Plant Evo-Devo and Morphological Research by Rolf Sattler and Collaborators ------------------

The Relevance of the Phytomorphological Research of Rolf Sattler and Collaborators to Plant Evo-Devo Research.

Rolf Sattler

In plant evo-devo (evolutionary-developmental biology) the evolution of plants is seen as a succession of more or less modified ontogenies (Minelli 2018, Rutishauser 2020). Thus, the study of plant development serves as a basis for the elucidation of plant evolution. Since our research has been developmental, it added empirical data to evo-devo research. Furthermore, it provided an analysis and re-evaluation of morphological core concepts of developmental plant biology that are relevant to the reconstruction of plant evolution.

Although in evo-devo genetic analysis of plant development is much emphasized, morphological concepts such as root, stem, and leaf are still used, and therefore our morphological research that questions the general validity of these concepts remains relevant to evo-devo in rather fundamental ways that I shall outline below.

This article consists of two parts:

1. A brief discussion of plant evo-devo in relation to plant morphology.

2. The relevance of our morphological research for plant evo-devo research.
More specifically,
I show to what extent the results of our morphological research appear compatible with evo-devo, and how in some ways - scientifically and philosophically - they offer broader and deeper avenues for evo-devo research.

Part 1

Plant Evo-Devo (Evolutionary Developmental Biology)

Nowadays, at most universities plant morphology is no longer taught as a separate major discipline. To a great extent it has been integrated into what is called evo-devo (Ambrose and Purugganan 2013). MorphoEvoDevo (Wanninger 2015) emphasizes the morphological aspect of evo-devo.

According to plant evo-devo, the evolution of plants is seen as the evolution of plant development. Long ago, in his
Phylogenie der Pflanzen (1959), “a standard work that has no equal in the international literature” (Karl Mägdefrau), and as far as I know has not yet been translated into English, Walter Zimmermann referred to this process as hologeny, the evolutionary succession of more or less modified ontogenies. More recent treatises of plant evo-devo include those of Minelli (2018) and Rutishauser (2020).

Plant evo-devo "involves a synthesis of molecular developmental genetics, evolutionary biology and plant morphology" (
Cronk 2008). The emphasis is on the elucidation of the role of genes in the organism’s development and evolution. Researchers want to know which genes are turned on or off during the development of body parts. For example, which genes are turned on or off during leaf development? According to classical plant morphology, one might postulate that there are at least some unique genes involved in the development of roots, stems, and leaves, or, in other words, that the mutual exclusivity of the three major organ types is reflected in the mutual exclusivity of gene activity. However, so far molecular genetics has not confirmed this postulate. On the contrary, molecular genetics has provided evidence in support of continuum morphology and partial homologies in addition to total homologies. Rutishauser and Isler (2001:1190-1193) reviewed molecular genetic evidence in support of the continuum between leaf and shoot categories. Molecular genetic investigations by Hirayama et al (2007) showed that the phylloclade of Ruscus aculeata "is not homologous to either the shoot or the leaf, but that it has a double organ identity," which means that it combines shoot and leaf processes (as has been documented on purely morphological grounds by Cooney-Sovetts and Sattler (1987). James (2009, p. 17) stated that "it is now widely accepted that... radiality [characteristic of most stems] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!" Also on the basis of molecular genetic research, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties” (see also Uchida et al. (2010) and Tsukaya (2014)).

Thus, results of evo-devo research support at least some aspects of continuum morphology, partial homology, and partial homeosis (see below) and these morphological concepts are in turn inspiring and relevant to further evo-devo research. Thus, research in evo-devo and morphology can be mutually beneficial.

Evo-devo also emphasizes process, but as far as I can see, it has not yet completely overcome the structure-process dualism that implies a static element. According to process morphology that transcends this dualism, structures do not
have processes, they are seen as processes (see my webpage on Plant Morphology and Fundamentals of Plant Morphology and Plant Evolutionary-Devo). A process-morphological version of the theory of anaphytes (anaphytosis) could provide a novel perspective on plant morphology and plant evo-devo (Sattler 2019).

Besides process morphology, other findings and ideas of morphology have not been sufficiently integrated into evo-devo research. The rich morphological literature offers much that could be relevant to evo-devo. For example, phytonic theories (see Cusset 1982) and repeating polymorphic sets or refrains (Meyen 1987) could provide additional interesting frameworks for investigation. More awareness of philosophical assumptions underlying evo-devo research could deepen our understanding. For example, an awareness of the phenomenon of emergence could enlarge the scope of the molecular genetic reductionism that tends to prevail in evo-devo research. Investigations into the role of self-organization and morphogenetic or morphic fields could also be beneficial. Morphogenetic fields may be understood in terms of a physical basis as a chemical pattern, an electromagnetic field, or an attractor pattern of a complex, non-linear process, or they may be conceived as having no physical basis (Pietak 2011: 101-106). The latter view has been elaborated by
Rupert Sheldrake (2009, 2012). Although not materialistic and mechanistic as mainstream biology, Sheldrake’s view is scientific in the sense that it is testable and has been confirmed through many observations and experiments.

Inclusion of the environment would expand evo-devo to eco-evo-devo (Tauber 2010). Recognition of the complementarity principle (both/and logic) could enlarge evo-devo perspectives. It could lead to a more inclusive vision that has been referred to as inclusionality. Finally, an awareness of the limitations of language and mathematics (a form of language and logic) could open vistas beyond evo-devo and science toward the unnamable, the mysterious, the spiritual, the sacred. For “whatever you might
say something "is", it is not" (Korzybski). Therefore, whatever we might say spirit is, it is not. And whatever we might say a plant, a leaf, or a flower is, it is not. Ultimately a plant, a leaf, a flower, and spirit "are" the un-speakable, the mystery that cannot be named. (I placed "are" in quotation marks to avoid a logical contradiction. But, in any case, the unnamable transcends logic and language).

In Part 2 of this article I shall elaborate in much more detail the issues discussed in Part 1 and I want to show how the research carried out in my laboratory and with many collaborators can be relevant to evo-devo research. I shall proceed chronologically.

