Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/170

Rh 1GO BOTANY [REPRODUCTIVE ORGANS In Ferns III Equi- Amongst Ferns the oosporc germinates and produces an asexual generation. The embryo plant formed from this spore is a simple mass of cellular tissue, no cotyledons being formed. This, then, is an Acotyledonous embryo (fig. 54). The same happens in other vascular Crypto gams, which are therefore also Acotyledonous. The embryo so formed has a primary rneristcm, with an apical cell, by division of which it increases in size, roots being formed downwards and a leaf-bearing axis upwards, upon which, eventually, the fructification (fig. 341) is formed. This consists in Ferns of the production of either stalked or sessile spore-cases or sporangia, from the epidermis on the back (Aspidium) cr edges (Pteris) of a leaf (frond), or upon a special branching axis (Osmunda), or a spicate one (Ophioglossum). These, when situated on the frond, aro arranged in definite clusters or sort (fig. 312), cither 312. Fig. 341. Fin. 311. Frond (fructiferous leaf) of a Fern (XiphoMus), showing sori, or round clusters of sporangia at the ends of veins. As these sori are on the back of the frond, the Fern is called dorsiferous. FIG. 342. Sorus or cluster of sporangia of a Fern (Asp/ilium trifpUatum). The sporangia are covered l&amp;gt;y an indusium or involucre, derived from the epidermis of the frond. The involucre is round (orbicular), and attached by its centre in an umbilical manner. The annulate sporangia are seen at the lower edge of the involucre. rounded or elongated. With the sporangia arc usually associated jointed cells or parapliyses. Frequently the sori are covered by a thin cellular layer forming an indusium or involucre, or the margin of the frond folds over them, forming a false indusium. Within the sporangium spores are produced, and when ripe these escape by the rupture of the sporangium, this in many cases being provided for by the formation, at a definite point, of an annulus or ring (fig. 343) of unequally thickened cells. Hence some Ferns are annulate, others cxannulate. The cell-wall of each spore is divisible into an outer layer or exospore, and an inner or cndospore. When the spore reaches the soil it germinates, and after a longer or shorter period produces the sexual generation. In this process the variously marked exospore is ruptured by the enlarging endospore, which divides so as eventually to form a flattened cellular expansion or prothallus (fig. 344) with small cellular rhizoids. Upon this, usually -on the under surface, the sexual organs, consisting cf antheridia and archegonia, are produced. A tendency to dicecism is observed in pro- thalli, frequently producing only one kind of sexual organs. By the mutual influence of those sexual organs an oospore is formed, from which a new asexual generation arises. Thus in Ferns the alternation consists of two dissimilar generations, a sexual or prothallial generation, and an asexual generation. In Equisetacere the same alternation of two dissimilar generations occurs. Upon an asexual generation the fruc tification is formed at the apex of a leaf-bearing axis. Peltate hexagonal stalked scales (fig. 345) are arranged in close whorls round tho apex of the axis, forming a pyramidal head (fig. 340, /), the surface of the scales being Fig. 343. Fig. 344. Fig. 345. FIG 343. Mature sporangium of (he Male Fern (Lattrea Filir-mas). It is supported on a stalk p, some of tho cells of which form an elastic ring or anuulus a round the spore-case. The spore-case 5 opens at the side to dis charge tho spores. FIG. 344. Young plant of a Fern (Pteris paJcacea), showing the commencement of (he sporangiferous frond /, arising from the impregnated ovule in tho archcgonium, the prothallium p being still attached. FIG. 345. Polygonal scale s of a species of Horse tail (Eqtrisetum), bearing mem branous sacs t, which open on their inner surface to discharge spores. directed tangentially to the axis. Round the margin of tho inner surface of these scales the sporangia are produced projecting towards the axis. When ripe each sporangium Fig. 347. L. 340. Fijr. 349. Fig. 351. FIG. 346. Fructification of a species of Horse-tail (Eqiiisctwn maximum ). The stalk is surrounded by a series of membranous sheaths s s, which are fringed by numerous sharp processes or teeth. The fructifiraf ion / is at tho extremity of the frond, in the form of a pyramidal mass of polygonal scales, bearing spores on their undcr-surfacc. FIG. 347. Large spores of Club Moss (Lycopodium) enclosed in an involucre. It is sometimes called an oophoridium in consequence of containing germinating bodies (ovules). It represents the female organ. The involucre opens trans versely to discharge the spores (macrospores). FIG. 348. Macrosporc discharged from the oophoridium of aLycopod (Selnginella Jtfertensii), with the outer coat removed to show the young cellular prothallium p at the upper end. FIG. 349. Vertical section of a small portion of tho prothallium and upper part of the large spore of a Lycopod (Selaginella denticulata), showing the embryo e, developed from a central cell of one of the archegonia a, carried down by the growth of the suspensor, so as to be embedded in the cellular tissue at the upper part of the spore. FIG. 350. Fructification of a species of Peppcnvort (ManVea Fdbri). The sporo- carp s opens to give out a peculiar cellular cord or peduncle p, which at first was curved in a ring like manner. This cord bears spike-like fructification/, consisting of antheridia and pistillidia enclosed in sacs. FIG. 351. Archegonium a of aRhizociirp (I ilu/aHa globulifera} cut vertically after impregnation, showing tho prothallium, with tho embryo in its interior in sn advanced state. This embryo gives origin to the leafy stem. opens towards the stalk of the scale, and the spores are shed, each having attached to it four clavate filaments sometimes