Articulation Morphology of Plants and Plant Evo-Devo: An empirical, dynamic, all-inclusive, and unifying approach based on open growth, ramification and articulation, inspired by the theory of anaphytes
Rolf Sattler
Latest update on December 28, 2025
Abstract
In Articulation Morphology, inspired by the theory of anaphytes that was first proposed in 1843, ramification is the key principle in plant morphology in the open growth of plants. 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 While the theory of anaphytes included tenets that are now considered outdated, Articulation Morphology - proposed here as a modern version of this theory - retains and develops only those aspects that remain valid and fundamentally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, namely 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 controversial and limited 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. Furthermore, articulation morphology is unifying because articles constitute a fundamental morphological unit that applies to all plants from algae to bryophytes and vascular plants, whereas organ-centred classical mainstream morphology lacks such a fundamental unifying unit for all plants. The central concept of articulation morphology is no longer homology but transformation - the transformation of ramification and articulation. Owing to this fundamental shift and to its empirical, dynamic, all-inclusive, and unifying foundation, articulation morphology may be regarded as a new paradigm for plant morphology. From this perspective, plant evo-devo becomes the investigation of the development and evolution of ramification and articulation.
Keywords: fundamentals of plant morphology; plant evo-devo; plant morpho evo-devo; homology in plant morphology; ramification in plants; process morphology; approaches to plant morphology; complementarity in plant morphology; conceptual foundations of plant morphology; philosophy of plant morphology
Introduction
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 interpreted as 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 these three organ categories (Rutishauser 2016, 2020; Rutishauser et al. 2008; Sattler and Rutishauser 2023; Classen-Bockhoff 2024). As a result, persistent debates have arisen about the categorical assignment of these controversial structures. These controversies remain unresolved because they amount to pseudo-problems: attempts to categorize structures that do not fit the categories (for examples, see below). Articulation morphology offers a way out of this impasse. Inspired by the theory of anaphytes (anaphytosis), it may be regarded as a modern extension of this theory. It provides an empirical, dynamic, all-inclusive, and unifying approach based on the open growth of plants that occurs through ramification and articulation: the formation of articles (segments such as internodes and simple leaves). Since it is based on the observable processes of ramification and articulation, articulation morphology is inherently empirical and dynamic. By accommodating all structures, including those that do not fit within the categorical framework of classical mainstream morphology, it is all-inclusive. Moreover, by introducing the concept of the article (or segment) that applies to all plants from algae to bryophytes and vascular plants, it achieves a unification that is lacking in classical mainstream morphology. However, the alternation of generations is generally recognized as a unifying principle of bryophytes, vascular plants, and even algae. Alon Israeli emphasizes this unification of the Viridiplantae in his yet unpublished paper “A new conceptual framework in plant evolution.”
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, pseudo-problems of classical mainstream morphology do not arise (Schultz 1943; Schultz-Schultzenstein 1867). According to this theory, plant morphology results from two fundamental processes: ramification (branching) and articulation. Both processes are directly observable. We can observe that during the development of plants, ramification occurs; it 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, whereas an internode is an article between successive ramifications. 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 capable of forming 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 the homology associated with it, 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, one could also speak of open morphology, emphasizing open growth, which leads to ramification and articulation. Open Morphology can also be understood as being open toward other complementary approaches to morphology (see below).
Besides implying a morphological theory and homology, mainstream morphology often invokes demarcations between organs 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 (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 indeed the whole plant - is 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). However, phytomers - consisting of a node, the internode below, the leaf, the axillary bud and even roots if 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 at the cost of drawing a boundary between the stem and the leaf. Other models of the shoot avoid one discontinuum, only 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 ramify, the segment between successive ramifications constitutes an article. Since articulation morphology 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.
Even between organs, no boundary can be observed. Yet in mainstream morphology, a boundary is often assumed, but some mainstream morphologists acknowledge its non-existence. 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 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: in mainstream morphology, organs are defined in terms of a morphological theory 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 shortcomings and pseudo-problems of the classical theory, which cannot account for structures that do not fit into its categorical framework. Claβen-Bockhoff (2024) avoids these pseudo-problems by acknowledging that extreme forms cannot be accommodated by mainstream morphology. However, she retains organs as the fundamental morphological units, and organs are delimited by the classical theory and homology. Thus, a compound leaf is an organ homologous with a simple leaf. This tenet has led to seemingly unresolvable controversies (Arber 1950; Rutishauser and Sattler 1997; Lacroix et al. 2003) – controversies that are surpassed by articulation morphology because it is not based on organs but on the observable processes of ramification and articulation. Thus, we can observe that a compound leaf is a system of articles, whereas a simple leaf represents only a single article, and in this sense, they are not homologous.