Part 2

Phytomorphological Research by Rolf Sattler and Collaborators and its Relevance to Plant Evo-Devo Research

On the Broader Context: The History of Plant Morphology

First some remarks on the history of plant morphology because it is an often overlooked treasure trove of data, concepts, and theories. It has been comprehensively illustrated by Gérard Cusset in an excellent article, in which he enumerated and discussed 37 morphological principles:

Cusset, G. 1982. The conceptual bases of plant morphology. In: Sattler, R. (ed.)
Axioms and Principles of Plant Construction. The Hague/Boston/London: Martinus Nijhoff/ Dr. W Junk Publishers, pp. 8-86 (also published in Acta Biotheoretica, Volume 31A).

When I studied botany in the 1950s, I was taught mainly what is referred to as classical plant morphology - “classical” because it has one of its major roots in Goethe’s famous booklet on the metamorphosis of plants. But as I carried out my morphological research, I became increasingly aware of the limitations of this kind of plant morphology, and I realized that a number of other botanists, especially in the 19th and 20th centuries, were also critical of the classical doctrine. Even Goethe himself entertained alternative views (see, for example, Rutishauser and Sattler 1985, and Sattler 2018, cited below). But the simplicity of the classical view, which is based on the root-stem-leaf trinity of plant construction, seemed so appealing that alternative views have been ignored or suppressed. As a result, students of plant biology received a very one-sided education and indoctrination. This one-sidedness still persists today, although there seems to be more awareness of the limitations of the classical model of plant construction. Cusset’s article, referred to above, as well as our research cited below, makes it very clear that classical plant morphology is only one view besides others. Hence, research in plant evo-devo that relies to a great extent on molecular genetics also needs to consider other morphological concepts and frameworks besides classical morphology.

Toward a Continuum Morphology of Plants

Mainstream plant morphology, that is, classical plant morphology, appears categorical: any organ of flowering plants must be either a root, or a stem, or a leaf, or a homologue of one of them. I found that although many organs can be fitted into these categories, others appear more or less intermediate between them . As a result, one can see the diversity of plant form as a continuum that spans all the categories. I first proposed this idea in my paper:

Sattler, R. 1966. Towards a more adequate approach to comparative morphology. Phytomorphology 16: 417-429.

Subsequently, I provided a much more detailed elaboration of continuum morphology, especially with regard to the shoot, that is, stems, leaves, and emergences (outgrowths of the organs) (roots were not completely ignored since they may also occur in shoots):

Sattler, R. 1974. A new conception of the shoot of higher plants. Journal of Theoretical Biology 47: 367-382.

In retrospect I think that a more appropriate title for his paper would have been “The continuum conception of plant morphology.” I realized that - as I already mentioned in my paper - the idea of a continuum was not totally new. Other authors in the 19th and 20th centuries had recognized intermediates between categories. But since their findings and views had been largely ignored and suppressed by the scientific community of plant biologists, I was not sufficiently aware of all of my predecessors in continuum morphology. I learned much about them during a stay in Prof. Gérard Cusset's laboratory at the University of Paris in 1978.

In my
Biophilosophy book (1986, p. 105) I presented the continuum model of the shoot in the form of a pyramid, that is, a tetrahedron in which shoots, stems, leaves, and emergences occupy the four corners of the tetrahedron and intermediate structures the space between the corners. Dr. Ming Anthony and I extended this model to pathological plant structures and referred to it as the pyramid model:

Anthony, M. and Sattler, R. 1990. Pathological ramification of leaves and the pyramid model of plant construction. Acta Biotheoretica 38: 165-170.

Finally, in 1992 I published a paper with Dr. Bernard Jeune that established quantitatively, by means of multivariate analysis, the continuum between the categories of root, shoot, stem, leaf, emergence, and their homologues:

Sattler, R. and B. Jeune. 1992. Multivariate analysis confirms the continuum view of plant form. Annals of Botany 69: 249-262.

As far as I know, this was the first quantitative confirmation of the morphological continuum that spans not only the categories of the three types of organs of classical plant morphology but also three of its organizational levels: the organ system (shoot), organs (roots, stems, and leaves), and outgrowths of organs (emergences). I am most grateful to Dr. Bernard Jeune for his collaboration on this project. Without his mathematical expertise I could not have accomplished this difficult undertaking. Its results went far beyond my first paper on continuum morphology (1966) - cited above - in which I used a very simple example to indicate the continuum between the stem and leaf categories.

Using a different mathematical approach and mostly different plant species, Gérard Cusset corroborated the continuum view of plant form (Cusset, G. 1994. A simple classification of the complex parts of vascular plants. Botanical Journal of the Linnnean Society 144: 229-242). Jeune and Sattler (1992) also confirmed the continuum in terms of process morphology (see below).

Although I worked mainly at the levels of organs systems, organs, and outgrowths of organs, it must be pointed out that the morphological continuum is not restricted to these levels. It can also be found at other levels of organization such as the levels of tissues and cells. For example, we demonstrated a continuum between different types of perforation plates in the cells of vessels (the water-conducting structures in plants) and pointed out its significance for the vexing problem of the origin of flowering plants:

Muhammad, A.F. and Sattler, R. 1982. Vessel structure of Gnetum and the origin of angiosperms. American Journal of Botany 69: 1004-1021.

Despite increasing evidence for continuum morphology, Kaplan's Principles of Plant Morphology (2022), the most comprehensive volume of plant morphology in the 21st century, still relies exclusively on the concepts of classical plant morphology (see Sattler, R. 2022. Kaplan's Principles of Plant Morphology: A Critical Review. The Botanical Review 88: 257-270.

Leaf Development

Our investigations of leaf development provided much evidence for a continuum between simple leaves, stems and shoots. The following papers demonstrated so-called “leaves” that approach the symmetry of stems:

Sattler, R., Luckert, D. and Rutishauser, R. 1988. Symmetry in plants: leaf and stipule development in Acacia longipedunculata. Canadian Journal of Botany 66: 1270-1284.

