and claimed that not all flowers are homologous with each other because some of them are monaxial whereas others are polyaxial. Thus, comparing a monaxial flower with a polyaxial one would be like a comparison of a classical flower with an inflorescence. According to Motte (1944, 1946), Melville (1962, 1963) and others, the flower of the Brassicaceae (Cruciferae) is a polyaxial system. From the point of view of continuum and process morphology, monaxial and polyaxial systems are no longer mutually exclusive categories ; rather a dynamic continuum can be envisaged between them, i.e. between inflorescence and (monaxial) flower (e.g. Rutishauser and Sattler, 1985 ; Sattler, 1988, 1992, 1994). Some of the homeotic mutants such as the floricaula ( flo) mutants of Antirrhinum investigated by molecular geneticists appear to support this view (Coen and Carpenter, 1993) and naturally occurring taxa also confirm it (e.g. Sattler, 1988, 1992 ; Leroy, 1993). One reason, largely overlooked by molecular geneticists, for why flowers of many taxa are more or less intermediate between a monaxial and polyaxial system is because individual stamens or stamen fascicles (superposed to petals in several taxa) are more or less intermediate between a phyllome and a branchlet (Rutishauser and Sattler, 1985). Sattler and Jeune (1992) demonstrated this relationship quantitatively using principal component analysis based on eight structural and developmental parameters. For example, the stamen of Comandra umbellata is almost halfway between a typical phyllome (leaf) and a typical caulome (stem), i.e. its distance from the typical phyllome is 2±43 and that from the typical caulome is 2±54. The stamen fascicle of Hypericum hookerianum is closer to a typical shoot (2±05) than to a typical phyllome (2±62). The stamens of Arabidopsis have not been analysed in this fashion, but there is good evidence that they are also somewhat intermediate between a typical phyllome and a typical caulome because in early developmental stages they exhibit radial symmetry as in caulomes (e.g. Bowman, 1994). Thus, morphologically, they are not totally homologous to a phyllome such as a sepal. Implicitly or explicitly claiming a total morphological correspondence of all floral appendages is a gross oversimplification. This contrasts strikingly with the precision in the comparison of DNA, RNA, and protein sequences which is usually quantitative (e.g. ‘ the flo and lfy genes encode proteins that are 70 % identical to each other ’ (Coen and Carpenter, 1993). A rectification of this discrepancy of precision between the molecular and morphological levels appears highly desirable if we want to achieve a more adequate understanding of plant development. Concretely, with regard to stamens, this may imply, for example, that we investigate more precisely the relation between gene action and processes they share with branchlets on one hand and phyllomes on the other. In other words to work out the mosaic of genetic and morphogenetic relations in stamens and other structures (such as the gynoecium) seen as combinations of processes at different organizational levels ranging from the biophysical molecular to the organismal– environmental levels (see also Greyson, 1994). Continuum and process morphology are not only relevant in concrete research situations of genetics and molecular. biology, but also relate to the general theoretical framework of these disciplines. This relationship may be illustrated with regard to heredity. Heredity is generally understood to be the result of the passing on of genes, i.e. material particles which contain the hereditary information. This particulate view of heredity has been criticized by process-oriented biologists. A process view of heredity has been suggested as an alternative (Ho, 1988 ; Oyama, 1988, 1989). According to this view, ‘ heredity—a name given to the observed constancy of reproduction—must ultimately be looked upon as a process, and not as some material which is passed on from parent to offspring ’ (Ho, 1988). This means that heredity resides in the complex interrelations of processes that comprise the organism–environment system. Morphogenetic process combinations, the focus of process morphology, are integral aspects of the whole dynamic system. Thus, process morphology is intimately related to the process view of heredity, which in turn has broader