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 fundamentally changes 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, especially categorical (classificatory) homology in terms of morphological categories. Thus, the problem or pseudo-problem of the categorical assignment of controversial structures does not arise in articulation morphology. However, if one wishes to compare 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 emphasized above, the primary aim of articulation morphology is not morphological homology but transformation. Fossils, biogeography, 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 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 is equivalent to an article because they result from a single ramification. However, a pinnate leaf, which is considered one organ according to mainstream morphology, is a system of articles. In flowers, the difference between mainstream morphology and articulation morphology is even more striking: stamens and carpels are systems of articles, not leaf homologues.
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. 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 that 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 invoked 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 “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, pp. 82, 139-141). 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.
Stem scales in Ledermanniella of the Podostemaceae (Moline et al. 2007), also sometimes referred to as leaves, are not vascularized and thus can be considered enations (emergences). They are inserted irregularly around the stem, whereas the vascularized leaves are arranged in a distichous order.
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, from which side roots arise 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). 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 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 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, these plants are not sedentary: as one end continues to grow, it slowly and imperceptibly moves across the surface of the ground.
The earliest 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 wide applicability and 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" (Bell 1991, Minelli 2015, Rutishauser 2005, 2016, 2020, Rutishauser and Isler 2001, Rutishauser et al. 2008). As Bell (1991) emphasized, misfits are "misfits to a botanical discipline [such as classical morphology], not misfits for a successful existence.”
One example of misfits is 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 unproblematic: they simply represent dorsiventral articles. If 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 forced into them.
In Ruscus aculeatus, in addition to axillary phylloclades, terminal phylloclades occur: the shoot apical meristem (SAM) is directly transformed into a doriventral structure that resembles a leaf.
Another example of a misfit is found in Chisocheton tenuis (Meliaceae), 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 as an elaborated system of articles resembling 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 1995; 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. This articulate view is important and significant because it supersedes the persistent debate over whether a compound leaf constitutes a single organ homologous with a simple leaf or a shoot or partial shoot. Nonetheless, if desired, questions of homology may be considered. Some molecular geneticists have argued that morphological homology can be inferred from gene expression (e.g. Hasson et al. 2010). However, a structure such as a compound leaf is much more than genes. It cannot be reduced to genes alone. Hence, morphological homology cannot be inferred from gene expression (Rutishauser 2020, 3.8). That said, gene expression and molecular networks represent an important aspect of plant development. For example, KNOX1 expression normally occurs in the shoot apical meristem (SAM). “Reactivation of KNOX1 expression during leaf development has been found in a large number of plants with compound leaves, where it marks the positions where leaflets will appear” (Minelli, p. 131)
Returning to morphology, 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 and unnecessary. As Wake (2007) pointed out, debates over homology - in this case, whether the frond is homologous to a stem or leaf - are irrelevant to understanding developmental transformation during evolution and consequently distract from this central issue. Since the developmental transformation of Wolffia deviates drastically from the typical pattern of flowering plants, Classen-Bockhoff (2024, pp. 354-5) included its frond among extreme forms that do not fit the categories of classical morphology, and Minelli (2015) accordingly referred to it as a misfit. Yet for articulation morphology, the frond of Wolffia is not a misfit but merely an uncommon pattern of articulation. There are no misfits in articulation morphology - everything fits. 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 therefore 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 depend 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 of the root-stem-leaf model, the leaf skin model, or the metameric model.
4. Articulation morphology is more comprehensive than mainstream morphology. Unlike mainstream morphology, articulation morphology 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 the 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 categories such as root, stem or leaf.
8. Contrary to mainstream morphology, in articulation morphology, homology is no longer the central concept. Instead, the focus is on transformation – transformation of ramification and articulation. However, if desired, questions of homology may still be pursued, especially if the notion of partial homology is included.
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 more widely that, like articles, organs cannot be demarcated by boundaries, since 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). Using fuzzy logic in addition to Aristotelian identity and either/or logic.
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 the 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 all structures are homologous with one or another mutually exclusive category, such as root, stem and leaf. Aristotelian either/or logic is thereby transcended 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 (e.g., Kaplan 2022; Sattler 2022). 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. Because it focuses on higher-level units such as branches and the whole plant, it may encounter the problems or pseudo-problems of classical morphology only rarely.