Rutishauser, R. and Sattler, R. 1986. Architecture and development of the phyllode stipule whorls in
Acacia longipedunculata: controversial interpretations and continuum approach. Canadian Journal of Botany 64: 1987-2019.

So-called compound “leaves” exhibit shoot features during their development. For this reason they would be called more appropriately leaf-shoot intermediates. We demonstrated the combination of leaf and shoot features for two species:

Rutishauser, R. and Sattler, R. 1997. Expression of shoot processes in leaf development of Polemonium caeruleum. Botanische Jahrbücher für Systematik 119: 563-582.

Lacroix, C.R. and Sattler, R. 1994. Expression of shoot features in early leaf development of
Murraya paniculata (Rutaceae). Canadian Journal of Botany 72: 678-687.

Lacroix et al. (2003) confirmed our conclusions for other species. And more recently, on the basis of molecular genetics, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties.” (see below in Part 2).

Utricularia (bladderworts) we demonstrated a variety of leaf and shoot-like structures that cannot be fitted into the categories of classical plant morphology because they show a combination of features of different categories, including roots (see Sattler & Rutishauser (2023) and other papers by Rutishauser and Sattler in my list of publications).

Homology in Plants

Homology has been called the central concept of comparative morphology and therefore remains important in evo-deo. In classical morphology, whose basic concepts are still used in evo-devo, homology is conceived in terms of either/or. But in continuum morphology homology becomes a matter of degree, and thus, for example, a structure may be partially homologous to a stem, a leaf, and even a root. Such homologization seems inconceivable in classical morphology where an organ must be homologous to either a stem or a leaf or a root. Because of this insistence on categorical homology, homologization in classical terms gave rise to endless debates about the homology of intermediate structures. But, as I pointed out in the following two publications, if homology is seen as a matter of degree, these futile debates can be resolved:

Sattler, R. 1984. Homology – a continuing challenge. Systematic Botany 9: 382-394.

Sattler, R. 1994. Homology, homeosis, and process morphology in plants. In: Hall, B.K. (ed.)
Homology: The Hierarchical Basis of Comparative Biology. New York: Academic Press, pp. 423-475.

Although homology as a matter of degree appears compatible with modern research in plant evo-devo and molecular genetics, as far as I know, it has not always been sufficiently recognized explicitly.

Fuzzy Logic in Plant Morphology

Quantitative or partial homology and continuum morphology involve fuzzy logic, whereas categorical homology and classical morphology are based on Aristotelian either/or logic. According to fuzzy logic, membership in a class is a matter of degree. Hence, a structure may be, for example, a 50%, 80%, or 100% leaf. I discussed fuzzy logic in my essays on Either/Or Logic and Beyond and in Healing Thinking through Fuzzy Logic. And I also briefly mentioned the importance of fuzzy logic (or fuzzy thinking) in my article on Agnes Arber for whom thinking in terms of degree was of great importance:

Sattler, R. 2001. Some comments on the morphological, scientific, philosophical and spiritual significance of Agnes Arber’s work and life. Annals of Botany 88:1215- 1217.

For a much more detailed discussion of Agnes Arber’s thinking in terms of degree (that is, fuzzy thinking – a term she did not use, as far as I know, because fuzzy logic was invented after her death) see the excellent article by Rutishauser, R. and Isler, B. (2001). Developmental genetics and morphological evolution of flowering plants, especially bladderworts (Utricularia): Fuzzy Arberian morphology complements classical morphology. Annals of Botany 88:1173-1202.

For a general discussion of fuzzy logic and its importance in practically all aspects of life see the excellent book by Bart Kosko. 1993.
Fuzzy Thinking. The New Science of Fuzzy Logic. New York: Hyperion.

The words ‘fuzzy’ and ‘fuzziness’ appear distasteful for many people and especially for many scientists, I guess because these words are often equated with inexactness and vagueness, and science is supposed to be exact. However, fuzzy logic, as a quantitative approach, offers more exactness and precision than either/or logic. And continuum morphology, which reflects fuzzy logic, offers more precision than categorical classical morphology because instead of forcing the whole diversity of structures into mutually exclusive categories, it captures the whole range of structural variation. I guess if Zadeh, the inventor of fuzzy logic, would have called it ‘continuum logic,’ it would have been more palatable to many people; but Zadeh chose the name 'fuzzy logic.'

Although fuzzy logic appears compatible with modern research in plant evo-devo, as far as I know, it has not been sufficiently recognized explicitly.

My Critique of Essentialism

Categorical thinking and essentialism appear closely related. I became aware of the pitfalls of essentialism early on in my morphological work and pointed them out in my paper “A new conception of the shoot of higher plants (1974, p. 369)” (see above in the section on Continuum Morphology). Later on in 1977, I elaborated on essentialism in a paper I presented at an international congress in Japan:

Sattler, R. 1977. Essentialism in plant morphology. Proceedings No. 3, XIVth International Congress of the History of Science. Tokyo & Kyoto, pp. 464-467.

More recently, I also discussed essentialism in my paper and website on Philosophy of Plant Morphology (see below).

Classical morphologists such as, for example, Kaplan (2022) tend to embrace essentialism consciously or more often subconsciously. When they encounter structures that do not fit the classical categories, they often insist that these “misfits” represent the essence of one of the categories. For example, for a structure that appears intermediate between a stem and a leaf they would argue that it is essentially a stem or a leaf. They might not agree which one it is, but they agree that it must be one or the other. Thus, the underlying belief or philosophy of essentialism that dates back to Plato and Aristotle is used - consciously or subconsciously – to force the whole diversity of structural variation into fixed categories or essences. To my mind, this implies a disregard of empirical findings in favour of the preconceived notion of categories or essences. Can we still call it science when misfits (intermediate structures) can no longer question the general validity of the categorical, essentialistic approach?

Evolutionary biologists generally reject essentialism. I have shown, however, that to some extent evolutionary plant morphology may also be trapped in essentialism because evolutionary plant morphologists often use mutually exclusive morphological categories and the use of such categories may imply essentialism.

