Page:The New International Encyclopædia 1st ed. v. 16.djvu/593

* PTERIDOPHYTES. 517 PTERIDOPHYTES. plants are often called eiy])lo;^ams. pteridopliytes are often called vascular cryptogams. 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 observation. This body bears the sex-organs (antheridia and archegonla, 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 cons|)icu- ous but sexless fern plant or sporophj'te ( Fig. 1 ) , iipon the under surface of the leaves of which FlO. 2. STRUCTURE OF EQUI8ETUM. 1. prothailiuin : 2. longitudinal section of an archego- nium : 3, section of an antheridium ; 4, a sporoph.vll with 5 sporangia visible; 5, a spore, showing elaters ; 6. a sperm. asexual spores are produced (Figs. I, 5). When these fall to the ground and germinate they pro- duce prothallia. and thus complete the cycle. Pteridophytes differ from bryophytes (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 pteridophvtes are very large, spirally coiled, and bear numerous cilia for swimming (Figs. 1, 4). The feriia (Filicales) are the most prominent Jitrridophytes in the present flora, numbering about 4.500 species, chiefl.v tropical. In habit they aie mainly terrestrial, but some 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- niously veined, usuall.v compound, which bear on the under surface very numerous spore-eases (sporangia), and uncoil from the bud (circi- nate). The horsetails (Equisetales). which were foniierly very abimdant and included large trees, now comprise only about twenty-five small or straggling forms, well marked bv their jointed and fluted stems, the absence of foliage leaves, and the terminal conical structure ('strobilus'). consisting of spore-bearing leaves (sporoph,vlls, q.v.). each of which bears five to ten spore-cases (sporangia) on its lower surface (Fig. 2). The club-mosses (Lycopodiales), which comprise about .500 species and were formerly much more abundant and conspicuous, are characterized bv their slender, trailing, branched stems, thickl.v covered by small foliage leaves, and by a stro- bilus consisting of sporophylls, each bearing a ^yn^<>y Fig. 3. belagixella. 1, arcliegonium with egg: 2. antheridiuni with sperms; 3, upper part of female gametopbyte, with heavy mega- spore wall. single sporangium upon the upper side (Figs. 3 and 4 ). The most important feature of the group is that Selaginella, the largest genus, is heterosporous. See Hetebcspokt. Probably the greatest interest to the botanist in connection with the pteridophvtes lies in the h.vpothesis 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 Fig. 4. sf:i,Ai;iNELl.A. a. megaspore ; b, prothaliiunt ; c. rhizoids ; rf, embryo sporophyte. the horsetails, but opinion is divided between the ferns and club-mosses as probable ancestral forms with the burden of testimony apparently in favor of the former, at least in some cases. For fuller details, see Ferns: EguisETrM; Lycopodiaxes. Consult books cited under Morphology, all of which treat more or less fuU.v of this group. Also: Campbell, Mosses and Fern^ (New York,