2. Evolutionary Developmental Morphology (Evo-Devo Morphology)
Evolutionary Developmental Morphology (according to Classen-Bockhoff 2024) explains the vast majority of vegetative structures according to the basic organs of classical morphology, but - like Rutishauser (2005, 2016, 2020) - recognizes the existence of extreme forms (misfits according to Rutishauser) 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 extreme forms of Classen-Bockhoff’s morphology are absorbed into this continuum, and therefore cease to be exceptions or misfits
5. Process Morphology
Process morphology (Jeune and Sattler 1992, Rutishauser 2020) conceives structures as combinations of morphogenetic 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 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 largely overlooked in mainstream morphology. Articles are described in terms of process morphology as process combinations or, for practical purposes, as structures corresponding to process combinations. In articulation morphology, all morphological patterns are an expression of ramification and articulation. The most common patterns are those 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, including 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 considerable 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. Gene expression may be influenced by epigenetic factors. Even the experimenter may play a role in gene expression, a phenomenon 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 the improvement of energy acquisition (photosynthesis) and adaptations to environmental stress.
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
Complementarity
All nine approaches complement one another to various degrees. 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, p. 355), by not forcing extreme forms (misfits) into the classical 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 by incorporating misfits into a continuum of structures or process combinations so that they are no longer misfits (Rutishauser 2020). Although classical and continuum morphology have been conceived as subclasses 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 in a different way to a better understanding of plant morphology. For a more complete understanding of the nine approaches we must examine homology.
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 among these definitions, but one might venture to suggest that they all revolve around the ideas 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 evolutionary 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 does not. Problems – or rather pseudo-problems - arise when an organ does not fit any of the categories, for example, if it is intermediate between two categories. In such cases, endless debates have ensued over which of the two categories the organ should be forced into. To end such futile debates, Classen-Bockhoff (2024) acknowledges that there are extreme forms, such as Wolffia, that do not fit the classical categories and therefore cannot and should not be homologized with them. Furthermore, she concluded that flowers are de novo structures not comparable to the vegetative shoot and that therefore the sporangiophores of flowers are not homologous to leaves (phyllomes). 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), such 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. Hirayama et al. (2007) demonstrated that during the development of the phylloclade of Ruscus aculeatus, genes are expressed that are normally expressed in both the shoot apex and leaves. 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 one or another category does not arise. There is only one fundamental category: the shoot. As such, her theory constitutes a unifying framework like Goethe’s and Schad’s leaf theories (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, it is not necessary to ask whether compound leaves are homologous with simple leaves or partial shoots. The primary concern is tracing transformations of ramification and articulation. Nevertheless, homology may be considered after the 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 evolutionary developmental and causal morphology invoke homology only to a limited extent.
Articulation Morphology, Continuum and Process Morphology
In addition to homology, the relationship of articulation morphology to continuum and process morphology requires clarification. 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 sense, articulation morphology is a uniquely dynamic approach based on the fundamental process of open growth, which implies the equally fundamental processes of ramification and articulation. It investigates how these processes manifest during ontogeny and how ontogeny has changed during evolution, that is, within the evo-devo framework.
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 combinations, demonstrating 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, articulation morphology can be seen as surpassing both continuum and process morphology because, in addition to the horizontal dimension, it also incorporates 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 for 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 visually and interpreted intellectually. Therefore, Arber (1950, p.211, 1954) understood morphology as a synthesis of the eye and the mind. Materialists maintain that only the physical realm exists, and the mind is seen as an epiphenomenon of the brain - hence, fundamentally also physical. Yet much evidence points to a reality beyond the material, 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 therefore 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, 2010) concluded that whatever you say about reality is not reality itself. Language functions like a map: useful for navigation, but ultimately not the territory it represents: “the map is not the territory” (Korzybski, ibid.)
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 an abstraction that captures the empirical, dynamic, all-inclusive and unifying aspect of the reality of plants. An awareness of this limitation can lead to a deeper understanding of plants, from which morphology itself emerges. In this light, contemplating a leaf or a flower may open a doorway to a more profound reality.
In this sense, articles of articulation morphology 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). Articulation Morphology provides a conceptual framework for morpho evo-devo. Within this framework, it investigates morphological changes in ramification and articulation in the evolution of development. 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 organs, articulation morphology emphasizes the transformation from a single ramification in simple leaves to additional ramifications in compound leaves. What matters are the ramifications and the resulting articulation. Therefore, instead of or in addition to enquiring about the homology of organs, articulation morphology poses the more fundamental question of how patterns of ramification and articulation have changed during ontogeny and phylogeny.
Although change in ramification and articulation is central to evolution, we also have to recognize the prevalent repetition of these processes in both development and evolution. The same or similar articles, such as a particular leaf type, may be produced repeatedly along the stem and may persist over long evolutionary periods. Only some patterns of ramification and articulation change, whereas many others are retained over long periods.
