Family 2. Psilotaceæ. The sporangia are placed on the apex of short, two-leaved stems, as 2–3, seldom four, small capsules. Small herbs, with angular stems; leaves small, simple, and one nerved. Only four species.—Psilotum (Madagascar, Moluccas, Sandwich Islands, etc.) is destitute of roots, their place being supplied by special underground stems which bear a few modified leaves, very much reduced, especially when buried deeply in the soil. Three species.—Tmesipteris (Australia), one species.
Sub-Class 2. Selaginelleæ (Heterosporous Lycopodinæ).
Micro-and macrospores. The prothallia are very much reduced, especially the male; the female does not leave the spore. The leaves are ligulate.
Fig. 233.—Germination of the microspores of Selaginella: A the spore rendered transparent, seen from above. In the interior is seen the prothallium (f), and the first divisions of the antheridium (a, b, c, d); in B the spore-wall is removed and all the spermatozoid-mother-cells formed; in C, the microspore has opened and the spermatozoids and the mother-cells are escaping together.
The sexual generation. In the MICROSPORES are formed: (1) a very small “vegetative” cell, representing the vegetative part of the prothallium (f in Fig. [233] A, B), and (2) a cell many times larger and which divides into a number (4–8) of primordial cells, each of which divides into four spermatozoid-mother-cells, though all of these may not develope spermatozoids. On germination, when the spore-wall is ruptured, the spermatozoids and spermatozoid-mother-cells are ejected into the water.
The SPERMATOZOIDS in Selaginella are elongated and club-shaped, with two cilia (Fig. [234]); but in Isoëtes lacustris they are spirally-twisted threads which differ from all other spermatozoids by having a bunch of cilia at each end; the other species of Isoëtes have cilia only at the anterior end.
The MACROSPORES. Shortly after the macrospores have been set free, or in Selaginella, while still enclosed in the sporangium of the mother-plant, they germinate and soon become filled with the cellular tissue of the prothallium, and even in Selaginella the archegonium begins to be formed before the rupture of the spore-cell-wall has commenced (Fig. [235] A).
Fig. 234.—Spermatozoids of Selaginella: b with a remnant of cytoplasm.