Articulation Morphology of Plants and Plant Evo-Devo: An empirical, all-inclusive, unifying, process-oriented approach based on open growth, ramification and articulation, inspired by the theory of anaphytes
Rolf Sattler
Latest update on November 24, 2025
Abstract
According to the theory of anaphytes (anaphytosis), first proposed in 1843, in the open growth of plants, ramification is the key principle in plant morphology. It engenders articulation: the formation of articles, called anaphytes. While the theory of anaphytes included tenets that are now considered outdated, Articulation Morphology, proposed here as a modern version of this theory, retains and develops only those aspects that remain valid and fundamentally important: ramification and articulation. In this view, plants are articulated wholes: systems of articles formed through ramification. These articles are understood dynamically as process combinations according to process morphology. For practical purposes, they may be described in traditional structural terms such as root, stem, leaf, or leaflet, but without implying a morphological theory such as the classical root-stem-leaf theory of mainstream morphology. Hence, articulation morphology is strictly empirical, solely relying on the observable processes of open growth, ramification and articulation. In contrast to classical mainstream morphology, which often fails to accommodate atypical or deviant structures, articulation morphology is all-inclusive: even the most deviant structures can be understood as deviant patterns of ramification and articulation. The central concept of articulation morphology is no longer homology but transformation - the transformation of ramification and articulation. From this perspective, plant evo-devo becomes the investigation of the development and evolution of ramification and articulation.
Classical Mainstream Morphology
Classical morphology remains predominant in mainstream plant morphology (Kaplan 2022, Sattler 2022), although it has been surpassed (e,g., Sattler 1994, Rutishauser 2020, Classen-Bockhoff 2024). According to classical morphology, plants such as flowering plants consist of three kinds of organs: root, stem (caulome), and leaf (phyllome) (Braun 1851, Troll 1954, Kaplan 2022). Thus, any organ we encounter must be either a root, a stem (caulome), or a leaf (phyllome). However, some structures deviate so much from the common pattern that they cannot be clearly assigned to any one of the three kinds of organ categories (Rutishauser 2016, 2020, Rutishauser et al. 2008, Sattler and Rutishauser 2023, Classen-Bockhoff 2024). As a result, endless debates have persisted and continue to arise about the assignment of these controversial organs. These controversies remain unresolved because they appear to be pseudo-problems: attempts to categorize structures that do not fit the categories (for examples see below).
The Theory of Anaphytes (Anaphytosis)
In the theory of anaphytes (anaphytosis), which was developed by C. H. Schultz, who is also known as Schultz-Schultzenstein, such pseudo-problems do not arise (Schultz 1943, Schultz-Schultzenstein 1867). According to this theory, plant morphology is the result of two fundamental processes: ramification (branching) and articulation. These two processes are observable. We can observe that during the development of plants, ramification occurs, which leads to articulation: the formation of articles, called anaphytes, which arise after each ramification and between successive ramifications. For example, a simple leaf is an article that does not undergo further ramification, while an internode is an article between successive ramifications. The process of the continued formation of anaphytes is called anaphytosis. Hence, Schultz-Schultzenstein named his theory anaphytosis. Like other authors, I refer to it more simply as the theory of anaphytes. Besides the fundamental processes of ramification and articulation, Schultz (1843) included theoretical claims in his theory of anaphytes that today appear outdated. He regarded anaphytes as individuals that can form a whole plant. I do not endorse this claim and other speculative components of his theory. I endorse only its factual basis of ramification and articulation.
The theory of anaphytes has been almost completely forgotten. Only very few authors have referred to it (see Cusset 1982, Rutishauser and Sattler 1985, Sattler 2018, 2019). Foster, originally the senior author of the well-known textbook Comparative Morphology of Vascular Plants (Foster and Gifford 1974), concluded late in life that “a theory, in some way analogous to that of the anaphytes, was most valuable” (quoted by Cusset 1982, p. 46). Nevertheless, Kaplan (2022), his most prominent student, like most mainstream morphologists, ignored - or seemed unaware - of the theory of anaphytes and remained committed to classical morphology, although the morphological foundation of the theory of anaphytes provides a more inclusive and comprehensive framework.
Articulation Morphology
Organisms can be partitioned into different kinds of parts (Winther 2011). Thus, plants can be partitioned into organs or articles (anaphytes). Organs are established, if not through questionable non-observable boundaries, then at least through a morphological theory such as the classical root-stem-leaf theory and theoretical implications of homology, whereas articles are distinguished on the basis of the observable process of ramification. The organ-based approach of mainstream classical morphology has been solidified by the prevalent idea that plants consist of three fundamental organs (“Grundorgane” in German), as promoted by Troll, the influential German morphologist (Troll 1937-1943, 1954) and later by Kaplan, the influential American morphologist, who, like most mainstream morphologists, adopted Troll’s categorical framework (Kaplan 2022, Sattler 2022). In contrast to the organ-based approach of mainstream morphology, according to articulation morphology - a modern version of the theory of anaphytes that I am proposing here - plants are articulated (or segmented) wholes: systems of articles generated by open growth, which leads to ramification and articulation: the formation of an article from each ramification and between successive ramifications. In this view, open growth, ramification, and the formation of articles constitute the most fundamental processes in plant morphology and plant evo-devo. Instead of ‘articles,’ one could refer to segments, structural units or simply structures defined by ramification and differentiated in many ways. These include thallus segments, telomes, roots, segregation products of the shoot apical meristem (SAM), arising from lateral and axillary meristems, fractionation products of the reproductive apical meristem (RAM), and rare structures such as the fronds of the Lemnaceae and haustoria (see below).
Morphology based on ramification and articulation could be called ramification morphology or articulation morphology. I prefer the latter because it emphasizes more strongly the difference to mainstream morphology, which also addresses ramification but often in a more restrictive way concerning the formation of branches such as axillary branches. In contrast, the process of ramification in articulation morphology refers to the formation of new primordia regardless of which articles they produce. Hence, in articulation morphology, ramification is used in a much broader and more fundamental sense than in mainstream morphology. This approach recognizes that structures such as compound leaves, stamens and carpels are also ramified.
Instead of articulation morphology, we could refer to open morphology, emphasizing open growth, which leads to ramification and articulation. Open Morphology can also be seen as being open toward other complementary approaches to morphology (see below).