Switching from the organ-based approach of classical mainstream morphology to one centred 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, an organ homologous with a leaf. In contrast, according to articulation morphology, ramification at the floral apex produces a carpel primordium that develops into an article, which I prefer to call a gynoecial appendage (Sattler 2024). As this gynoecial appendage develops, further ramification leads to the formation of ovules: first the nucelli and subsequently the integument(s). Thus, the formation of a carpel (a gynoecial appendage bearing ovules) involves three or four successive ramifications, resulting in four or five kinds of articles: the gynoecial appendage, the placenta, the nucellus, and one or two integuments. From this perspective, 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 reorientation opens a new direction for evo-devo research: instead of analyzing the carpel as a single organ, we enquire about the ramifications and the resulting articulations: where is the placenta - an article - formed? How does it ramify, and which articles result from the ramification? Since these questions concern morphology, the fundamental importance of morphology in evo-devo research becomes evident. Thus, the initial questions are framed in morphological terms, and here we have the choice between the categorical concepts of mainstream morphology, laden with problematic assumptions of homology, or concepts of articulation morphology that refer to directly observable morphogenetic processes leading to the formation of articles.
Replacing organs with articles affects not only morpho evo-devo but also developmental genetics, which plays a central role in evo-devo besides morphology. This shift directs the analysis of gene expression and molecular networks from organs to articles, and by analyzing articles – instead of organs – new insights can be gained. For example, with regard to the gynoecium, some developmental geneticists have taken a first step in this direction. Mathews and Kramer (2012) recognized “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. A further step has been taken in the elucidation of the developmental genetics of the nucellus and integuments (Jiang et al. 2023). Sattler (2024) also took a first step, but from a morphological point of view, by distinguishing the gynoecial appendage from the placenta or ovule (in gynoecia with only one ovule; see also Zhang et al 2019). Articulation morphology allows a further step by recognizing the nucellus and the integument(s) as additional articles.
Although articulation morphology is fundamentally a morphological approach, it can be integrated with molecular genetics, which means that articles would be understood not only morphologically, in terms of morphogenetic processes, but also at the molecular level, in terms of gene expression and regulatory networks. For example, in many species with compound leaves, leaflets can be described as articles that exhibit dorsiventral, determinate growth and arise along the leaf margin in zones of KNOX genes reactivation (Hay and Tsiantis 2010). However, such integration would be beyond articulation morphology, whose major goal is to conceive morpho evo-devo in terms of articles rather than organs and to direct investigations of developmental genetics toward articles as the observable, all-inclusive and unifying structural units of all plants. By doing so, articulation morphology can reveal aspects beyond 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 and provide an empirical basis. Organs, by contrast, are defined within the framework of a morphological theory and its associated homology concept, such as the classical root-stem-leaf theory of mainstream morphology, which is controversial and limited. 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.
Mainstream morphology remains useful and complementary to articulation morphology, but it cannot accommodate structures that deviate from its 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 by encouraging new questions in terms of ramification and articulation. On this basis, an integration of articulation morphology with molecular genetics may also be explored.
Because it is all-inclusive, articulation morphology is also unifying since the notion of the article applies to all plants from algae to bryophytes and vascular plants, whereas the notion of the organ divides plants into those with organs and those without, such as algae and the telomic fossils. This division obscures the structural unity underlying all plant construction. Even within vascular plants, the organ concept creates a division between organs and other structures such as leaflets, stipules, enations and hairs, whereas the notion of the article applies to all structures.
If one wishes to compare 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 may 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 fundamentally changes 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). And their proponents have often been more or less critical, or even hostile, toward one another. More recently, 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 also recognizes its complementarity with other morphological approaches, including classical mainstream morphology. Furthermore, it goes even one step further since it offers a factual, non-controversial foundation that can serve as a common ground for all morphologists and evo-devo plant biologists, regardless of their theoretical preferences.
Briefly, the significance of articulation morphology is at least two-fold. First, it transcends the rigidity of the categories of classical mainstream morphology that may create pseudo-problems. Second, it introduces an empirical, dynamic, all-inclusive, and unifying approach: empirical and dynamic because it is based on the observable processes of open growth, ramification and articulation; all-inclusive because it includes all exceptions (misfits), which thereby cease to be exceptions; and unifying because it implies the concept of the article – a structural unit that applies to all plants, from algae to bryophytes and vascular plants, but has been largely overlooked because of the organ-centered approach of mainstream morphology. Articulation morphology thus highlights the fundamental significance of this concept for plant morphology and plant evo-devo.
Acknowledgements: I am very grateful to Regine Classen-Bockhoff, Daniel Faccini, Alon Israeli, 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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