I never denied that morphological categories have a limited usefulness because many, if not most structures fit more or less the categories. However, intermediate structures don’t fit, and therefore continuum morphology appears more encompassing than categorical morphology. Categorical morphology can be seen as a special case of continuum morphology that seems appropriate for the extremes in the continuum (see Lacroix et al. 2005).

As noted above, molecular genetics also supports the continuum view. For example, James (2009, p. 17) stated that "it is now widely accepted that... radiality [characteristic of most stems] and dorsiventrality [characteristic of typical leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!"

In contrast, when molecular geneticists discuss, for example, organ identity essentialism may be implied.

“Fusion” in Plant Morphology

The concept of fusion plays an important role in plant morphology. Classical plant morphologists distinguish two kinds of fusion: 1. Fusion that can be observed (post-genital fusion), and 2. Fusion that by definition cannot be observed (congenital fusion). The latter is often used to make aberrant configurations fit into the positional relationships required by classical morphology. For example, according to classical morphology, lateral shoots are supposed to arise in the axils of leaves. If they arise on the leaf, they are often said to be congenitally fused with the leaf so that in reality they would have arisen in the axil. But this fusion cannot be observed. The observable processes were elucidated in two epiphyllous species by Dickinson and Sattler (1974) and (1975) (see my list of publications). Timothy Dickinson also published an excellent review article on the phenomenon of epiphylly (structures on leaves): Dickinson, T. A. 1978. Epiphylly in Angiosperms. Botanical Reviews 44:181-232.

In 1978, I published the following paper:

Sattler, R. 1978. ‘Fusion’ and ‘continuity’ in floral morphology. Notes from the Royal Botanic Gardens Edinburgh 36: 397-405.

In this paper I tried to elucidate the observable processes that underlie the so-called congenital fusions. In this way I recovered the empirical basis where it had been lost or obscured through classical plant morphology. And I pointed out the processes that change from one ontogeny to subsequent ontogenies during evolution, that is, the processes that are of concern to evo-devo research.

Heterotopy in Plant Morphology

One phenomenon that has been lost or obscured through the concept of congenital fusion is heterotopy, which implies a change in the position of structures. Together with my students and collaborators, I provided empiriral evidence for heterotopy in several publications such as, for example:

Dickinson, T.A. and Sattler, R. 1974. Development of the epiphyllous inflorescence of Phyllonoma integerrima (Turcz.) Loes.: implications for comparative morphology. Bot. J. Linn. Soc. 69: 1-13.

I discussed the significance and implications of heterotopy, especially for homologization and evo-devo in plant biology (see, for example, the above paper on "A new conception of the shoot of higher plants," 1974).

Instead of using the term heterotopy, molecular geneticists refer to ectopic gene expression when genes are expressed in different locations.

The Gynoecium

Heterotopy may occur anywhere in the plant body. However, it is especially important in the gynoecium of flowers, where it has often been obscured through the application of the concept of congenital fusion. In 1974, I published a paper in which I introduced a gynoecial morphology that does not rely on unobservable congenital fusions and elucidates developmental changes during evolution:

Sattler, R. 1974. A new approach to gynoecial morphology. Phytomorphology 24:22-34.

In retrospect, I would prefer the title “An empirical approach to gynoecial morphology” because this title would underline that this approach is based on observable data. As a result of these data, the floral concept of classical morphology had to be revised in a rather fundamental way. My students and I provided much empirical evidence for this revision (see my book Organogenesis of Flowers (1973), Macdonald and Sattler (1973), Posluszny and Sattler (1976), Pauzé and Sattler (1979), Sattler and Perlin (1982), and Sattler and Lacroix (1988), quoted in Morphological Development of Flowers).

Sattler and Perlin (1982) and Sattler and Lacroix (1988) rescued the carpel concept to some extent by a redefinition. Traditionally a carpel is defined as a folded megasporophyll, that is, an appendage that bears and encloses ovule(s). Consequently, gynoecia whose ovules are not born on the gynoecial appendages are acarpellate (without carpels). But they may also be considered carpellate (having carpels), if we redefine the carpel as an appendage that encloses ovule(s) and that may or may not bear the ovule(s). Thus, cauline ovules (that are born on the floral axis) can be recognized and it is not necessary to postulate in principle unobservable congenital fusions. Hence, this redefinition renders gynoecial morphology more empirical.

However, some gynoecia such as those of
Stylidium (Sattler 1974) remain acarpellate (without carpels) even according to the redefined carpel concept. Therefore, we have to conclude that the flowers of angiosperms may be carpellate or acarpellate, although he majority are carpellate according to the redefined carpel concept.

As far as I know, the diversity of gynoecial construction, especially acarpellate gynoecia, have not been sufficiently explored in evo-devo research that tends to imply the classical carpel concept (in the sense of a folded megasporophyll), and therefore evo-devo has not caught up sufficiently with the redefinition of the carpel as I proposed it and as it has been endorsed by Greyson (1994), Leins and Erbar (2010, in their glossary only), and others.

Diversity of Floral Development and Floral Concepts

In most textbooks and research publications the classical concept of the flower is taken for granted. According to this concept, a flower is a modified monaxial shoot that consists of sterile and fertile appendages (leaf homologues). The sterile appendages, if present, are tepals or sepals and petals, whereas the fertile appendages are stamens and carpels.

Investigations of the
diversity and complexity of floral development have shown that the classical concept of the flower often is not supported by empirical evidence (see my publications and my book Organogenesis of Flowers). Therefore, alternative concepts have been proposed. These concepts that are often ignored have been summarized and applied to the flowers of Myrica gale by

Macdonald, A. D. and Sattler, R. 1973. Floral development of Myrica gale and the controversy over floral concepts. Canadian Journal of Botany 51:1965-1976.

We discussed these concepts with regard to Myrica gale because the female flowers of this species are not easily understood in classical terms. Other floral concepts, especially that of Leon Croizat, provide interesting perspectives. I am not aware of any other publication that applies all of these floral concepts to any other species of angiosperms.

Sattler and Jeune (1992), cited above, found that the continuum model of the shoot also applies to flowers, which means that floral organs cannot always be clearly assigned to mutually exclusive categories. Especially stamens turned out to be often more or less intermediate between typical phyllomes and caulomes or shoots (Sattler 1988, see below).