As mentioned already, one difference between the organs of mainstream morphology and the articles of articulation morphology is that the latter is based on the observable process of ramification, whereas the former is based on a morphological theory and homology and often relies on demarcations that are questionable because there are no clear-cut boundaries between organs understood as morphological body parts (Minelli 2021). By drawing boundaries differently, five different models of plant construction have been distinguished (see Rutishauser and Sattler 1985, pp. 420-424, Classsen-Bockhoff 2024, pp. 487-489, Fig. 8.2). These models complement one another, yet they are all based on boundaries that do not exist in nature. The shoot and the whole plant are a continuum. Within this continuum, Howard (1976) highlighted the stem-node-leaf continuum. This continuum is acknowledged in the model that subdivides the plant into phytomers, which are also called modules (White 1979, Classen-Bockhoff 2024, p. 352), but phytomers - consisting of a node, the internode below, the leaf, the axillary bud and even roots if they are present - create a discontinuum in the stem from one internode to another. This discontinuum is avoided in the traditional root-stem-leaf model by acknowledging the continuity of the stem, but it draws a boundary between the stem and the leaf. Other models of the shoot avoid one discontinuum on;u to create another. In contrast, articulation morphology is not based on organs with disputed boundaries; it is based on articles - units that emerge through the observable process of ramification. These articles are not defined by boundaries; we simply observe that ramification occurs through the formation of a new growth centre (primordium), which at its base is continuous with the article on which it is formed. As this new primordium develops into a new article it acquires distinct properties and through these properties – not through a non-existent boundary – it becomes distinguishable from the article on which it arises. Hence, the distinction of articles does not rely on boundaries: we can indeed distinguish articles without drawing boundaries. If the newly-formed article does not ramify, it remains a single article. If it does, the segment between successive ramifications constitutes an article.
Even between organs no boundary can be observed. Yet in mainstream morphology a boundary is often assumed, but some mainstream morphologists acknowledge the non-existence of boundaries. Kaplan wrote: “The vagaries of defining the boundaries between stem and leaf components of the shoot underscore the fundamental developmental unity of these two elements and the artificiality of attempting to draw a rigid boundary between them” (Kaplan 2022, p. 5). Nonetheless, Kaplan insists that the shoot "consists of two major components: leaves and stem" (ibid., p. 4) and that "a flower is a reproductive short shoot bearing microsporophylls (stamens) … and megasporophylls (carpels) as its appendages or leaf homologues" (ibid., p. 1069). Thus, even if the attempt to draw boundaries is given up, a crucial difference between organ-based mainstream morphology and articulation morphology remains: organs are defined in terms of a morphological theory - such as the classical root-stem-leaf model – and homology, whereas articulation morphology does not depend on a morphological theory and homology. It is solely based on the observable processes of ramification and articulation, independent of any morphological theory and homology. Hence, it avoids the pseudo-problems of a morphological theory such as the classical theory that cannot deal with structures that do not fit into its categorical framework. And since it does not require drawing boundaries that do not exist in nature, it surpasses the complementarity of the five shoot models that are based on different, complementary boundaries.
In articulation morphology – which also could be called segmentation morphology - the central and most basic concept is no longer morphological homology but transformation: the transformation of ramification and articulation. This changes the most basic questions we ask. Instead of asking questions about morphological homology, we ask how patterns of ramification and articulation have changed during development and evolution. For this reason, articulation morphology may be considered a new paradigm of plant morphology. It changes fundamentally our way of thinking about morphology and consequently morphological investigation. As an example, consider compound leaves. In articulation morphology, the primary question is no longer whether they are homologous to simple leaves or partial shoots. Instead, we focus on the transformation of development (ontogeny) that occurred during evolution and we find that compared to simple leaves, compound leaves acquired additional ramifications and articulations (see below for more examples).
In evo-devo, evolution is seen as the transformation of ontogenetic development. In terms of articulation morphology, this means that during development (ontogeny) and evolution (phylogeny) articles are added, eliminated, or replaced by others that are more or less different. To investigate this transformation - that is, the evolution of development (ontogeny) - we do not need homology because articulation morphology investigates transformation directly without the interference of morphological homology and the associated morphological categories. Thus, the problem or pseudo-problem of the categorical assignment of controversial structures does not arise in articulation morphology. However, if one wants to make comparisons between articles it may be done using fuzzy set theory, where differences range from 0% toward 100%. 0% means no difference, hence sameness or identity. If one does not or cannot quantify the difference, one can refer to “more or less different,” or “more or less similar.” One could interpret the difference or similarity as the homology of articles, ranging from total to partial or combinatorial homology (Sattler 1994, Minelli 2016, 2023). However, as noted above, the primary aim of articulation morphology is not morphological homology but transformation. Fossils, biography, ecology and developmental genetics may provide data that help to determine the direction of transformation during evolution. Other types of homology – beyond the morphological – may also play a role in the elucidation of evolution (Minelli 2018, Ochoterena et al. 2019).
How do we describe articles? I propose describing articles as process combinations according to process morphology (Sattler 1990, 1992). Considering processes such as growth duration and growth distribution resulting in radial, bilateral, or dorsiventral growth (see Sattler 1998), articles may differ in growth duration and may arise as radial, bilateral, or dorsiventral primordia. In the latter case, they may be oriented in the same plane as the article on which they are formed or in a transversal plane, that is, in a plane perpendicular to the plane of the parent article. For example, teeth in serrated leaves arise in the plane of the leaf, whereas leaflets may be formed in a plane perpendicular to that of the leaf (Arber 1950, p. 116, Rutishauser and Sattler 1997).
For convenience, familiar terms may be used for the process combinations, and new terms may be coined when necessary. For example, the process combinations that constitute what we commonly call a simple leaf may be referred to as a simple leaf as long as we keep in mind that a leaf is not just a static structure but a process combination. Other terms of mainstream morphology such as root and internode may also be used, and thus a limited continuity between mainstream morphology and articulation morphology is possible.
However, the difference between mainstream morphology and articulation morphology is more fundamental than may initially seem. Whereas some articles correspond to organs of mainstream morphology, others do not. For example, a simple leaf, an internode, or a root are equivalent to an article because they result from just one ramification. However, a pinnate leaf, which is considered one organ according to mainstream morphology, is a system of articles. For flowers, the difference between mainstream morphology and articulation morphology is even more striking: stamens and carpels are systems of articles, not leaf homologues, since leaf homologues are not defined in terms of ramification and articulation. Classen-Bockhoff (2016, 2024) referred to them as sporangiophores.
