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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. International Association for Plant Taxonomy (IAPT) is collaborating with JSTOR to digitize, preserve and extend access to Taxon.More than probably any other group of plants, the pteridophytes, particularly the group of ferns, have long remained problematical from the point of view of taxonomy. This is scarcely surprising, as in this group the search is still continuing for a suitable set of catholic morphological features which are amenable to precise description, and which indicate the possible trends of evolution within the group. Also, the vast majority of these plants (nearly 90% of the number of known species) are strictly tropical with no temperate representatives, and these were morphologically almost unknown until recently. However, the revival of interest in the morphology of these plants in the present century has brought to light a wealth of information about them. The monumental morphological investigations undertaken by Prof. F. O. Bower early in the century proved beyond doubt that the taxonomic criteria used by earlier workers were inadequate to express phylogeny. The modern concept that taxonomy should also express phylogeny led to a reconsideration of the classification of these plants, and during the past three decades no less than seven systems came to be proposed (Christensen, 1938;Ching, 1940; Holttum, 1947; Copeland, 1947;Reimers, 1954; Alston, 1956; Pichi-Sermolli, 1958). All these differ from each other in quite substantial features, mainly due to the differences in the relative significance attributed to different morphological features by the various authors; the addition of fresh morphological data has also played a significant role in interpreting relationships of many groups.Of late, there have been significant additions to our knowledge relating to several morphological criteria of the pteridophytes. Among these are the chromosome counts and details of prothallial morphology, spore morphology and anatomy, which are now known for a vast number of genera. An analysis of these and other morphological criteria suggests that the current interpretations of taxonomy and evolution of the homosporous ferns are possibly not totally acceptable. A new scheme for the classification of these plants is therefore presented here:

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SUMMARY Morphology of the spores, prothalli and juvenile leaves of 6 spp. of Pellaea. and 2 spp. of Notholaena is described. The spores are trilete and possess a crassimarginate laesura. The exine is psilate and there is a distinct psilate perine in Notholaena. In most species of Pellaea the exine is psilate, but there is a perinous ornamented outer layer closely adhering to it: the exine is spinulose and a perinous layer is absent in P. falcata and P. rotundifolia. The germ filament is composed of short, thick cells. A prothallial plate is formed by longitudinal divisions of the anterior cells when the germ filaments are 4–10 cells long. In some species, the terminal cells of the filament are sluggish and are often pushed aside by expansion of cells behind them. Young prothalli are usually ameristic but a sluggish obconical apical cell may be differentiated in some species. As the prothallus grows, the apex becomes notched and cordate. Cells behind the apical notch then constitute an apical meristem. Formation of a midrib is initiated when the prothalli are 5–6 weeks old. P. doniana, P. falcata and P. rotundifolia are sexually reproducing species and develop cordate naked prothalli with thin midribs bearing sex organs. The others are apogamous and the prothalli, soon after the development of a cordate apex, become irregular in shape. Apogamous development of the sporophyte is initiated by the formation of a multicellular cushion on the lower surface of the thallus, generally behind the apical notch: on highly lobed thalli the major lobes may develop a separate sporophyte on each. The cushion develops glandular hairs and paleae similar to those on the sporophyte and grows out as a flat, hood‐like protuberance in which a 3 ‐sided meristematic cell may soon be developed. The hood grows out as the 1st juvenile leaf and often a second leaf is soon developed near its base. A stem apex is differentiated by cells close to the leaf base and further leaves are formed. No roots are produced till the sporophyte develops 3 or 4 leaves. Vascular elements are developed in the apogamous cushion early in development and often extend into the prothallial tissue below. The lamina of the early juvenile leaves is nearly spatulate and entire or faintly lobed: it is naked in the first few leaves, but later, glandular hairs spread from the stipe upwards over the lamina.