As in the vegetative region, the continuum in the reproductive region is a dynamic continuum, that is, a continuum of process combinations. This view, which I called process morphology, I first proposed for floral morphology and then extended it to plant morphology in general (see below under Towards a Process Morphology of Plants).

I seems that plant evo-devo research could benefit from more awareness of different floral concepts.

Homeosis in Plants

Homeosis plays an important role in evo-devo research. It means that at the site of one part of a plant another part or feature(s) of another part of that plant are expressed. Thus, for example, an axillary branch may be replaced by a leaf or a structure with some features of a leaf and a branch. Such cases cannot be understood in classical terms but require a broader framework of continuum morphology and the notions of partial or quantitative homology. I have discussed these and other implications of homeosis in the following papers:

Sattler, R. 1988. Homeosis in plants. American Journal of Botany 75:1606-1617.

Sattler, R. 1994. Homology, homeosis, and process morphology in plants. In: Hall, B.K. (ed.)
Homology: The Hierarchical Basis of Comparative Biology. New York: Academic Press, pp. 423-475.

Cooney-Sovetts and Sattler (1987) and Lehmann and Sattler (1992, 1993, 1994, 1996, 1997) investigated many cases of homeosis and demonstrated their significance for plant development and evolution (see my list of publications).

Developmental Hybridization in Plants

Developmental hybridization, leading to developmental mosaics, can be considered a special case of homeosis in which features of different structures are combined. If the different structures are homologous in the classical sense (that is, if they belong to the same category), the hybridization can be understood within the framework of classical morphology. However, if the different structures are not homologous in the classical sense (that is, if they belong to different categories or do not fit categories), then the hybridization can be only understood in terms of continuum morphology and its associated concepts of partial or quantitative homology. Examples of this type of developmental hybridization are compound leaves such as pinnate leaves that combine features of leaves and branches and thus become mosaics. The following papers present evidence for this phenomenon (see also Lacroix et al. 2003 and Rutishauser et al. 2008):

Sattler, R. and Rutishauser, R. 1992. Partial homology of pinnate leaves and shoots. Orientation of leaflet inception. Botanische Jahrbücher für Systematik 114: 61-79.

Lacroix, C.R. and Sattler,R. 1994. Expression of shoot features in early leaf development of Murraya paniculata (Rutaceae). Canadian Journal of Botany 72:678-687.

Rutishauser, R. and Sattler, R. 1997. Expression of shoot processes in leaf development of
Polemonium caeruleum. Botanische Jahrbücher für Systematik 119:563-582.

I consider the above publications important for several reasons: 1. They illustrate the much neglected phenomenon of developmental hybridization (although that term was not used in all of these publications), 2. They support Agnes Arber’s partial shoot theory of the leaf that has not received the recognition it deserves, and 3. They show that developmental hybridization is not restricted to rare "misfits" but occurs also in common structures such as compound leaves.

Investigations of the molecular genetics of compound leaves also support our morphological conclusions.
Eckardt and Baum (2010) stated that “it is now generally accepted that compound leaves express both leaf and shoot properties.”

Towards a Process Morphology of Plants

I have been interested in plant development for a long time and, like my colleagues, I did not realize that the way we studied development was dynamic only to a limited extent. It still involved some static presuppositions because although we recognized that structures change during development, we tacitly assumed that they retain their categorical status. For example, we could see that a leaf changes as it develops, but we took it for granted that it must remain a leaf. Thus, we were locked into a dualism of something that could change versus something – the category or essence – that remained static.

The process morphology that I began to develop in the late 80s is meant to eliminate this dualism that perpetuates static categorical assumptions. Therefore, this approach is not based on structural categories such as root, stem, and leaf. It is based on processes, and the diversity of plant form is seen as a diversity of process combinations. Ontogenetic as well as phylogenetic change is seen as a change in process combinations, not as a change in the properties or processes of structures.

It seems that many people find it difficult to see the difference between traditional developmental and evolutionary studies and process morphology. One way of emphasizing this difference is the following: According to the common way of thinking, structures
have processes, hence there are structures and processes, whereas according to process morphology structures are seen as process(es), hence there is only process and thus the structure/process dualism is overcome.

I first developed process morphology for floral development in a short paper that I presented in a symposium at the 14
th International Botanical Congress in Berlin, Germany:

Sattler, R. 1988. A dynamic multidimensional approach to floral morphology. In: Leins, P. , Tucker, S,C. and Endress, P.K. (eds) Aspects of Floral Development. Stuttgart: Cramer, pp. 1-6.

Subsequently, I generalized the process-morphological approach for all plant form:

Sattler, R. 1990. Towards a more dynamic plant morphology. Acta Biotheoretica 38:303-315.

Sattler, R. 1992. Process morphology: structural dynamics in development and evolution. Canadian Journal of Botany 70: 708-714.

These papers demonstrate how morphological processes may change during ontogeny and phylogeny, which is the topic of evo-devo. In collaboration with Prof. Rolf Rutishauser I showed how it works in challenging concrete situations:

Sattler, R. and Rutishauser, R. 1990. Structural and dynamic descriptions of the development of Utricularia foliosa and U. australis. Canadian Journal of Botany 68:1989-2003.

Dr. Bernard Jeune and I demonstrated quantitatively that the diversity of process combinations forms a dynamic continuum. Hence, process morphology is also continuum morphology:

Jeune, B. and Sattler, R. 1992. Multivariate analysis in process morphology. Journal of Theoretical Biology 156:147-167.

In my last morphological publication I used process morphology as a framework to bridge and transcend two very different views on the evolution of plant form, namely Wolfgang Hagemann’s view and Walter Zimmermann’s telome theory:

Sattler, R. 1998. On the origin of symmetry, branching and phyllotaxis in land plants. In: Jean, R.V. and Barabé, D. (eds) Symmetry in Plants. Singapore: World Scientific, pp. 775-793.