Since ramification is fundamental to articulation morphology, we have to distinguish different modes of ramification: dichotomous, lateral, and axillary ramification. Dichotomous ramification is common in liverworts such as Marchantia and the earliest telomic fossils such as Rhynia and Cooksonia. Whereas the articles in liverworts are dorsiventral, those in the earliest telomic fossils are mostly of radial symmetry, but a continuum from radial to dorsiventral symmetry has been documented (see Sattler 1998). In the telomic fossils it is most obvious that they consist of articles. An article arising from a single ramification that does not ramify further is called a telome, whereas an article between two successive ramifications is called a mesome, Both telomes and mesomes are telomes in the broad sense. Thus, a telomic fossil is a telome truss that consists entirely of telomes in the broad sense.
According to the telome theory (Zimmermann 1952, 1959, 1965), the diversity of vascular plants has evolved through elementary processes. These processes produced different patterns of ramification and articulation. The elementary process of overtopping leads from dichotomous to lateral ramification, whereas the elementary process of planation describes the change from three-dimensional to two-dimensional ramification. The elementary process of fusion has been implied in leaf formation, which has, however, led to criticism (Stein and Boyer 2006, Beerling and Fleming 2007). But Zimmermann himself (1959, p. 105, 1961) pointed out already that “fusion” should not be understood as an actual fusion but rather as a basipetal shift of growth.
To the extent that the telome theory explains the diversity of vascular plants, it corresponds to articulation morphology because telomes are articles, not organs. However, the telome theory is more limited than articulation morphology because it does not address plant structures such as enations, leaves of bryophytes and algae, and it is difficult or impossible to apply it to more highly evolved vascular plants such as seed plants in which individual telomes are usually no longer recognizable. Moreover, intercalary meristems leading to zonal growth play an important role, especially in flowers. For example, inferior ovaries may be formed through intercalary growth. Interprimordial growth leads to a continuity between primordia (often described as a “fusion”), as, for example, in the formation of a sympetalous corolla (for a more comprehensive discussion of this complex topic see Sattler 1978).
Whereas dichotomous ramification is rare in vascular plants, lateral ramification is widespread. Additionally, axillary ramification is characteristic of seed plants. Acknowledging this, Sachs (1882) proposed a schematic ground plan of seed plants, which was adopted by classical morphologists such as Troll (1954) and Kaplan (2022). It comprises a root system and a shoot system with a stem, leaves, and axillary buds. Hence, it represents the trinity of the classical organ categories: root, stem (caulome), and leaf (phyllome). Although this ground plan was meant for all seed plants, it applies only to a limited extent where organs coincide with articles such as simple leaves.
There appears to be a continuum from leaves to leaflets, stipules, enations, and hairs (Rutishauser and Isler 2001, Fig. 39, Rutishauser and Sattler 1986, Arber 1950, p.141). Articles span this whole continuum. Since hairs are often formed much later than the more prominent structures, we may distinguish two phases of ramification: an initial phase of the more prominent structures - traditionally often referred to as organogenesis - and a later phase of hair formation, but the two phases may overlap.
For a more complete description of ramifications, the spatial arrangement of articles has to be taken into account. If we refer to phyllotaxy in articulation morphology, this notion must be understood in a wider sense: not only as the positioning of leaves on the shoot apical meristem (SAM) but as the arrangement of all articles. These articles may arise sequentially (as in spiral phyllotaxy) or simultaneously (as in whorled and decussate phyllotaxy). Deviations from these patterns occur, especially in flowers.
Any particular plant or group of plants may be described by its sequence of ramifications and the resulting articulations. Thus, articulation morphology, based on open growth, encompasses the whole life cycle of plants. In seed plants, the embryo is typically bipolar, differentiating into two opposite poles: the root pole and the shoot pole (for exceptions see Claβen-Bockhoff 2024, pp. 599-600). The root pole develops into the main root, and subsequently side roots are formed through ramification and articulation. One, two, or more cotyledons (articles) arise from the apical region of the embryo lateral to the shoot pole. The hypocotyl (representing one article), is formed between the cotyledon(s) and the first side root. It unifies the two poles, creating oneness in duality. The shoot pole develops into the shoot apical meristem (SAM). As it ramifies through lateral and axillary meristems, it also forms internodes, each representing one article. Lateral meristems usually develop into simple leaves (each consisting of one article) or compound leaves (each comprising a system of articles), but occasionally uncommon structures (representing one or more articles) occur (see below). Axillary meristems normally produce a side branch, generated by a new SAM, but occasionally also other structures such as phylloclades (consisting of many or only one article) occur in the axillary position and rarely even the SAM may become transformed into a phylloclade (see below). Usually much later, hairs - minute articles - may be formed on various structures. Furthermore, common (typical) and uncommon (atypical) structures (consisting of one or more articles) may arise in various deviant positions. For example, typical roots may be formed on the hypocotyl and highly deviant structures such as haustoria may develop on roots, stems, and leaves (Claβen-Bockhoff, 2014, pp. 641-647). As in Daoism, where Yin comprises the Yang and vice versa, so the root retains the potential to develop shoots and the shoot may form roots.
In angiosperms, the SAM eventually becomes transformed into an inflorescence meristem, a floral unit-meristem, or directly into a floral meristem (Claβen-Bockhoff 2024). Usually, the first ramifications of the floral meristem produce the members of the perianth, each corresponding to one article, followed by the sporangiophores, which consist of a system of articles (Claβen-Bockhoff 2024).
Pteridophytes and bryophytes also exhibit a polar organization. In addition to a shoot, most Pteridophytes have roots, but they are not derived from an embryonic root pole opposite the shoot pole, as in seed plants. Even those plants lacking roots are polarized. Thallose liverworts such as Pellia epiphylla exhibit a vertical polarity: the lower side of their dorsiventral thallus faces the earth and its upper side the sky. In addition, the thallus is polarized in the horizontal plane: it grows at the front end while decaying at the rear, thus displaying a life and death pole. This dynamic grants it the potential for immortality. Contrary to most plants, such plants are not sedentary: as one end continues to grow, it slowly and imperceptibly moves across the surface of the ground.