ramifications such as the transcendence of gene-centred biology and the nature} nurture dichotomy (e.g. Ho, 1988 ; Oyama, 1989 ; Gray, 1992 ; Hubbard and Wald, 1993 ; Goodwin, 1994 ; Holdrege, 1996). P H Y S I O L O G Y A N D M O R P H O L O GY Physiology deals with the functioning or activity of plants and how it is affected by various factors such as minerals, hormones and genes. Since many of these factors also influence morphological features, morphology is an integral aspect of plant physiology. This is obvious in the titles of many physiological publications such as the following (our italics) : ‘ Influence of magnesium deficiency on rates of leaf expansion, starch and sucrose accumulation, and net assimilation in Phaseolus ulgaris ’ (Fischer and Bremer, 1993) ; ‘ Characterization of calcium}calmodulin—dependent protein kinase homolog from maize roots showing light-regulated gravitropism ’ (Lu, Hidaka and Feldman, 1996). From these and many other titles it is obvious that morphology is relevant to physiology. In many other physiological publications regardless of whether morphological terms are used in the title or not, reference to morphology is made directly or indirectly. Indirect reference may be due to the methodology used. For example, if physiological factors such as hormones are extracted from leaves or stems, reference to morphological concepts is implied. Releance of non-classical morphological concepts and conceptual frameworks Most of the physiological research that relates to morphology is based on concepts of classical plant morphology such as root, stem, leaf, branch, inflorescence and flower. This tradition is kept alive by research publications as well as textbooks which usually refer only to classical morphology. One of the few exceptions to this trend is Taiz and Zeiger’s (1991) Plant physiology. They included a short paragraph on the phytomer with a drawing of this morphological unit. However, it is incorrectly delimited because while the phytomer’s axillary bud (in the axil of its leaf).

(5) Sattler and Rutishauser—Releance of Morphology is excluded, the axillary bud of the preceding phytomer is included. In any case, the reference to the non-classical phytomer concept remains rather inconsequential, since this concept is not applied in the remainder of the book that deals exclusively with classical concepts as do almost all other books and publications in the field. Although classical morphology is useful and adequate to a great extent, it is limited (e.g. Rutishauser and Sattler, 1985). Therefore, it is important to complement it with nonclassical concepts and approaches which present other perspectives that may be of interest to plant physiologists and are obscured by classical morphology. One non-classical concept is the above mentioned ‘ phytomer ’, more commonly referred to as ‘ metamer ’ (White, 1984). A metamer comprises an internode, node, leaf (or leaves, if there is more than one leaf at the node), the axillary bud(s) of this leaf or leaves, and roots, if present. One could see a metamer as an assemblage of organs of classical morphology, but one can see it also as an alternative conceptual dismemberment of the whole plant in which the stem-leaf boundary is not drawn (Rutishauser and Sattler, 1985 : Table 1). In this latter perspective, the metamer represents the stem-nodeleaf continuum (Howard, 1974). It may be significant for physiological research to take into consideration this stemnode-leaf continuum, i.e. plant metamerism. Watson and Casper (1984) reviewed research that supports this continuum from a physiological point of view. They noted that metamers may exhibit a degree of autonomy with regard to physiological properties such as assimilate or carbon distribution. ‘ Thus, metameric units in Phaseolus—consisting of a section of stem, a trifoliate leaf, and the associated, laterally borne reproductive branch—function as internally integrated and relatively autonomous physiological units ’ (Watson and Casper, 1984). These units correspond to Adam’s (1967) ‘ nutritional units ’ (Watson and Casper, 1984). In other plants, other ‘ semiautonomous, integrated physiological units ’ (IPUs) may occur. ‘ Clearly, not all plants are equally subdivided ; rather, they exist somewhere on a continuum from total integration to highly localized sectorialization ’ (Watson and Casper, 1984). Even if it should