SOME PTERIDOSPERM STEMS AND FRUCTIFICATIONS WITH PARTICULAR REFERENCE TO THE MEDULLOSAE1 ROBERT W. BAXTER2 INTRODUCTION Theophrastus, one of the first to attempt the classification of plants, recognizedas his major groups, trees, shrubs, and herbs. Although this division was long agorealized to be an artificial one it nevertheless provided then, as it does today, con-venient categories for different types of plant habit. Since the study of paleobotanyinvolves not only the search for the ancestral types of present-day plants but alsoattempts to visualize and illustrate the gross appearance of past floras we may stillfind it convenient to use these major habit groupings in classifying fossil plants. We now know that the Carboniferous forests were made up of the tree-likePitys, Cordaites, Lepidodendron, Sigillaria and Calamites, which attained diametersof several feet and reached 100 feet or more in height. Growing among these treeswere numerous plants of creeping, climbing, and shrubby habit, characterized ingeneral by small stems with little or no secondary growth. It is also generally truethat this last group (ferns and seed-ferns) had developed large leaves or fronds(megaphyllous) while the larger tree-like plants were generally small-leaved(microphyllous). The term "microphyllous" is used in this paper in a broad sense toinclude not only the groups lacking leaf gaps but also those living and fossil gymno-sperms (Coniferophyta, Arnold, 1948) in which the seeds are stem-borne (Sahni's(1920) Stachysperms) and the leaves are small, simple, linear or fan-shapedgrowths, borne in dense spirals or whorls on the trunk and branches. In plant evolution the early development of a single large trunk (tree-like) seemsto have some correlation with microphyllous habit, as the development of numeroussmall branches of equal size (shrubby) may be correlated with megaphyllous form.Accordingly, it seems possible that a classification based on external form and sizemay, considered in relation to the origin of plant groups, not be entirely withoutsome phylogenetic meaning.3 As might be supposed, the larger tree-like fossils 1An investigation carried out in the graduate laboratory of the Henry Shaw School of Botanyof Washington University and submitted as a thesis in partial fulfilment of the requirements forthe degree of Doctor of Philosophy in the Henry Shaw School of Botany. 2Assistant Professor of Botany, University of Kansas, Lawrence. 3While it is not our present intention more than to suggest the possibility of the foregoing state-ments, there is ample evidence that the Coniferophyta (Ginkgoales, Taxales and Coniferales) havebeen trees and microphyllous since their origin from the Pityeae and Cordaiteae in Upper Devonianand Carboniferous times (Arnold, 1948). The other groups of Carboniferous trees (Calamites,Lcpidodendron, etc.) are, on the other hand, represented today by only a few isolated genera inthe so-called fern allies. Thus it would seem that the only possible ancestral forms of the mega-phyllous gymnosperms, and possibly the angiosperms, must be sought in the shrubby, climbing orherbaceous undergrowth of the Paleozoic forests. In this habit group through profuse equivalentstem ramifications the telome units would be provided for the development of the large frond andbroad leaf, which when fertile may have evolved into sporophylls or carpels (Wilson, 1942). Issued September 30, 1949. (287)
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