The earlierst fossil land plants probably had a dorsiventral thallus resembling that of thallose liverworts, growing horizontally close to the surface of the ground. Subsequently, this prostrate growth became increasingly vertical (erect) as seen in extant liverworts such as Hymenophyton flabellatum (Schilperoord 2011, p. 62) and the telomic plants. This shift toward verticality was crucial for the evolution of the morphological diversity of vascular plants, culminating in the towering trees of seed plants, spanning ground and sky, earth and heaven, which the Chinese Daoists understand as an expression of Yin and Yang. According to Hagemann (1976, 1991) and Schilperoord (2011) fossil liverworts gave rise to vascular plants, whereas the predominant view holds that vascular plants evolved from fossil telomic plants (Classen-Bockhoff 2024). Sattler (1998) proposed a synthesis of these opposing views.
Research in plant architecture complements articulation morphology, especially with regard to higher-level units such as branches and the plant as a whole. The different architectural models of trees (Hallé et al. 1978, Barthélémy and Caraglio 2007) can be understood as strange attractors that are inherently fuzzy, thus requiring fuzzy logic for their proper analysis (Oldeman and Vester 1995).
Because of its practical terminology, classical mainstream morphology (Kaplan 2022, Sattler 2022, Sattler and Rutishauser 2023) also remains useful. However, it is limited in scope, as it cannot adequately account for structures that do not fit its categories. These structures have been referred to as "misfits" (Minelli 2015, Rutishauser 2005, 2016, 2020, Rutishauser et al. 2008). "Misfits" are "misfits to a botanical discipline [such as classical morphology], not misfits for a successful existence” (Bell 1991). One example of misfits are phylloclades, such as the sterile phylloclades of Semele androgyna of the Asparagaceae (Cooney-Sovetts and Sattler 1987). Their homology has been debated for centuries. However, for articulation morphology, they are not problematical: they simply represent dorsiventral articles. Thus, if we assume that the ancestors of Semele bore axillary branches, then during evolution these branches (a system of articles) have been replaced by a single dorsiventral article (except in the fertile phylloclades where the article bears inflorescences - systems of articles). In articulation morphology, the question of homology does not arise. We do not ask whether the single dorsiventral article is homologous with a leaf or a branch. Instead, we ask how development has changed during evolution (evo-devo). Endless debates about the homology of these structures are not helpful and appear futile because they are based on a pseudo-question – the assumption that a structure that does not fit the categories must nonetheless be fitted.
In Ruscus aculeatus, in addition to axillary phylloclades, terminal phylloclades also occur: the SAM (shoot apical meristem) is directly transformed into a doriventral structure that resembles a leaf.
Another example of a misfit is found in Chisocheton tenuis, where lateral structures in leaf positions exhibit indeterminate growth and bear inflorescences and vegetative shoots on their adaxial side (Fisher and Rutishauser 1990). Again, the homology of these structures has been debated for a long time without any final resolution (Fisher and Rutishauser 1990). Transcending homologization, articulation morphology understands them simply an elaborated system of articles that resembles pinnate leaves, but with indeterminate growth. Thus, during evolution pinnate leaves have acquired indeterminate growth that is typical of shoots.
Even typical pinnate leaves remain controversial (Rutishauser and Sattler 1997, Lacroix et al. 2003). They too cannot be clearly fitted into the leaf category. But from the perspective of articulation morphology, they simply represent an article that produces a secondary set of articles in a distichous arrangement.
An extreme example of a morphological misfit is Wolffia arhiza, the smallest vascular plant on Earth. It forms flattened, rootless structures, often referred to as fronds (Lemon and Posluszny 2000). Homologization seems impossible. But without attempting homologization, we can understand how the development (ontogeny) of Wolffia has evolved. Instead of forming a root and shoot pole, the embryo develops directly into a dorsiventral frond (article), without the intervention of a shoot apical meristem (SAM). Subsequently, additional fronds (articles) are formed through budding, which involves ramification and articulation, again without the intervention of a shoot apical meristem. Understanding these processes does not depend on identifying homologies. What matters is the transformation of development during evolution, not morphological homology – the assignment of organ categories. As Wake (2007) pointed out, debates on homology - in this case, whether the frond is homologous to a stem or leaf - are irrelevant to the investigation of developmental transformation during evolution and consequently are a distraction from these central issues. Since the developmental transformation of Wolffia deviates drastically from the usual pattern of flowering plants, Classen-Bockhoff (2024) included Wolffia in her list of exceptions to categorical classical morphology, and Minelli (2015) described it as a "misfit" that does not fit within the classical categories. Yet from the perspective of articulation morphology, Wolffia is neither an exception nor a misfit; it is only an uncommon pattern of articulation. There are no exceptions or misfits in articulation morphology. Hence, articulation morphology is more fundamental and more comprehensive than categorical organ-based morphology. Its principal advantages may be summarized as follows:
1. Ramification and articulation are intrinsic to open growth, the most fundamental process of plant morphology. As such, the formation of articles – the direct result of ramification and articulation - is fundamental. Although mainstream morphology acknowledges the importance of open growth, it has largely overlooked articles - the most fundamental morphological units – by leaping directly to the homology of organs.
2. Articles are directly observable, being empirically defined by ramification, whereas organs rely on a morphological theory and homology, such as the classical root-stem-leaf model.
3. Articles are continuous with one another. Organs are also continuous, but are often delimited by boundaries that do not exist in nature. For example, the definition of a “leaf” differs depending on whether one adopts the boundaries the root-stem-leaf model, the leaf skin model, or the metameric model.
4. Articulation morphology is more comprehensive than mainstream morphology. It accommodates all observed plant structures without exceptions. Structures that deviate from the common pattern are not misfits but simply uncommon ramifications and articulations. Changes in ramification and articulation are the explanation for these uncommon patterns.
5. Articles occur in both thalli and the cormus, whereas organs are confined to the cormus. Therefore, the notion of the article provides a unifying principle for all plants from algae to bryophytes to vascular plants.
6. Articles span all levels of organization, from structures that mainstream morphology calls organs (e.g., leaves) to leaflets, stipules, enations, and hairs, whereas organ-based morphology is limited to the level of organs.
7. Articles are not confined to mutually exclusive categories, whereas organs are traditionally classified as belonging to either one category or another such as stem or leaf.
8. From the perspective of articulation morphology, the focus in plant evo-devo is on changes in ramification and articulation. Homology is no longer the most basic issue, although it may still play a role in the elucidation of evolution.
