[Illustration: FIG. 234.--Spermatozoids of _Selaginella_: _b_ with a remnant of cytoplasm.] [Illustration: FIG. 235.--Macrospore of _Selaginella_: _A_ longitudinal section, before the rupture of the wall, six weeks after being sown. The endosperm (_e_) has not yet filled the entire chamber. Cell-formation is still proceeding in the lower part of the spore. The endosperm and prothallium (_f f_) are separated by a distinct line (diaphragm). _B_ Germinating macrospore seen from outside: _s_ wall of the spore; _æ_ archegonia.] [Illustration: FIG. 236.--Archegonia of _Selaginella_: _A_ unripe, in longitudinal section; _c_ the central cell; _k_ neck-canal-cell, which is wedged in between the two-storied neck-cells; _B_ ripe; _u_ ventral canal-cell; _C_ seen from above, open. It will be noticed that the neck is formed of two tiers of four cells each.] The ARCHEGONIA are constructed on the same plan as those of the other Archegoniatæ, but are quite embedded in the prothallium (Figs. 235 _æ_, 236). =The asexual generation= varies very much in the different orders. [Illustration: FIG. 237.--_Isoëtes lacustris_ (slightly diminished): _st_ the stem; _r_ roots; _b_ leaves.] [Illustration: FIG. 238.--_Isoëtes lacustris_. Longitudinal section through the base of the leaf with a microsporangium. The edge of the groove, in which the microspangium is placed, is continued as a thin covering which envelopes the sporangium. The inferior edge of the ligular groove (_L_) forms a lip (_J_); _t_ sterile cell-rows (trabeculæ) which divide the sporangium into compartments; _l_ vascular bundle.] [Illustration: FIG. 239.--_Selaginella inæqualifolia_. Cone in longitudinal section; microsporangia are seen on the left side, macrosporangia on the right (most frequently each with four macrospores).]