Introduction to Pteridophytes and Gymnosperms

There is a broad consensus about the delimitation of vascular plants as against thallophytes, and their major subdivisions are well-established. Living pteridophytes are conveniently subdivided into (true) ferns and fern allies, the latter formed by lycopods, horsetails and psilotates. Likewise, among gymnosperms the separation of a microphyllous coniferophytalean line (encompassing conifers and ginkgoids) from a megaphyl-lous cycadophytalean line has hardly been challenged since the time of Coulter and Chamberlain (1910), with the only exception that the Gnetales were originally not included in the cycadophytines, while at present they are usually considered as part of them. However, if we look back in time it appears that the fossil record confronts us with forms that blur this clear-cut picture, resulting in controversial interpretations of the interrelationships and the phylogeny of major groups of both pteridophytes and gymnosperms.

Although this book deals principally with living tracheophytes and takes fossils into account only where they contribute towards our understanding of extant plants, some reference to fossils is germane.

As to the classes of the pteridophytes as defined by neobotanists, various intermediate forms are known from the Devonian and Lower Carboniferous. Thus the characters of lycopods and horsetails are shared by forms such as Eleulherophyllum, those of horsetails and ferns by Ibyka, and of ferns and lycopods by Enigmophyton, although the evaluation of these and similar fossils is a matter of continuing palaeobotani-cal inquiry. The Psilotaceae, often considered to belong to a primitive class of fern allies, have more recently been suggested as being closer to true ferns, although chemical evidence (see next section) does not favour this. Morphological evidence casts doubt on the status of Ophioglossaceae and Marattiaceae as true ferns.

As to gymnosperms, comparable situations exist. Firstly, the concept of a profound cleft between, and consequently a very distant phylogenetic origin of, cy-cadophytinous and coniferophytinous forms has been disturbed by the discovery of fossils combining characters of both lineages, such as Phylladoderma, Eo-ginkgoites, and others. Secondly, the discovery of pro-gymnosperms has brought to light a probable ancestor for both these major gymnospermous groups. This is most remarkable in view of the numerous characters common to coniferophytes and cycadophytes, such as the occurrence of lignified guard cells, the presence of transfusion tissues in the leaves, the presence of bifla-vonoids (see next chapter), the near absence of polyploidy in favour of the presence of centromeric fission or fusion of chromosomes, and so on. Such sharing of characters would be rather enigmatic if both groups really had a distant origin.

It is still a matter of debate among palaeobotanists whether or not primarily "platyspermic" seeds are of utmost importance for tracing gymnosperm phylogeny, whether platyspermic and "radiospermic" seeds originated independently from each other, and whether such characters are relevant to the position of forms such as Ephedra. Suffice it to say that the clear-cut subdivisions in pteridophytes and gymnosperms as they exist today are certainly the result of the great age of these groups, which has led to the deepening of the gap between them, notwithstanding the - geologically speaking - more recent and active diversification of some of their components, such as the "younger" fern families.

Thus the following major classification of pteridophytes and gymnosperms, forming the basis of the present work, is largely a compromise between convenience and present knowledge. The specific problems inherent in this classification are discussed in the introductory sections for pteridophytes and gymnosperms and the treatments of individual families.

0 0

Post a comment