Despite these differences between articulation morphology and categorical organ-based morphology, the gulf between the two could be partially bridged through the following adjustments in categorical organ-based morphology by
a. Recognizing that, like articles, organs cannot be demarcated by boundaries, as there is a continuum within a plant between organs such as the stem and leaf. This renders unnecessary the distinction of different shoot models based on different boundary placements (as explained above).
b. Acknowledging that some organs, such as compound leaves, constitute a system of structures, where one structure gives rise to other structures, commonly called leaflets. Although “leaflet” literally means “small leaf,” mainstream morphology defines a leaflet as a segment of a leaf rather than an independent leaf. Apart from considerations of homology, recognizing the equivalence of “segment” and “article” could provide a conceptual bridge between the mainstream view and articulation morphology.
c. Accepting that mutually exclusive organ categories are too rigid to encompass the full diversity of plant forms. If homology is still pursued, allowing, alongside categorical total homology, also partial homology (Sattler 1994), factorial or combinatorial homology (Minelli 1998, 2016, 2018).
As an alternative to mainstream morphology, one may redefine the leaf in an inclusive sense that transcends classical categories. In his botanical notes from his Italian journey, Goethe suggested this broader perspective with his hypothesis: "Alles ist Blatt” (All is leaf). Thus, a leaf that absorbs water we call a root, and a leaf of radial symmetry we call a stem (for the exact German quote see Schad 2005, p. 211). Schad (ibid.) expressed this broader view by defining the leaf as "jedes potenzreiche Grundgewebe" (any ground tissue rich in potential). In this sense, all plant structures, including those of algae, can be understood as leaves (Schad, personal communication). As Goethe pointed out, the simplicity of just one fundamental unit (instead of two or three basic categories) makes possible the greatest diversity (see Bortoft 1996, p. 80). If instead of “any ground tissue rich in potential,” we refer to a growth centre or primordium rich in potential, we arrive at the notion of the article, a unit arising from a primordium that may differentiate in a multitude of ways to produce the diversity of all plant forms. Like the leaf in the broad sense, the article constitutes a fundamental unit of plant construction. Thus, the diversity of plant forms arises through a differentiation of this basic unit, leaving no exceptions or misfits. In this unitary framework, it is no longer necessary to ask whether a structure is homologous with either this or that category, since mutually exclusive categories such as root, stem and leaf have been superseded. Aristotelian either/or logic is thereby transcended, especially where it leads to pseudo-questions.
Nine Approaches to Plant Morphology
To place articulation morphology into a broader context, I will briefly outline nine approaches to plant morphology, and then, in the next section, I will compare articulation morphology with the approaches most closely related to it.
1. Classical Morphology
Classical Morphology asserts that all structures, at least in seed plants, can be understood in terms of the root-stem-leaf model. This means that any organ we encounter must be a homologue of either a root, a stem, or a leaf. However, the categorization of atypical structures remains controversial. Despite these difficulties, classical morphology is still predominant in mainstream morphology.
Plant architecture (Hallé et al. 1978, Barthélémy and Caraglio 2007) may be seen as an extension of classical morphology. Since its focus is on higher-level units such as branches and the whole plant, it may encounter only rarely the problems or pseudo-problems of classical morphology.
2. Developmental Morphology
Developmental Morphology (according to Classen-Bockhoff 2024) explains the vast majority of vegetative plant structures according the categories of classical morphology, but recognizes, like Rutishauser (2005, 2016, 2020), that there are exceptions (misfits) that cannot be fitted into the classical categories. Flowers are considered de novo structures to which the categories of classical morphology do not apply.
Plant architecture is compatible with this approach.
3. Arber’s Partial-Shoot Theory
Arber’s Partial-Shoot Theory recognizes only one morphological category – the shoot (Arber 1950). According to this theory, leaves, leaflets, leaf lobes, even hairs and roots are partial shoots (ibid., 132-135, 140-142, 159). Since partiality is a matter of degree, Rutishauser and Isler (2001) introduced the term Fuzzy Arberian Morphology (FAM) in contrast to categorical classical morphology (ClaM).
4. Continuum Morphology
Continuum Morphology (Sattler and Jeune 1992, Rutishauser 2020) has two aspects: 1. A continuum within any individual plant since there are no clear-cut boundaries between parts of a plant such as roots, stems, and leaves, and 2. A continuum between the structural categories of classical morphology. The exceptions (misfits) of Classen-Bockhoff’s morphology are absorbed into this continuum, and therefore cease to be exceptions.
5. Process Morphology
Process morphology (Jeune and Sattler 1992, Rutishauser 2020) conceives structures as combinations of developmental processes. Τhese process combinations form a continuum as in continuum morphology. For practical purposes, process combinations may be referred to as structures.
6. Articulation Morphology
As in Continuum and Process Morphology, there are no exceptions (misfits) in Articulation Morphology. Like other approaches, articulation morphology is based on open growth, the most fundamental process that distinguishes plants from most animals. However, unlike mainstream morphology, articulation morphology recognizes that open growth implies ramification and articulation: the formation of articles, which have been overlooked in mainstream morphology. Articles are described in terms of process morphology as process combinations or, for practical purposes, as structures that correspond with process combinations. In articulation morphology, all morphological patterns are an expression of ramification and articulation. The most common patterns are those that are recognized by classical morphology. Those patterns that do not fit into the classical framework are simply unusual patterns of ramification and articulation. In contrast to other approaches whose central concept is homology, articulation morphology focuses on transformation: transformation of ramification and articulation during ontogeny and phylogeny.
Plant Architecture complements articulation morphology to the extent that it deals with whole plants and higher-level units such as branches.
7. Algorithmic Morphology
Algorithmic Plant Morphology involves modelling, simulation, and visualization of plant development using computational methods: computer graphics, formal language theory, and programming language design (Prusinkiewicz and Runions 2012, Runions et al. 2017, Di et al. 2021). As Minelli (2018, pp. 57-60) pointed out, this approach may involve “a view of the plant body very different from the traditional one, within which leaves, inflorescences, flowers, petals, stamens, etc. are ‘given’ - that is, represent (1) homologues … and (2) plant organs” (ibid., pp. 59-60). Instead, this approach focuses on “properties (e.g. ‘floweriness’ or ‘petalness’) with a specific …spatial distribution” (ibid., pp. 59-60). Algorithmic morphology may contribute to the causal analysis of plant development as, for example, in phyllotaxis research (Barabé and Lacroix 2020, Reinhardt and Gola 2022). Furthermore, algorithmic morphology has an aesthetic appeal (Prusinkiewicz and Lindenmayer 1996).
