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.
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.
Fig. 237.—Isoëtes lacustris (slightly diminished): st the stem; r roots; b leaves.
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.