turn out that the metameric integration is not widespread, it is still useful because it complements the classical view. Furthermore, it is of heuristic value because it allows us to ask different questions and pursue research in a different direction. The continuum view of plant form has been adopted to an even smaller extent in physiology than metamerism. Nonetheless, it is relevant. For example, as pointed out by Steeves and Sussex (1989), the formation of leaf-like appendages (in Begonia hispida var. cucullifera) without the intervention of a shoot apex directly on young leaves (Sattler and Maier, 1977) is relevant to the question of leaf determination. Since the leaf-like appendages in this Begonia variety form a continuum with typical trichomes, the question arises of how this continuum is based physiologically, i.e. which physiological factors contribute to its manifestation. Obviously, such a question cannot be asked unless the continuum between leaves and trichomes is recognized in the first place. If this continuum is seen dynamically, i.e. as a continuum of morphogenetic process combinations, then. 575. the further question arises as to how these process combinations relate to physiological processes. Process morphology is not only relevant to physiology in many concrete research situations, but also affects the general relation of physiology and morphology. Traditionally, physiology is said to deal with function, and morphology with structure. As pointed out, for example, by Woodger (1967), this distinction has led to debates on whether function determines structure or ice ersa. Woodger’s process view based on Whitehead’s process philosophy (e.g. Birch and Cobb, 1981) and process morphology help to overcome such debates and the structure-function (structure-process) antithesis from which they arise. If structure is seen as process, then it is no longer opposed to function which is also process. From this point of view an organism is seen as process or activity. Morphology deals with the morphogenetic aspects of this activity, whereas physiology also investigates the activities that accompany morphogenesis such as metabolism. Since these latter activities are integrated with the morphogenetic processes, it is inappropriate to ask whether the morphogenetic processes determine the physiological ones or ice ersa. Both are aspects of an integrated dynamic system. Process morphology has further philosophical ramification with regard to physiology. Traditionally, physiological processes are said to occur within structures such as roots, stems, leaves or the whole plant. If, however, structures are seen as process, or, more specifically, as process combinations, then there is only process or activity. There is no longer an agent (or substance) to whom the processes could be ascribed. Thus, we find ourselves in a similar situation as in physics. As Capra (1983) put it : ‘ There is motion but there are, ultimately, no moving objects ; there is activity but there are no actors ; there are no dancers, there is only the dance ’. E C O L O G Y A N D M O R P H O L O GY Ecology deals with the relations between organisms and their environment. In autecology or physiological ecology the focus is on the interaction of individual organisms and environmental factors such as light, temperature, gravity, etc. (e.g. Billings, 1966 : Fig. 2-1). In population ecology the emphasis is on how populations relate to the environment. Population genetics is part of, or closely related to, this branch of ecology. Community ecology deals with whole communities including the environment. In all of these fields of ecology, morphology plays a role. This is most obvious in autecology or physiological ecology. Many publications report on the interaction of plant structures with the environment or analyse the ways in which environmental factors affect particular structures. The following are examples (our italics) : ‘ Leaf anatomical responses to light in five tropical Moraceae of different successional status ’ (Strauss-Debenedetti and Berlyn, 1994) ; ‘ Carbon fixation profiles do reflect light absorption profiles in leaes ’ (Evans, 1995). Ecomorphology, which deals with growth forms in relation to the environment, continues to contribute to our understanding of plants within environments (e.g. White, 1984 ; Hagemann, 1989 ; Marquis, 1996). During the past.