8. Causal Morphology
Causal Morphology investigates the causation of plant development on the background of the descriptive and comparative frameworks of the preceding approaches. Nowadays, the predominant causal analysis is through developmental genetics. Numerous genes affecting plant form have been identified. Epigenetic factors also affect gene expression. Even the experimenter may play a role in gene expression, which is known as the experimenter effect (Church 2018).
9. Functional Morphology
Functional morphology examines the functions of morphological traits. According to Bai (2017), the two major functions of plants are improvement of energy acquisition (photosynthesis) and adaptions to environmental stress.
Complementarity
Classical morphology can be considered complementary to continuum and process morphology because, although more limited, they seem appropriate and useful for the majority of structures and offer convenient terminology. Classen-Bockhoff (2024), by recognizing exceptions (misfits) that in classical morphology are forced into categories where they don’t belong, offers an important complement to classical morphology. Arber’s partial-shoot theory adds another perspective to our understanding of the leaf and the root. Continuum and process morphology extend classical and developmental morphology because they incorporate misfits into a continuum of structures or process combinations (Rutishauser 2020). Although classical and continuum morphology have been conceived as sub-classes of process morphology (Lacroix et al. 2005, Jeune et al. 2006), they can also be considered complementary. Likewise, articulation morphology and the other approaches to plant morphology are complementary: Articulation morphology emphasizes transformation during development (ontogeny) and evolution, algorithmic morphology contributes computer modelling, causal morphology analyzes developmental causation, and functional morphology is concerned with the functions of structures. Thus, each approach contributes differently to a better understanding of plant morphology. At least some of the nine approaches overlap and the list is not exhaustive. Other approaches such as the metameric (phytomeric) model (the modular approach) could be added, and Plant Architecture might be considered a separate approach.
Homology
Homology is widely regarded as the most basic and central concept of morphology, yet there is no consensus on how it should be defined (Hall 1994). Since Owen’s (1943) classic definition as the sameness of organs, many different definitions have proliferated (Hall 1994, Ochoterena et al. 2019). It is difficult to find a common denominator for this array of definitions, but perhaps one could venture to suggest that they all revolve around the idea of correspondence and similarity or sameness with or without reference to common ancestry. Throughout this essay, “homology” refers specifically to morphological homology, even when not explicitly stated.
How do the nine approaches relate to homology, if at all? In classical morphology and developmental morphology (according to Classen-Bockhoff 2024), homology is defined as sameness or essential similarity. Thus, if two organs belong to the same morphological category, such as stem or leaf, they are homologous. This kind of homology is based on either/or logic: an organ either belongs to a category or not. Problems – or rather pseudo-problems - arise if an organ does not fit any of the categories, for example, if it is intermediate between two categories. In such cases, endless debates have resulted into which of the two categories the organ should be forced. To end such futile debates, Classen-Bockhoff (2024) acknowledges that there are exceptions (misfits) such as Wolffia that do not fit the categories and therefore cannot and should not be homologized. Furthermore, she concluded that homology does not apply to flowers and their sporangiophores, which she regards as de novo structures not comparable to the vegetative shoot. This highlights the limitations of the classical homology concept. If, however, we accept the concept of partial, factorial or combinatorial homology (Sattler 1994, Minelli 2018), then misfits can be accommodated as structures that are partially homologous to more than one category. For example, the phylloclade of Ruscus aculeatus can be seen as partially homologous to an axillary shoot and a leaf (Cooney-Sovetts and Sattler 1987). This combinatorial view of homology is implied in continuum and process morphology (Sattler 1994). And it has also been recognized in causal morphology. For example, during the development of the phylloclade of Ruscus aculeatus genes are expressed that are normally expressed in the shoot apex and leaves (Hirayama et al. 2007). Therefore, these authors concluded that “the phylloclade is not homologous to either the shoot or the leaf, but that it has a double identity” (ibid.). This represents a shift from either/or logic to both/and logic (Sattler 2018), thus offering a more inclusive understanding of homology.
Arber’s Partial-Shoot Theory does not explicitly rely on homology (Arber 1950). Since, according to her theory, all structures are a partial shoot, the question of whether they are homologous to this or that category does not arise. There is only one fundamental category: the shoot. As such, her theory is a unifying framework like Goethe’s and Schad’s leaf theory (see above).
Similarly, functional morphology and algorithmic approach do not rely on homology. The primary aim of the latter is to generate the widest possible range of morphological patterns. However, once these patterns have been generated, they could be compared in terms of total and partial homology.
In articulation morphology, homology is likewise not foundational. Understanding evolution as changes in ramification and articulation, does not require homology; for example, we do not need to ask whether compound leaves are homologous with simple leaves or partial-shoots. The primary concern is tracing transformation of ramification and articulation. Nevertheless, homology may be considered after transformation has been analyzed
To sum up, although homology is still widely regarded as the most basic and central concept of morphology, among the nine morphological approaches discussed above, Arber’s partial-shoot theory, algorithmic morphology, functional morphology and articulation morphology operate without recourse to homology, whereas developmental and causal morphology invoke homology only to a limited extent.
Articulation Morphology, Continuum and Process Morphology
Articulation Morphology includes both continuum and process morphology (Sattler 1974, Sattler 1992, Rutishauser 2020) and also goes beyond them. While continuum morphology relies on the classical categories as a reference system and demonstrates a continuum between them (Sattler and Jeune 1992), articulation morphology operates independently of these classical categories. It can, however, retrospectively reveal that common patterns of ramification and articulation correspond with classical categories. It may also be possible to devise a continuum morphology that does not rely on the classical categories as a reference system.
Process morphology can be understood in at least two ways:
1. As in continuum morphology, the categories of classical morphology are used as reference points, but here they are understood as process combinations. They are linked through a continuum of process combinations, which leads to a dynamic continuum (Jeune and Sattler 1992).
2. Alternatively, process morphology and process combinations can also be understood independently of the reference system of classical morphology. In this broader context, articulation morphology is a uniquely dynamic approach based on the fundamental process of open growth that implies the equally fundamental processes of ramification and articulation. It explores how these processes manifest during ontogeny and how ontogeny has changed during evolution, hence evo-devo.
