The authors analyze Takhtajan's system of classification of the Angiosperms in relation to the principles of evolutionary and cladistic systematics. It is shown that Takhtajan belongs to the evolutionary school: he identifies the ancestors of some taxa, he accepts polytomous branching and he groups taxa on the basis of primitive as well as derived character states. Takhtajan's notion of weighted similarity does not appear to be based on objective criteria, when determining the weight and evolutionary status of characters.After a summary (...) of the modifications brought out by Takhtajan in his 1980 version, the weak and strong points of evolutionary systems as a whole are emphasized. In these, the delimitation of taxa and their filiation are difficult to refute since they do not rely on precise criteria, which would ensure continuity and uniformity within a given system. However, these systems have some flexibility, which allows them to incorporate readily new information, a feature unfortunately missing in cladistic classifications. In fact, while it does have weaknesses from a theoretical point of view, the system of Takhtajan gives us an idea of flowering plants phylogeny that appears to be one of the most complete at the present time from both analytical and synthetic standpoints. (shrink)

The meaning of the concept of fusion is discussed in relation with the works of Payer and those of Van Tieghem. It is pointed out that there is a difference, at the theoretical level, between the concept of fusion congénitale as defined by Payer and the concept of concrescence congénitale formulated by Van Tieghem. The former is inobservable by definition, while the latter deals with intercalary growth. For Van Tieghem, anatomy can prove the existence of fusion, even if we do (...) not see it during ontogenesis.We distinguish three complementary methods for explaining the unions of organs: ontogenetic, typological and phylogenetic. We have attempted, not so much to defend one or other of these methods, as to show that they often invoke very different interpretations for the same morphological phenomena. It is probable that only an analysis of the writings of 19th century botanists will clarify the concept of fusion and more generally the epistemology of plant morphology. (shrink)

The authors constructed an algorithm that relates leaf contour to the ratio between the two parts of the leaf using the function θ’=nθm, wherem is the allometric exponent. Using this model, it is possible to simulate the contour of symmetrical or asymmetrical leaves. The authors hypothesize that the portion of the leaf contour that agrees with the simulation is linked to a constraint imposed by the initial asymmetry of the leaf primordium. The final shape of the leaf results more from (...) a summation of cumulative processes through time than from a single process. In the analysis of foliar allometry, it is important to take into consideration the different phases of leaf development, since the nature of the allometric constants may change from one phase to the next, even though their numerical value may remain the same. (shrink)

In plant morphology, most structures of vascular plants can easily be assigned to pre-established organ categories. However, there are also intermediate structures that do not fit those categories associated with a classical approach to morphology. To integrate the diversity of forms in the same general framework, we constructed a theoretical morphospace based on a variety of modalities where it is possible to calculate the morphological distance between plant organs. This paper gives emphasis on shoot, leaf, leaflet and trichomes while ignoring (...) the root. This will allow us to test the hypothesis that classical morphology (typology) and dynamic morphology occupy the same theoretical morphospace and the relationship between the two approaches remains a question of weighting of criteria. Our approach considers the shoot (i.e. leafy stem) as the basic morphological structural unit. A theoretical data table consisting of as many lines as there are possible combinations between different modalities of characters of a typical shoot was generated. By applying a principal components analysis (PCA) to these data it is possible to define a theoretical morphospace of shoots. Typical morphological elements (shoots, leaves, trichomes) and atypical structures (phylloclades, cladodes) including particular cases representing ‘exotic’ structures such as the epiphyllous appendages of Begonia and ‘water shoot’ and ‘leaf’ of aquatic Utricularia were placed in the morphospace. The more an organ differs from a typical shoot, the further away it will be from the barycentre of shoots. By giving a higher weight to variables used in classical typology, the different organ categories appear to be separate, as expected. If we do not make any particular arbitrary choice in terms of character weighting, as it is the case in the context of dynamic morphology, the clear separation between organs is replaced by a continuum. Contrary to typical structures, “intermediate” structures are only compatible with a dynamic morphology approach whether they are placed in the morphospace based on a ponderation compatible with typology or dynamic morphology. The difference in points of view between typology and continuum leads to a particular mode of weighting. By using an equal weighting of characters, contradictions due to the ponderation of characters are avoided, and the morphological concepts of continuum’ and ‘typology’ appear as sub-classes of ‘process’ or ‘dynamic morphology’. (shrink)