(6) 576. Sattler and Rutishauser—Releance of Morphology. decades, studies of plant architecture have become increasingly integrated with ecology (e.g. Halle! et al., 1978 ; Watson and Casper, 1984 ; Sachs and Novoplansky, 1995). In population and community ecology the influence of morphology is less obvious. Also, much work has been carried out without reference to morphology. As White (1984) pointed out, ‘ in the huge literature of plant competition, the response [to the environment] is almost invariably expressed in terms of biomass ’. Here morphology is ignored and thus much detailed information is lost. There are, however, also competition studies that consider morphological parameters. For example, Darwinkel (1978) studied tiller demography and the effect of tiller density on grain production of winter wheat. Hunter and Aarson (1988), and others have referred to morphology in the context of studies on cooperation.. Releance of non-classical morphology White (1979, 1984) pointed out the significance of plant metamerism for ecology and other fields. According to this view, metameric plants such as seed plants are built up of segments, called metamers (see Introduction and Physiology section). Taking these metamers as the basic units of construction (instead of stems and leaves), may provide a different perspective. For example, Callaghan (1980) analysed productivity and nutrient allocation in Lycopodium annotinum from this point of view. Whereas most botanists consider a plant such as a tree to be an individual, some authors look upon each module (a shoot derived from one apical meristem) as an individual, and for others each metamer represents an individual. If metamers or modules are identified as individuals, then a tree is a population of individual plants. Disregarding somatic mutation, such a population consists of genetically identical individuals. White (1979, 1984) referred to such populations as ‘ metapopulations ’ in contrast to populations of genetically diverse individuals. The important conclusion is that once a plant is seen as a sort of population, then principles of population biology apply within a single plant such as a tree. What was previously an individual unit in a population, is now a population itself. Sachs, Novoplansky and Cohen (1993) concluded that ‘ most plants could be special and interesting intermediates between unitary organisms and clonal populations ’. Continuum morphology also plays a role in ecology. We may distinguish the following two aspects of continuum morphology : (1) the continuum of non-contiguous parts ; and (2) the continuum of contiguous parts. The continuum of non-contiguous parts has been confirmed using multivariate analysis (Jeune and Sattler, 1992 ; Sattler and Jeune, 1992). In this case, parts of different plants such as roots, stems, leaves and intermediate structures form the continuum or, more precisely, the heterogeneous continuum in which some areas, namely those of the typical structures, are denser than those of intermediate structures (Sattler, 1996). How does this finding relate to ecology ? What difference can it make to ecological analysis ? An example of the continuum of contiguous parts is the. stem-node-leaf continuum (Howard, 1974). In such a continuum the ‘ parts ’ are no longer separate realities. In a still more comprehensive view, the whole plant, comprising the root and shoot system, can be seen as a continuum. And finally, this organismal continuum can be understood as part of a continuous organism-environment system. In this sense, Etherington (1975) referred to the soil-plant-aircontinuum (SPAC). From this perspective the plant is no longer distinct from the environment. ‘ Plant ’ and ‘ environment ’ no longer exist as separate realities. Research conducted from this broader perspective has to integrate morphology and ecology. How this may be done can be illustrated through process morphology and process thought. As pointed out already, according to process morphology, structures are seen as process combinations. In two case studies (Sattler and Rutishauser, 1990 ; Jeune and Sattler, 1992), only morphogenetic processes have been considered. For example, a typical simple leaf was seen as a combination of axillant positioning and dorsiventral and determinate growth. It was pointed out, however, that processes at other organizational levels have to be included for a more comprehensive analysis (Sattler, 1992). Obviously, the morphogenetic processes that characterize the leaf are combined with processes at the molecular level and processes of the environment such as solar radiation, cosmic radiation, gravitation, etc. (e.g. Billings, 1966). In other words, the dynamics of a leaf are a combination of processes ranging from local to cosmic dimensions. In this sense, the leaf is integrated into a rather universal web of processes. It does not exist just locally as visual perception would have us believe. However, in conducting research we will have to confine ourselves to a limited number of processes, which is one reason why science cannot attain the absolute truth. Nonetheless, it is important to keep in mind the cosmic connection.. EVOLUTIONARY BIOLOGY AND M O R P H O L O GY Charles Darwin considered morphology the ‘ very soul ’ of natural history which to him was the basis of evolutionary biology. Today, molecular evolution has become a prominent field. Nonetheless, we also want to know how whole organisms and populations of organisms evolved. Since morphology is an important aspect of whole organisms, it continues to play a major role in evolutionary biology. This is reflected in many publications in this field (e.g. Zimmermann, 1959 ; Croizat, 1960, 1962 ; Meyen, 1973, 1987 ; Stevens, 1984 ; Stidd, 1987 ; Cronquist, 1988 ; Hay and Mabberley, 1991, 1994 ; Takhtajan, 1991 ; Mabberley and Hay, 1994). The reader who scans bibliographic sources will notice frequent reference to morphological concepts. A few recent examples of research publications are the following (our italics) : ‘ Floral structures and evolution of primitive angiosperms : recent advances ’ (Endress, 1994) ; ‘ Flowers in Magnoliidae and the origin of flowers in other subclasses of the angiosperms. I. The relationships between flowers of Magnoliidae and Alismatidae ’ (Erbar and Leins, 1994)..