Continuum morphology (Sattler and Jeune 1992) and process morphology (Jeune and Sattler (1992) did not address ontogeny, but only compared typical and atypical structures or process combination and demonstrated that they are linked through a continuum. Since articulation morphology incorporates ontogeny from the embryo to the mature plant, it is more comprehensive than continuum and process morphology but includes them since articles form a continuum of process combination.
If the continuum of continuum and process morphology is conceived as extending horizontally and ontogeny as vertically, then articulation morphology can be seen to surpass both continuum and process morphology because in addition to the horizontal dimension it incorporates also the vertical dimension, thereby making it more comprehensive.
Homology - both total and partial – has been implied in continuum and process morphology (Sattler 1994). In continuum morphology, structures are totally or partially homologous (Sattler and Jeune 1992), whereas in the dynamic continuum, it is the process combinations that show total and partial homology (Sattler 1994). However, it may be possible to conceive of a continuum and process morphology that does not imply homology. In articulation morphology, transformation, not homology, is the most basic concept: transformation of patterns of ramification and articulations. Subsequently, these patterns may be compared in terms of total and partial homology, thus linking articulation morphology with the dynamic continuum – the dynamic interpretation of continuum morphology.
Shared Background of the Morphological Approaches
The morphological approaches, different as they are, nonetheless share a common background. This background is often taken for granted and many morphologists may not even be aware of it, but it is relevant to the way we understand facts. For naïve realists, facts exist independently of us, whereas to critical observers, facts constitute a consensus that is due to a common background. This common background is not easily elucidated, but several of its key dimensions can be outlined:
1. Morphology deals with the physical aspect of plant form. This aspect of form is perceived through visually and interpreted intellectually. Therefore, Arber (1950, p.211, 1954) understood morphology as a synthesis of the eye and the intellect or mind. Materialists maintain that only the physical realm exists and mind is seen as an epiphenomenon of the brain - hence, fundamentally also physical. Yet much evidence points to a reality beyond the material that has been referred to by different names such as spirit (e.g., Ravindra 1991) or consciousness (e.g., Greene 2009, Hoffman 2019). Although morphology is restricted to physicality, it is important to realize that physical form – the very subject of morphology - emerges from a more encompassing reality. Contemplating forms in nature - such as a flower - may lead us to an awareness of this deeper reality.
2. The physical aspect of plant form is described and analyzed in terms of the categories of space and time, which do not exist independently of us but are our common way of perceiving reality. Mystics and some poets have long recognized that space and time do not constitute ultimate reality. In Siddhartha, Hermann Hesse (1951) wrote: “Time is not real…And if time is not real, then the dividing line that seems to lie between this world and eternity…is also an illusion.” Similarly, William Blake concluded:
To see the world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour
Plant morphologists may therfore realize that the common experience of plant form in terms of space and time emerges from a deeper reality of infinity and eternity.
3. Morphologists rely on language to describe and analyze plant form. However, words and concepts cannot fully capture reality as it is; they are an abstraction from reality. Abstracting means selecting some features while omitting others. Therefore, Korzybski (1933) concluded that whatever you say about reality is not reality. Language functions like a map: useful for navigation, but ultimately not the territory it represents: “the map is not the territory”
How does all this relate to articulation morphology? I emphasized that, in contrast to organs that are based on a morphological theory, articles are factual. However, this does not mean that they represent ultimate reality. They represent a consensus of morphologists that is based on a widely shared background – but a consensus that captures only an aspect of the reality of plants. An awareness of these limitations can lead to a deeper understanding of plants - from which morphology itself emerges. In this light, contemplating a leaf or a flower can open a doorway to a more profound reality.
Articulation morphology, in this sense, may be seen as an expression of the profound wisdom of the Heart Sutra, which, in Tanahashi’s (2014) translation, states:
Form is boundlessness;
boundlessness is form.
Since articles are continuous within the whole plant, their form is boundless, yet this boundlessness manifests as the form of articles. Thus, there is no contradiction between boundlessness and articulation, which is indicated by saying that the plant is an articulated whole, a unity in multiplicity, in which form and boundlessness coincide.
Beyond its morphology, a plant extends into its environment. Thus, it is continuous with the soil and the air, the earth and the sky. Ultimately, it is interconnected with everything and thus is integrated into the universe, the most inclusive whole.
Evo-Devo and Articulation Morphology
According to evo-devo, the evolution of plants is the evolution of plant development (Minelli 2018, Rutishauser 2020). Morpho evo-devo emphasizes the morphological aspects of evo-devo (Wanninger 2015, Petrone-Mendoza et al. 2023). Within this framework, articulation morphology investigates morphological changes in ramification and articulation. The evolution of land plants involved a shift from dichotomous to lateral ramification and eventually also to axillary branching. In many cases, laterally formed articles gave rise to additional articles - for example, in compound leaves. Whereas in mainstream morphology simple and compound leaves are considered homologous, articulation morphology emphasizes transformation from one ramification in simple leaves to additional ramifications in compound leaves. What counts are ramifications and the resulting articulation. Hence, the challenge in evo-devo is to explain the change in ramification and articulation - observable processes that do not depend on a controversial morphological theory and homology. Therefore, instead of asking how organs such as stamens and carpels changed, the more fundamental question becomes how and why ramification and articulation changed.
Although a change in ramification and articulation is of central importance in evolution, we have to recognize also the prevalent repetition of ramification and articulation in both development and evolution. For example, the same or similar articles, such as a particular leaf type, may be produced repeatedly along the stem. Similarly, much repetition occurs during evolution. Only some patterns of ramification and articulation change, whereas many others are retained over long periods.
