PTERIDOPHYTES, ter'id-6-fits (Xeo-Lat. nom. pl., from Gk. rrepts, ptcris, fern + Our6v, phylon, plant). The fern-plants, one of the four great divisions of the plant kingdom, next in order of rank to the highest group, the flowering or seed plants (spermatophytes). There are three great divisions of pteridophytes: ( 1 ) ferns (Fili cales), (2) horsetails ( Equisetales), and (3) club-mosses (Lyeopodiales). The pteridophytes have a well-developed vascular system, which is entirely absent in the bryophytes, the next group below them. This system of conducting and strengthening tissues is correlated with the at tainment of much greater size and larger foliage display by the fern plants than by the moss plants. It associates them with the seed plants (spermatophytes). Since plants below the seed plants are often called cryptogams, pteridophytes are often called vascular cryptogam:. There is also a well-marked alternation of generations (q.v.), which may be illustrated by the life-his tory of a common fern. When a fern spore ger minates it produces a green, flat, usually heart shaped body (prothallium), so small that it es capes ordinary o6zervation. This holy bear. the sex-organs (autheridia and archegonia, Figs. 1, 2. 3). It is therefore the sexual plant or game tophyte and is entirely independent. The egg pro duced and fertilized upon this prothallium (game tophyte) germinates and produces the conspicu ous but sexless fern plant or :porophyte (Fig. 1), upon the under surface of the leaves of which about 500 species and were formerly much more abundant and conspicuous, are characterized by their slender, trailing, branched stems, thickly covered by small foliage leaves, and by a :tro bilus consisting of sporophylls, each bearing a asexual spores are produced (Figs. 1, 5). When these fall to the ground and germinate they KO duce prothallia, and thus complete the cycle. Pteridophytes differ from hryophytes (q.v.) espe cially in that their gametophytes are leafless and inconspicuous. and the sporophytes are prominent, leafy, and nutritively independent of the gameto phyte. Further. the sperms of pteridophytes are very large, spirally coiled, and bear numerous cilia for swimming (Figs. 1, 4).
The ferfis (Filicales) are the most prominent pteridophytes in the present flora, numbering about 4500 specie:. chiefly tropical. In habit they are mainly terrestrial, but sonic tropical forms are epiphytic (perching) and one aberrant group, the 'water ferns' float or are rooted in water. The peculiar characters are the horizontal sub terranean stem, which sends to the surface com paratively few large leaves (fronds), dichoto mously veined, usually compound, which hear on the under surface very numerous spore-cases (sporangia). and uncoil from the bud (eirci nate). The horsetails (Equisetales). which were formerly very abundant and included large trees, now comprise only about twenty-five small o• straggling forms, well marked by their jointed and fluted stems, the absence of foliage leaves. and the terminal conical structure l'strobilus'). consisting of spore-bearing leaves (spo•ophylls. q.v.), each of which hears five to ten spore-cases (sporangia) on its lower surface (Fig. 2), The club-mosses ( Lyeopod ia les) , which comprise single spo•angiuum upon the upper side (Figs. 3 and 4). The most important feature of the group is that Selaginella, the largest genus, is heterosporous. See HETEBOSPORY.
Probably the greatest interest to the botanist in connection with the pteridophytes lies in the hypothesis that seed-plants have been derived from them, to prove which has been the purpose of much inconclusive investigation. There is gen eral consent that seed-plants have not come from the horsetails, but opinion is divided between the ferns and club-mosses as probable ancestral form. with the burden of testimony apparently in favor of the former, at least in some cases. For fuller details, see FERNS KQUISETt'M: LYCOPODLkLES.
Consult books cited under Jlonuuen.onY, all of which treat more or les. fully of this group. Also: Campbell, Mosses and Ferns (New York, 1895) : Atkinson, The Biology of FCrliR (New York. 1894). For works on classification, see under TAXONOMY.