(7) Sattler and Rutishauser—Releance of Morphology Releance of continuum and process morphology Evolutionary changes, especially in natural populations, are often characterized in terms of changes in gene frequencies. Although this ‘ bean bag ’ view offers some insight, it is limited. Evolution is a change in successive ontogenies and therefore the study of ontogeny including its morphogenesis is crucial for an understanding of evolution. In classical morphology, structural changes in successive ontogenies are usually seen in terms of modifications within structural categories. Thus, for example, the evolutionary modification of roots, stems and leaves have been investigated. Although this somewhat categorical view has provided insights into evolutionary change, it is limited because during evolution developmental pathways (or portions of them) of different structural categories may be combined (e.g. Lodkina, 1983 ; Poethig, 1988). Sattler (1988) referred to such combination as ‘ developmental hybridization ’ because it leads to hybrid structures that combine features of different structural categories (e.g. Sachs, 1982 ; Meyen, 1984, 1987 ; Brugger and Rutishauser, 1989). Instead of ‘ hybrid structures ’, some plant biologists have referred to developmental mosaics (e.g. Rutishauser, 1995) which may be only another term for the same concept. In this context, what is most important to us is the concept not the term designating the concept. The concept under consideration is non-classical in the sense that hybrid or mosaic structures cannot be assigned to structural categories. For example, phylloclades in the Asparagaceae are neither leaf, nor stem, nor shoot homologues, but more or less intermediate between these categories because they combine features of leaf, stem and shoot to varying degrees depending on the particular case (see Cooney-Sovetts and Sattler, 1987 ; for other examples of hybrid or mosaic structures see Sattler and Rutishauser, 1990 ; Rutishauser and Huber, 1991). Mosaic or hybrid structures thus form part of the heterogeneous continuum between typical representatives of structural categories (Sattler and Jeune, 1992). Instead of perceiving this continuum in static or static}dynamic terms (Sattler and Jeune, 1992), it can be seen dynamically in terms of process morphology (Jeune and Sattler, 1992). From this point of view, mosaic or hybrid structures are process combinations which arose by the combination of processes that are normally part of the process combinations characteristic of structural categories such as leaf and stem. Developmental hybridization is often ignored because it is thought that it occurs only in very few exceptional taxa and some artificially induced mutants. Recently, it has been shown, however, that even common structures such as pinnate leaves may have been formed through this evolutionary process (Fisher and Rutishauser, 1990 ; Sattler and Rutishauser, 1992 ; Lacroix and Sattler, 1994 ; Rutishauser, 1995 ; Rutishauser and Sattler, 1997). Pinnate leaves such as those of Murraya paniculata and other taxa exhibit morphogenetic processes typical of shoots as well as processes characteristic of leaves. If we follow the common assumption that pinnate taxa in angiosperms evolved from primitive angiosperms with simple leaves, then we can conclude that during the evolution of the pinnate taxa some morphogenetic processes of shoot process combinations. 577. have become expressed in the sites of process combinations characteristic of typical leaves. As noted, this conclusion depends on the assumption that the taxa with pinnate leaves evolved from ancestors with simple leaves. Even if this assumption should turn out to be incorrect or improbable, the fact remains that the pinnate leaves investigated thus far combine morphogenetic processes of both simple leaves and shoots. Therefore the pinnate leaves remain part of the dynamic continuum between shoots and simple leaves (Rutishauser and Sattler, 1997). The extent to which developmental hybridization has played a role in evolution must be assessed by future studies. It seems more prevalent than generally acknowledged. One reason why this evolutionary process has often been overlooked or ignored is due to the fragmentation underlying studies in so-called character phylogeny (Hay and Mabberley, 1991 ; Mabberley and Hay, 1994). In these studies, whole organisms are first conceptually dismembered into parts representing structural categories