Fig. 328.
Selaginella with three
fruiting spikes.
(Selaginella apus.)
Fig. 329.
Fruiting spike
showing large
and small sporangia.
Fig. 330.
Large
sporangium.
Fig. 331.
Small
sporangium.
585. Sporangia.—On examining the fruiting spike, we find as in lycopodium that there is but a single sporangium in the axil of a fertile leaf. But we see that they are of two different kinds, small ones in the axils of the upper leaves, and large ones in the axils of a few of the lower leaves of the spike. The microspores are borne in the smaller spore-cases and the macrospores in the larger ones. [Figures 329-331] give the details. There are many microspores in a single small spore-case, but 3-4 macrospores in a large spore-case.
586. Male prothallia.—The prothallia of selaginella are much reduced structures. The microspores when mature are already divided into two cells. When they grow into the mature prothallium a few more cells are formed, and some of the inner ones form the spermatozoids, as seen in [fig. 332]. Here we see that the antheridium itself is larger than the prothallia. Only antheridia are developed on the prothallia formed from the microspores, and for this reason the prothallia are called male prothallia. In fact a male prothallium of selaginella is nearly all antheridium, so reduced has the gametophyte become here.