Switching from an organ-based approach to one centered on ramification and articulation can change the questions we ask and the insights we obtain. For example, consider carpels. In mainstream morphology, a carpel is interpreted as a closed megasporophyll, that is, a leaf homologue. In contrast, according to articulation morphology, ramification at the floral apex produces a carpel primordium that develops into an article that I prefer to call a gynoecial appendage (Sattler 2024). As the gynoecial appendage develops, further ramification leads to the formation of ovules: first the nucelli, then the integument(s). Thus, the formation of a carpel (a gynoecial appendage with ovules) involves three or four ramifications resulting in four kinds of articles: the gynoecial appendage, the nucellus, and one or two integuments. Therefore, contrary to the organ-based approach of mainstream morphology, the question in articulation morphology is no longer whether the carpel is a leaf homologue. The question is how ramification and articulation have changed. This opens a new direction for evo-devo research: instead of analyzing the carpel as a single organ, we must enquire about the ramifications and the resulting articulations: Where is the placenta, which is an article, formed? How does it ramify, and what articles result from the ramification? Since these questions concern morphology, the fundamental importance of morphology in evo-devo research becomes evident. Before turning to analyses of developmental genetics, which are important in evo-devo, the initial questions are framed in morphological terms, and here we have the choice between categorical concepts of mainstream morphology, laden with problematical assumptions of homology, or concepts of articulation morphology that refer to directly observable processes, which lead to the formation of articles. By focusing on articles - the fundamental units of plant morphology - articulation morphology can reorient analyses of developmental genetics.
Notably, even without reference to the theory of anaphytes, which has been largely forgotten, some researchers have already moved toward its modern version that I call Articulation Morphology. Thus, from the perspective of developmental genetics that is central in evo-devo, Mathews and Kramer (2012) concluded “that the carpel is a complex organ consisting of a foliaceous appendage and the placenta,” that is, two articles: the foliaceous appendage and the placenta. On morphological grounds, Sattler (2024) reached the same conclusion. In both cases, the complexity arises through ramification and articulation.
Thus, articulation morphology provides a coherent framework for integrating morphology and developmental genetics in evo-devo. By focusing on articles rather than organs, it can reveal aspects overlooked by organ-based approaches and thereby open new avenues for evo-devo research.
Conclusions
Like animals, plants exhibit polarity; but unlike animals, plants have open growth, which may continue throughout their life. Open growth leads to ramification, and ramification leads to articulation: the formation of an article from each ramification and between successive ramifications. Since ramification and articulation are intrinsic to open growth, open growth constitutes the most basic and distinctive process of plant construction. It entails continuous development leading to the genesis of articles. Thus, plant development can be understood as continuous articulated growth and differentiation. Growth and differentiation are two basic processes in process morphology besides decay and dedifferentiation (Sattler 1990, 1992).
According to articulation morphology – which could also be called segmentation morphology - a plant is an articulated (or segmented) whole: a system of interconnected articles arising through ramification. In contrast to mainstream morphology, whose fundamental units are organs, in articulation morphology the fundamental units are articles. Articles originate as growth centres (primordia) in continuity with preceding articles. They are directly observable since ramifications can be observed, and in this sense, they are not controversial. They provide an empirical basis. In contrast, organs are defined within the framework of a morphological theory, such as the classical root-stem-leaf theory of mainstream morphology, which is controversial and limited. However, although articulation morphology is based on articles instead of organs, a connection to organs can be made. If an article does not ramify further, it may correspond to an organ. Thus, for example, a simple leaf is an article and also an organ. However, if a leaf produces leaflets, it corresponds to a system of articles. Articles between ramifications, such as internodes, do not correspond to an organ.
Ramification can also be understood as an expression of differential growth, since differential growth may give rise to articulation. And the development of the articles can be seen as a process of differentiation. Thus, instead of referring to a plant as an articulated whole, one could conceive of it as a differentiated whole that produces articles. Instead of ‘articles,’ one could refer to segments, structural units, or simply structures defined by ramification – that is, by differential growth. Articulation can then be described as structuration: the formation of structures. These structures are understood dynamically as process combinations, and they encompass the whole continuum from organs to hairs (Rutishauser and Isler 2001, Fig. 39).
Mainstream morphology remains useful and complementary to articulation morphology, but it cannot accommodate structures that deviate from its rigidly categorical, organ-based framework. In contrast, articulation morphology is all-inclusive: even the most deviant structures can be understood as deviant patterns of ramification and articulation. Thus, articulation morphology provides a more comprehensive framework that can open new avenues for evo-devo research.
Being all-inclusive, articulation morphology is also unifying because the notion of the article applies to all plants from algae to bryophytes and vascular plants. In contrast, the notion of the organ divides plants into those with organs and those without organs such as algae and the telomic fossils. This division obscures the structural unity underlying all plant construction.
If one wants to make comparisons between articles, one may apply fuzzy set theory, according to which differences range from 0% toward 100%, which means “more or less different” in ordinary language. One might interpret the difference, which corresponds to degrees of similarity, as the homology of articles, ranging from total to partial homology, with total homology being 0% difference (sameness). However, in articulation morphology, the central and most basic concept is no longer morphological homology but transformation: the transformation of ramification and articulation. This changes the most basic questions we ask. Instead of asking questions about morphological homology, we ask how ramification and articulation have changed during development and evolution – a shift in focus that changes fundamentally our way of thinking about morphology and consequently morphological investigation. Thus, articulation morphology investigates transformation directly without the interference of morphological homology, whereas mainstream morphology uses morphological homology as its most basic and central concept and infers the transformation of development during evolution within the framework of morphological homology with all its ensuing unresolved problems and controversies.
Throughout the history of plant morphology, a wide range of morphological theories and concepts have been proposed (Cusset 1982). Their proponents have often been more or less critical, or even hostile to each other. However, at least some morphologists have become more tolerant, viewing different approaches as complementary rather than mutually exclusive (Rutishauser and Sattler 1985, Classen-Bockhoff 2024). Articulation morphology can go even one step further since it offers a factual non-controversial foundation that can serve as a common ground for all morphologists, regardless of their theoretical preferences.
Briefly, the significance of articulation morphology is at least two-fold: first, it moves beyond the rigidity of the categories of classical mainstream morphology that may create pseudo-problems; and second, it introduces an empirical, all-inclusive, unifying, process-oriented approach: empirical because it is based on the observable processes of open growth, ramification and articulation; all-inclusive and unifying because it includes all exceptions (misfits), which thereby cease to be exceptions, and process-oriented because it is based on the fundamental process of open growth that implies ramification and articulation: the formation of articles, whose existence has been largely overlooked because of the organ-centered approach of mainstream morphology. This essay seeks to reintroduce these articles – that could also be referred to as segments, structural units or simply structures defined by ramification – and to highlight their fundamental significance for plant morphology and plant evo-devo.
Acknowledgements: I am very grateful to Regine Classen-Bockhoff, Daniel Faccini, Bruce Kirchoff, Alessandro Minelli, and Rolf Rutishauser for their critical evaluations of the first versions of this essay, which contributed to its improvement and expansion.
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