Annals of Botany 80: 571–582, 1997 REVIEW ARTICLE The Fundamental Relevance of Morphology and Morphogenesis to Plant Research ROLF SATTLER and ROLF RUTISHAUSER Biology Department, McGill Uniersity, Montreal, Quebec, Canada H3A 1B1 and Institut fuX r Systematische Botanik und Botanischer Garten, UniersitaX tZuXrich, 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. recognize, of course, that external and internal form are INTRODUCTION intimately related. The purpose of this paper is two-fold: (1) What is plant morphology? Literally, the term ‘mor- to show that morphological concepts are used or implied in phology’ is derived from two Greek roots: morphe, which all major disciplines of plant biology such as genetics, means form and}or structure, and logos, meaning discourse molecular biology, physiology, ecology, systematics, evol- or investigation. Thus, plant morphology is the investigation utionary biology, etc. Therefore, these disciplines are more of plant form and}or structure. This can be interpreted in or less influenced by morphology; and (2) to show that the either a narrow or a broad sense (e.g. Sattler, 1978). In the disciplines of plant biology are affected to some extent by narrow sense, morphology refers only to external form (see, progress in plant morphology which comprises new em- for example, Bell, 1991). In the broad sense, it comprises pirical data, new concepts and conceptual frameworks, and form and structure at all organizational levels, i.e. the form new ways of thinking. Whereas the relevance of new em- and structure of whole plants, organs, tissues, cells, cell pirical data is generally recognized, theoretical and philo- organelles, molecules, etc. Thus, morphology sensu lato sophical innovations are often overlooked. Many plant includes anatomy and even structural biochemistry. biologists believe that the basic conceptual framework and Regardless of whether morphology is defined narrowly or way of thinking of morphology were well established in the broadly, it deals with the change of form during time, 19th and early 20th century. Therefore, it is usually taken during ontogeny and phylogeny. Since morphogenesis is the for granted, that if reference to morphology is necessary, it development of form, especially during ontogeny, mor- must be in terms of classical morphology, i.e. in terms of the phology comprises morphogenesis. We included ‘morpho- categories root, stem, and leaf for most higher plants. Most genesis’ in the title of this paper for two reasons: firstly to textbooks and other publications reinforce this belief make it clear that we do not understand morphology in an because they tend to ignore more recent theoretical and extremely narrow and static sense according to which it philosophical innovations. Thus, the student is misled to would refer only to mature form or structure, and secondly think that fundamental progress in morphology has come to to emphasize that the dynamics of form and structure are of an end. special concern to us. However, there have been major theoretical and philo- The emphasis in this paper will be on plant morphology sophical innovations during last 30 years or so, some of the sensu stricto, i.e. external form as it changes during time. We most fundamental of which include: (1) the introduction of 0305-7364}97}110571­12 $25.00}0 bo970474 # 1997 Annals of Botany Company 572 Sattler and Rutishauser—Releance of Morphology the Lindenmayer languages or algorithms (L-Systems) (e.g. variate analysis (Jeune and Sattler, 1992). In this dynamic Lindenmayer, 1968, 1978) culminating in Prusinkiewicz’ continuum, the process combinations that we call typical computer simulations of plant form and development (e.g. roots, shoots, stems, leaves and trichomes are linked by Prusinkiewicz and Lindenmayer, 1990); (2) the development intermediate process combinations that share processes of of fractals, chaos theory and nonlinear dynamics and its the typical structures to various degrees. application to the study of plant form (e.g. Mandelbrot, All of the above fundamental innovations (and others 1982; Barabe! , 1991; Peitgen, Jurgens and Saupe, 1992; that have not been mentioned) can have an impact on Kaandorp, 1994; Corbit and Garbary, 1995; Oldeman and research in the various botanical disciplines that rely on Vester, 1996); (3) the elaboration of the metameric and morphological concepts. Since it is beyond the scope of a modular concept and the idea of the plant as a metapopu- single paper to examine the impact of all major innovations lation facilitating the integration of morphology and of the past decades, we shall focus on the relevance of the population biology (White, 1979, 1984); (4) the analysis of notion of the dynamic continuum, i.e. the continuum view plant architecture in terms of a continuum of architectural of plant form and process morphology. However, we shall models (Halle! , Oldeman and Tomlinson, 1978; Edelin, not totally ignore other innovations. 1991; Oldeman and Vester, 1996); (5) the elaboration of Process morphology and other fundamental innovations perspectivism (general principle of complementarity) ac- affect not only the outcome of research in other fields but cording to which contrasting or contradictory models (or may even change the questions that are asked and thus conceptual frameworks) are complementary (not antag- influence the direction in which research proceeds (Sattler, onistic) to each other (see Rutishauser and Sattler, 1985, 1990, 1993). For example, instead of asking ‘Is it this or 1987, 1989). An example is the complementarity of the that?’, one would ask ‘How is it related to this or that?’ classical model (according to which the shoot consists of (this or that being other processes or process combinations). stem(s) and leaves) and the metameric model that constructs Thus, a categorical world view is superseded (or at least the whole shoot of metamers only (Rutishauser and Sattler, complemented) by a dynamic relational view. If, in addition, 1985: Table 1 and below); (6) the rediscovery of homeosis it is recognized that relations are not only external but also which has led to an increased integration of molecular internal, this has further consequences that have been genetics and morphogenesis (see, for example, Meyen, 1973, elaborated by various process thinkers (e.g. Cobb, 1988; 1987; Cusset, 1982; Sattler, 1988, 1994; Rutishauser, 1989, Birch, 1990). 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 GENETICS, MOLECULAR BIOLOGY AND confirmation by multivariate analysis (Sattler and Jeune, MORPHOLOGY 1992; Cusset, 1994). Philosophically, this view is based on, Genotype and phenotype or compatible with, fuzzy logic and fuzzy thinking (Kosko, 1993): the world is not only seen in terms of ‘either-or’, Traditionally, genetics deals with the relationship of black or white, this or that, but in terms of ‘more or less’ of genotype to phenotype, genes to traits, and their inheritance. which the ‘either-or’ becomes a special extreme case. Thus, The phenotype and its traits are often morphological Aristotle’s basic logical law ‘A or not-A’ is superseded (see characters such as stem length, flower site, seed form. Thus, Kosko, 1993; for applications in plant development see morphology and morphological concepts play an important Rutishauser, 1995); (8) the development of the notion of role in genetic analysis. This has been so from the beginnings partial homology (Sattler, 1966; Meyen, 1973) and