The process morphological approach of the above publication provides a framework that allows us to examine the evolution of land plants (bryophytes and vascular plants) in a novel dynamic way. Although it did not include a genetic analysis, I consider it a prime example of evo-devo research liberated from the constraints of categorical classical plant morphology that still dominates mainstream plant morphology and even evo-devo research to some extent.

Finally, in a chapter of a book dedicated to Alessandro Minelli's 70th birthday, I contrasted structural and dynamic approaches and discussed how process morphology supersedes the structure-process dualism and therefore is more radically dynamic than other dynamic approaches that still refer to structures and thus imply a structure-process dualism.

Sattler, R. 2019. Structural and dynamic approaches to the development and evolution of plant form. In: Fusco, G. (ed.) Perspectives on Evolutionary and Developmental Biology. Padova University Press, Chapter 6, pp. 57-70.


In the following book chapter I presented a summary and synthesis of continuum and process morphology, homeosis and homology, including partial homeosis, partial and quantitative homology:

Sattler, R. 1994. Homology, homeosis, and process morphology in plants. In: Hall, B.K. (ed.) Homology: The Hierarchical Basis of Comparative Biology. New York: Academic Press, pp. 423-475.

Plant Morphology, Language and Linguistics

Towards the end of my career I became increasingly aware of how much the language we use influences the results of morphological research and scientific research in general. For example, most languages have a noun-verb structure. Thus, we say, ‘This leaf branches.’ When nouns refer to categories, they obscure and negate the continuum between categories. For example, when we refer to leaves or stems we imply categories, unless we stipulate that a structure may be more or less like a leaf and more or less like a stem, or unless we specify a structure quantitatively as, for example, a 80% leaf or a 20% stem.

Another consequence of the noun-verb structure is the structure-process dualism I referred to above in the section on process morphology. Verbs refer to action, to process, but nouns imply static. For example, a leaf may undergo all sorts of processes referred to by verbs, but it remains a leaf. Thus, the noun-verb structure imposes a static element on our description that does not seem to exist in nature. We could eliminate this distortion if we used a language based on verbs only. According to Benjamin Lee Whorf and others such languages actually exist (see, for example, Carroll 1956). Some Amerindian languages such as Nootka consist only of verbs, according to Whorf, and others such as Hopi use predominantly verbs where we use a noun-verb structure. Many linguists deny Whorf’s assertion. However, one can have intelligent sentences without a noun. For example, instead of saying, ‘The sun shines’, a Hopi would simply say ‘shining’, and thus would only refer to a process, not to an agent, the sun, that does the shining. Similarly, one need not postulate an agent, a creator for organic evolution and the evolution of the universe. Thus, as Whorf and others have pointed out, a process language gives us a very different perception of the world: not a world that is fragmented into agents (objects or entities) or a creator to whom processes are ascribed, but a world that appears simply dynamic. David Bohm referred to holomovement (movement within the whole). I briefly discussed these issues in my essay on
Process Language and in the following publication:

Sattler, R. 1993. Why do we need a more dynamic study of morphogenesis? Descriptive and comparative aspects. In: Barabé, D. et Brunet, R. (eds) Morphogenèse et Dynamique. Frelighsburg: Orbis, pp. 140-152.

Another issue concerning language has been much emphasized by Korzybski, (see my essays on Healing Thinking through Non-Identity (Korzybski) and Healthy Language-Behavior and Spirituality). Korzybski pointed out that language entails an abstraction from reality, not reality itself. Therefore, “whatever you might say something "is", it is not” (which is explained through his Structural Differential). For example, whatever we might say a leaf is, it is not. What is it then? It is the unnamable, un-speakable mystery. Thus, an understanding of language and its limitations leads us beyond science into the unnamable, which is beyond thinking (see also below the section on Plant Morphology and Spirituality).

The Fundamental Relevance of Plant Morphology

Although nowadays plant morphology – and morphology in general – is no longer taught as a separate discipline at most universities, it remains fundamentally relevant to practically all disciplines of plant biology, including plant evo-devo, because all disciplines rely on and refer to morphological concepts, if not explicitly at least implicitly. In the following publication we pointed out this relevance and showed how a lack of morphological knowledge can be detrimental to research in other areas such as evolutionary biology:

Sattler, R. and Rutishauser, R. 1997. The fundamental relevance of plant morphology and morphogenesis to plant research. Annals of Botany 80: 571-582.

Sattler, R. and Rutishauser, R. 2023. Fundamentals of plant morphology and plant evo-devo (evolutionary developmental biology). Plants 12(1), 118;, or

Plant Morphology and Philosophy

Plant evo-devo could also benefit from a greater awareness of its philosophical assumptions (see, e.g., Vergara-Silva 2003; Tauber 2010) and those of plant morphology. Often we are unaware of these assumptions. But as we render them explicit, we can examine them:

Sattler, R. 2018. Philosophy of Plant Morphology. Elemente der Naturwissenschaft 108: 55-79 (an expanded version on this website).

Sattler, R. 1986. Biophilosophy. New York: Springer, Chapter 5.

Sattler, R. 1978. What is theoretical plant morphology? In: R. Sattler (ed). Theoretical Plant Morphology. Acta Biotheoretica 27: 5-20 (Supplement: Folia Biotheoretica, No. 7).

One concept of central importance is the type concept. Morphologists, other scientists, and laypersons often refer to types. In doing do, it seems that they often imply classificatory types, which are mutually exclusive, that is, they imply either/or logic. In contrast, extreme types have fuzzy edges, that is, they imply fuzzy logic. If we use extreme types, we can see the continuum between morphological categories, and furthermore we can bridge the gap between classical morphology and continuum morphology as pointed out in the following paper:

Sattler, R. 1996. Classical morphology and continuum morphology: opposition and continuum. Annnals of Botany 78:577-581.

In this review, I have to acknowledge that many morphologists and philosophers have influenced me. If I had to single out one of them, which seems difficult, if not impossible, I would name Agnes Arber, especially her books The Natural Philosophy of Plant Form (1950), The Mind and the Eye (1954), and The Manifold and the One (1957). Some of her most basic ideas characterize also my thinking. In the following short article, I summarized these ideas: transformation (dynamics, process), wholeness (integration), thinking in terms of degree (fuzzy logic), complementarity (both/and logic), relativity, dialectics, complexity, humility, coincidence of contraries. I think these and related ideas constitute excellent guidance for research and living in the twenty-first century:

Sattler, R. 2001. Some comments on the morphological, scientific, philosophical and spiritual significance of Agnes Arber’s life and work. Annals of Botany 88:1215-1217.

