=The sexual generation.= When the spore germinates, the external covering (exospore) is ruptured, as in the Mosses. The internal cell-wall (endospore) grows out as a filament, which soon divides and gives rise to the prothallium, a flat, cellular expansion resembling the thallus of a Liverwort. In its fully developed state the prothallium is generally heart-shaped, dark green, and provided with root-hairs, and it attains a diameter of about one centimetre (Fig. 205). It is formed of one layer of cells, except along the central line near the anterior depression, where it becomes several layers of cells in thickness, forming the “cushion,” on the lower side of which the archegonia are developed. The antheridia are first formed; they are thus found on the oldest parts of the prothallium, on its edge, or among the root-hairs. The archegonia are developed later, and are therefore found near the apex. Several tropical Ferns have prothallia[18] deviating from this typical form; _Trichomanes_ (Order _Hymenophyllaceæ_) has filamentous, branched prothallia, which resemble the protonema of a Moss. Others, again, have strap-shaped prothallia, which resemble the thallus of certain Liverworts. [Illustration: FIG. 205.--Prothallium (_p p_) of Maiden hair (_Adiantum capillus veneris_) with a young plant attached: _b_ first leaf; _w′_ primary root; _w″_ adventitious roots; _h h_ root-hairs of the prothallium (× abt. 30).] [Illustration: FIG. 206.--Antheridia of Maiden-hair (× 550). _A_ Unripe; _B_ ripe, but unopened; _C_ open and ejecting the spermatozoids (_s_). Those which have been last ejected are still lying enclosed in their mother-cells, the others are coiled up and drag with them the cytoplasmic remains (_b_); _f_ cells of the prothallium.] The ARCHEGONIA have been already mentioned (p. 199, Fig. 201). The ANTHERIDIA are hemispherical or slightly conical bodies (Fig. 206). They consist, as in the Mosses, of a wall formed by one layer of cells, which encloses a number of spermatozoid-mother-cells (_A_ and _B_). The antheridia when ripe absorb water, and are ruptured, and the spirally-coiled spermatozoids liberated (Fig. 206 _S_). The spermatozoids have been observed to pass down the neck of the archegonium, and to fuse with the oosphere.