such as root, stem and leaf. Then parts belonging to the same category are compared and it is asked which is derived from which. For example, is the stamen derived from a phyllome (leaf homologue) ? Such questions preclude that anything else but leaf homologues could have been involved. They simplify matters and misrepresent the basics of evolution. It is well known that parts of organisms such as leaves do not evolve from each other. Whole organisms and populations of organisms evolve into each other. In each generation the whole organism is reconstructed with or without major modifications (e.g. Oyama 1988, 1989 ; Sattler, 1994 : Fig. 7). Thus leaves or other structures are rebuilt. During this reconstruction, processes may be combined in different ways and thus novelties may arise (e.g. Sattler, 1988). For example, at the sites where leaves arose in the ancestor, processes of both leaves and shoots may be combined. Recent work in developmental genetics has yielded many mutants that illustrate this phenomenon at different levels (e.g. Broadhvest et al., 1992 ; Sawhney, 1992). There is also evidence that combination of processes from combinations characteristic of different structural categories has occurred during phylogeny (e.g. Sattler, 1988, 1994). To recognize this evidence we have to go beyond the fragmentation of socalled character phylogeny and the categorical framework of classical morphology. Thus, continuum and process morphology become fundamentally relevant to the way we perceive structural diversity which forms the basis for hypotheses on evolutionary change. Process morphology is not only relevant to descriptive and reconstructive work on morphological evolution involving evolutionary processes. It is also relevant to other aspects of evolution such as the explanation of evolution in terms of evolutionary theory. A general discussion of evolutionary theory is beyond the scope of this paper. However, we want to draw attention to one rather significant aspect involving the formulation of questions that guide research. Traditionally, questions have been asked concerning the evolution of structures. From the point of view of process morphology one would rather ask : how have process combinations changed during evolution ? why have process combinations changed ? how and why has the.

(8) 578. Sattler and Rutishauser—Releance of Morphology. dynamic continuum changed ? Finally, since the dynamics of organisms and the environment are integrated, one would ask : how and why has the organism-environment system changed ? Since dynamics are more adequately represented by verbs than by nouns, a linguistic challenge is to develop a verb-based language that avoids nouns (Sattler, 1993). S Y S T E M A T I C S A N D M O R P H O L O GY Systematics has been defined as ‘ the scientific study of the kinds and diversity of organisms and of any and all relationships among them ’ (Simpson, 1961). ‘ Relationship ’ may be expressed in terms of similarity (as in numerical taxonomy) or presumed phylogenetic connection (as in phylogenetic taxonomy or cladistics). Regardless of how ‘ relationship ’ is seen, the basis for its evaluation is usually a list of characters and character states. Traditionally, these characters have been mainly morphological, for example, various morphological aspects of flowers, leaves and other structures. Today, molecular characters are often used. Nonetheless, morphological characters continue to play a fundamental role. When molecular data are used, they supplement morphological characters, or the systems derived from molecular data are tested against those based on morphological characters. It is interesting to know to what extent molecular systems are correlated with morphological ones. Since whole organisms and their structures contain more information than an assemblage of molecules including genes, relationships based on morphological characters supersede those founded on molecular data. This is due to emergence, a phenomenon that can be observed even at the inorganic level. For example, water has emergent properties that are absent in hydrogen and oxygen, its component parts. Similarly, any plant structure has emergent properties that cannot be observed in the constituent molecules. Therefore, a classification system based on morphological characters reflects information that is lacking in a system founded on molecular data (e.g. Sattler, 1986). In any case, a cursory look at modern