Research in evo-devo could benefit from a more profound and comprehensive knowledge of the biophilosophical literature.

Complementarity in Plant Morphology

Plant morphology has a very rich history. Many different concepts and theories have been proposed (see, for example, Cusset, G. 1982). As I mentioned already, I think that evo-devo research could benefit from a knowledge of these concepts and theories. They have often been perceived as antagonistic to each other: it has been assumed that one must be right and the others wrong. However, they can also be seen as complementary to each other, illuminating different aspects of plant form. Prof. Rolf Rutishauser, while working in my laboratory, explored complementarity in plant morphology in great depth and published three papers on this topic with me, demonstrating how different and even contradictory morphological concepts and theories complement each other because they capture different aspects of plant form:

Rutishauser, R. and Sattler, R. 1985. Complementarity and heuristic value of contrasting models in structural botany. I. General considerations. Botanische Jahrbücher für Systematik und Planzengeographie 107: 415-455.

Rutishauser, R. and Sattler, R. 1987. Complementarity and heuristic value of contrasting models in structural botany. II. Case study on leaf whorls:
Equisetum and Ceratophyllum. Botanische Jahrbücher für Systematik und Pflanzengeographie 109: 227-255.

Rutishauser, R. and Sattler, R. 1989. Complementarity and heuristic value of contrasting models in structural botany. III. Case study on shoot-like "leaves" and leaf-like "shoots" in
Utricularia macrorhiza and U. purpurea (Lentibulariaceae). Botanische Jahrbücher für Systematik und Planzengeographie 111: 121-137.

Complementarity implies both/and logic (see my essay on Healing Thinking through Both/and Logic, Buddhist and Jain Logic). Different, and even contradictory statements are accepted as different perspectives on the situation. Therefore, both/and logic and complementarity represent what is called perspectivism that underlines the many-sidedness of everything. The idea of many-sidedness has been articulated in Jain logic more than two thousand years ago and yet it has been barely incorporated into mainstream science and society. Increased awareness of many-sidedness could have the most beneficial effects on science and society: it could reduce conflicts and wars and thus lead to a more peaceful world (see, for example, Aidan Rankin. 2010. Many-sided Wisdom).

Research in evo-devo could also benefit from a more profound and comprehensive knowledge of different types of logic.

Plant Morphology and Spirituality

I hesitate to refer to spirituality because it could be misunderstood. I am not interested in a spiritual dogma or belief. To me spirituality (if I use this word) refers to the knowledge and experience of interconnectedness and oneness (non-duality). Agnes Arber’s last book is entitled The Manifold and the One (1957). In this book she beautifully examines how the manifold is related to oneness. As a plant morphologist, I investigated the manifold of plant form, and I also recognized that, as Nobel laureate Barbara McClintock put it, “basically, everything is one.” Any plant form is connected to everything else. This is especially evident when we recognize plant form as process, that is, process combinations. The process combinations that constitute, for example, a leaf are combined with the process combinations of the whole plant, which are combined with process combinations of the environment that includes animals and humans. Ultimately, these process combinations are combined with cosmic processes such as solar radiation and cosmic radiation. Therefore, we may say, for example, that a leaf or a plant is a cosmic event.

How does this cosmic event that includes us relate to spirituality? To many people spirituality is communion with spirit, and spirit is often seen as opposed to matter and form. From this perspective, morphology, the study of material form, cannot be related to spirituality. However, to non-dualists such as, for example,
Ken Wilber, spirit includes matter, or more precisely, it includes and transcends matter. This integral spirit and spirituality do not exclude form. They allow us to see the divine in form. Thus, form appears sacred.

But in whichever way we refer to spirit, we must remember
Korzybski’s insight that “whatever you might say something "is", it is not.” Therefore, whatever we might say spirit is, it is not. And whatever we might say a plant, a leaf, or a flower is, it is not. A plant, a leaf, a flower, and spirit are not the speakable, but the un-speakable, the mystery that cannot be named.

Another way of relating plant morphology to spirituality is through Buddhism, especially the Heart Sutra. According to the Heart Sutra, form is emptiness and emptiness is form (see, for example, Tenzin Gyatso, the Fourteenth Dalai Lama. 2005.
Essence of the Heart Sutra). "Form is emptiness" means that form lacks independent, intrinsic existence and therefore remains one with everything, with the whole universe. For example, a leaf or a plant have no independent, intrinsic existence. If they are seen as process combinations, these process combinations remain one with process combinations of the whole universe as noted above. Thus, morphological insight can become a path to oneness and wholeness. And wholeness is related to holiness, the sacred. We may talk about this, but rational, conceptual understanding has limits. We also have to feel it and be it.

English translations of the Heart Sutra vary. Lim Peng Eok (personal communication) translated the above passage as "The seen is the unseen. The unseen is the seen." Ordinarily, form is seen. But according to the Heart Sutra, it is also the unseen, beyond our sensual perception and our linguistic representation. Experiencing and being the unseen in the seen seems the greatest challenge as it transcends and yet includes morphological research on the way to awakening and liberation.

The following paper includes a discussion of the relation between plant morphology and spirituality:

Sattler, R. 1977. On “understanding” organic form. Sophia Perennis III: 29-50.

The late Professor T. Izutsu, author of Toward a Philosophy of Zen Buddhism (1977) invited me to write this paper.

The following paper is based on a talk in a symposium on "Divergence and Convergence of Sciences and Spirituality" at the 65th birthday celebrations of the Dalai Lama.

Sattler, R. 1999. Divergence and convergence of sciences and spirituality: life science and spirituality. Holistic Science and Human Values, Transactions 4:41-48.

The relation of general science and spirituality I discussed in the following paper:

Sattler, R. 2016. Science and Mystery. Holistic Science Journal 3(1): 49-53.