classification systems, both phenetic and phylogenetic, will easily convince us that morphology plays a major role in systematics (e.g. Cronquist, 1988 ; Takhtajan, 1991 ; Woodland, 1996). Although molecular data will be increasingly used, morphology cannot be abandoned if we want to retain maximum information. Releance of continuum and process morphology Although many quantitative characters are used in modern systematics, two-state characters are also commonly employed. An example of a two-state character is : flowers large (state 1) or small (state 2). Stevens (1991, 1996) has drawn attention to possible pitfalls of two-state and even multi-state characters. Whenever we deal with a continuum, dividing it into states is arbitrary and may lead to distortions as in the grading schemes employed at most universities : if the grade B ranges from 65–79, and A from 80–100, a student with a mark of 79 will receive a B, whereas students with grades of 80 to 100 will obtain an A. Thus two students. differing by 1 point will be separated into two different categories A and B, whereas students differing by 20 points will be in the same category. In plant systematics, distortion due to the use of two-state or multi-state characters is also a problem. Unfortunately, there are additional distortions that are revealed by continuum morphology. These distortions are due to the categorical framework of classical plant morphology. According to that framework, every structure has to be classified (homologized) into mutually exclusive structural categories. Since there is a continuum between these categories (Jeune and Sattler, 1992 ; Sattler and Jeune, 1992), structures intermediate between typical representatives of the categories must be forced into one or the other category. Thus, natural relationships are distorted. It should be noted that this problem is the same in both phenetic and phylogenetic systematics. The challenge for both is to surmount this problem through continuum morphology. There may be cases in which distortions influence the classification system to only a minor extent or not at all. This would have to be shown, however, through detailed analyses. In other instances, distortions may compound to such an extent that classification systems are greatly affected by them (Rutishauser, 1997). In extreme cases this is intuitively obvious without rigorous analyses. For example, in the genus Utricularia (bladderworts) there are species such as U. foliosa whose morphology deviates so much from the typical root-stem-leaf organization that there is ‘ no clear distinction between roots, stems and leaves ’ (Mabberley, 1987). If, in a systematic comparison with taxa of a more conventional morphology, we draw up a list of root, stem and leaf characters, distortion seems unavoidable. Arbitrary decisions would have to be made concerning the classification (homologization) of parts. For example, what according to one author is a leaf, to another one is a stolon (branch), or even part of a root-system (see Brugger and Rutishauser, 1989 ; Sattler and Rutishauser, 1990). Different phenetic or phylogenetic relationships would result depending on which homologization is accepted. There is a further problem that affects not only the comparison of Utricularia species, but also taxa of more conventional morphology. Plants can be conceptually dismembered in different ways (Rutishauser and Sattler, 1985 : Table 1). Besides the classical root-stem-leaf conceptualization, there are several others, one of them the metameric model according to which the plant shoot consists of metamers only. If the metameric or other models are used to score characters, a rather different list of characters may result. How would this affect classification ? We may indeed obtain different classifications even for rather conventional taxa if we used different conceptual dismemberments of the plants. This would have to be demonstrated, however, by rigorous analyses. In extreme cases such as Utricularia it is probable that different homologizations would have an impact on classification. Thus we would have to accept a number of different classifications or arbitrarily select one of them. Process morphology, however, presents another alternative. Instead of arbitrarily assigning plant parts to mutually exclusive categories such as stem or leaf, structures, or even whole plants, could be seen as process combinations and these process combinations could form the basis for the.

(9) Sattler and Rutishauser—Releance of Morphology construction of phenetic or phylogenetic classification systems (see also Hay and Mabberley, 1994). Continuum and process morphology have still broader implications for systematics. Usually the aim of systematics is to construct classification systems in which groups of organisms (taxa) are related hierarchically. In continuum and process morphology neither mutually exclusive groupings nor hierarchical structures are present. The notion of a continuum defies both. Relationships, however, can be established between any points in the continuum. Compatible with this view, phenetic continua of organisms or taxa could be constructed so that the emphasis would be again on relationship instead of a hierarchy of mutually exclusive groups. Since any organism or group of organisms can be related to many others, this approach leads to a relational or, more precisely, a multi-relational view which contrasts with the categorical and hierarchical view of nature. As many relationships are established, a network of relations results. Hence, the underlying structure is a net rather than a hierarchy. It seems that in many ways nets or networks are more adequate representations of nature than hierarchies, although the latter may have a limited usefulness (Sattler, 1986). The phylogenetic systematist may argue, of course, that phylogenetic systems are trees that correspond to hierarchies. We know, however, that at the species (or even genus) level reticulate evolution may occur (e.g. Rieseberg, 1991). At higher ranks, reticulation due to horizontal gene transfer has been documented (e.g. Syvanen, 1994). Therefore, even in phylogenetic systems nets may be a more adequate representation of natural relationships and hierarchies may be seen as somewhat simplified reconstructions. Future research will have to clarify to what extent nets and hierarchies are appropriate. In the meantime, it may be wise to keep both schemes in mind. They may indeed complement each other (on the notion of complementarity see Rutishauser and Sattler, 1985, 1987, 1989 ; Sattler, 1986). C O N C L U S I O NS It is often assumed that progress in plant morphology has (almost) come to an end and that, as a consequence, morphology is only of peripheral importance or none at all. This attitude manifests itself, for example, in the fact that at many universities, plant morphology is no longer taught or has been relegated to a peripheral position. As a result, most students of plant biology—and professors as well—are generally ignorant about morphology. From their point of view, such ignorance is harmless, since morphology is thought irrelevant to modern research in physiology, molecular biology, ecology and other fields. However, in this paper we show that plant morphology remains fundamentally relevant to almost all fields of plant biology. To pose questions, reference to morphological concepts is often necessary. For example, in molecular genetics, work on the basic significance of homeotic mutants entails reference to morphological concepts such as ‘ flower ’, ‘ petal ’, ‘ stamen ’, etc. Furthermore, knowledge of plant development is important. However, we do not want to claim that reference to morphology is always required. For. 579. example, an ecologist may restrict him}herself to biomass analysis which ignores morphology. But such analysis, although useful, provides only a very limited understanding. Reference to morphological concepts such as leaves, metamers or morphogenetic processes will greatly enlarge the scope and depth of ecological understanding (e.g. White, 1979, 1984). In recent decades, morphology has undergone fundamental conceptual, theoretical and philosophical changes (see Introduction). These changes lead to different questions and may redirect the course of research in morphology and other fields to which it is relevant. Therefore, a knowledge of recent fundamental innovations in morphology is important for progress in other fields. In this paper we have drawn special attention to the idea of a dynamic continuum and we have underlined the manifold implications and consequences of this idea for plant research. We do not claim that traditional morphology has been devoid of dynamic and continuum thinking. It has, however, been constrained by the assumption of a more or less rigid categorical framework. Continuum and process morphology transcend this constraint to a great extent and therefore allow questions and answers beyond the scope of traditional classical morphology whose concepts and theories are often taken for granted by researchers in non-morphological disciplines such as molecular genetics, ecology and evolutionary biology. Morphology is also relevant to ways of thinking in plant biology and biology in general. Since ways of thinking in biology affect the human condition and society, morphology contributes directly or indirectly to human and planetary welfare (e.g. Rutishauser and Sattler, 1985 ; Sattler, 1986, 1993 ; Kirchoff, 1995). A C K N O W L E D G E M E N TS This research was supported by grant A2594 from the Natural Sciences and Engineering Research Council of Canada (NSERC) to the first author. We thank Alexander Bernhard and Rajinder Dhindsa for critical comments on the first draft of this manuscript and Joanne Smith for typing all versions. 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