As we discuss evo-devo, it may benefit us as human beings to be aware of these deeper dimensions, although they are beyond the reaches of scientific investigation.

To conclude a personal comment: My morphological thinking began with my indoctrination into classical plant morphology. But already in the 60s my research led me to continuum morphology and in he 80s to process morphology. And all along since the 70s, as I became more familiar with Daoism and Buddhism (especially the Heart Sutra), I realized that form has no intrinsic, separate existence because of dependent origination that implies wholeness and oneness (that ultimately cannot be conceptualized). Dependent origination also involves impermanence, a central concept and experience in Buddhism, which appears compatible with a process oriented outlook on the world.

In other words, my investigations of the manifold of form, first in categorical terms and then in holistic dynamic ways, led me to the awareness of oneness and the recognition that the manifold and the one are not two but one as Agnes Arber so beautifully explained it in her book
The Manifold and the One, Chapter 5: The Coincidence of Contraries (see also my essay From Plant Morphology to Infinite Issues).


Ambrose, B. A. and Purugganan, M. (eds) 2013. The Evolution of Plant Form. Annual Plant Reviews 45.

Arber, Agnes. 1950. The Natural Philosophy of Plant Form. Cambridge: Cambridge University Press.

Arber, Agnes. 1954.
The Mind and the Eye.
Cambridge: Cambridge University Press.

Arber, Agnes. 1957.
The Manifold and the One.
London: John Murray.

Carroll, J.B. 1956.
Language, Thought, and
Reality. Selected writings of Benjamin Lee
. New York: Technology Press of MIT and

Cusset, G. 1982. The conceptual bases of plant
morphology. In: Sattler, R. (ed.)
Axioms and
Principles of Plant Construction
. The Hague/
Boston/London: Martinus Nijhoff/Dr. W. Junk
Publishers, pp. 8-86 (also published in Acta
Biotheoretica, Vol. 31A).

Eckardt, N. A. and Baum, D. 2010. The
Podostemad puzzle: The evolution of unusual
morphology in the Podostemaceae. The Plant
Cell 22: 2104.

Flannery, M. C. The many sides of Agnes

Greyson, R.I. 1994. The Development of Flowers. New York/Oxford: Oxford University Press.

Gyatso, Tenzin, the Fourteenth Dalai Lama.
Essence of the Heart Sutra. Boston: Wisdom Publications

Hirayama et al. 2007. Expression patterns of
class 1 KNOX and YABBY genes in Ruscus
aculeatus (Asparagaceae) with implication for
phylloclade homology. Development Genes and
Evolution 217:363-372. Epub 2007 Apr 12.

James, P. J. 2009. 'Tree and Leaf': A different
angle. The Linnean 25: 13-19.

Kaplan, D. R. 2022 (edited by C. D. Specht). Kaplan's Principles of Plant Morphology. CRC Press.

Kosko, B. 1993. Fuzzy Thinking. The new science of fuzzy logic. New York: Hyperion.

Lacroix, C., Jeune, B., and Purcell-Macdonald, S. 2003. Shoot and compound leaf comparisons in eudicots: dynamic morphology as an alternative approach. Botanical Journal of the Linnean Society 143: 219-230.

Lacroix, C., Jeune, B., and Barabé, D. 2005. Encasement in plant morphology: an integrative approach from genes to organisms. Canadian Journal of Botany 83: 1207-1221.

Leins, P. and Erbar, C. 2010.
Flower and Fruit. Stuttgart: Schweizerbart Science Publishers.

Meyen, S.V. 1987.
Fundamentals of
. London: Chapman and Hall (see
chapters on plant morphology).

Minelli, A. 2018. Plant Evolutionary Developmental Morphology. The Evolvability of the Phenotype. Cambridge University Press.

Pietak, A. M. 2011.
Life as Energy. Opening the
Mind to a New Science of Life
. Edinburgh:
Floris Books.

Rankin, A. 2010. Many-Sided Wisdom.
Winchester,UK/Washington, USA: O Books.

Rutishauser, R. 2020. EvoDevo: Past and future of continuum and process plant morphology. Philosophies 5(4), 41;

Rutishauser, R. and B. Isler. 2001. Developmental genetics and morphological evolution of flowering plants, especially bladderworts (Utricularia): Fuzzy Arberian morphology complements classical morphology. Annals of Botany 88:1173-1202.

Rutishauser, R., Grob, V. & Pfeifer, E. 2008:
Plants are used to having identity crises. Pages
194 – 213 in A. Minelli & G. Fusco (eds.)
Themes in Evolutionary Developmental Biology
Cambridge University Press, Cambridge.

Sheldrake, R. 2009.
A New Science of Life. 3rd
edition. London: Icon Books.

Sheldrake, R. 2012.
The Science Delusion.
Freeing the Spirit of Enquiry
. London: Coronet of
Hodder & Stoughton.

Tauber, A. I. 2010. Reframing developmental
biology and building evolutionary theory's new
synthesis. Perspectives in Biology and Medicine
53: 257-270.

Tsukaya, H. 2014. Comparative leaf
development in angiosperms. Current Opinions
in Plant Biology 17: 103-109.

Uchida, N. et al. 2010. Coordination of leaf
development via regulation of
KNOX1 genes.
Journal of Plant Research 123: 7-14.

Vergara-Silva, F. 2003. Plants and the Conceptual Articulation of Evolutionary Developmental Biology. Biology and Philosophy 18: 249-284.

Wanninger, A. 2015. Morphology is dead - long live morphology! Integrating MorphoEvoDevo into molecular EvoDevo and phylogenomics. Frontiers in Ecology and Evolution 3, Article 54.

Zimmermann, W. 1959. Die Phylogenie der
. 2nd edition. Stuttgart: Gustav Fischer

See also Plant Morphology, Morphological Development (Organogenesis) of Flowers, From Plant Morphology to Infinite Issues (including Ken Wilber and Korzybski), Philosophy of Plant Morphology, and Science: its Power and Limitations.

Latest update of this webpage on January 13, 2023.