=584.= Closely related to the club mosses are the selaginellas. These plants resemble closely the general habit of the club mosses, but are generally smaller and the leaves more delicate. Some species are grown in conservatories for ornament, the leaves of such usually having a beautiful metallic lustre. The leaves of some are arranged as in lycopodium, but many species have the leaves in four to six rows. Fig. 328 represents a part of a selaginella plant (S. apus). The fruiting spike possesses similar leaves, but they are shorter, and their arrangement gives to the spike a four-sided appearance. [Illustration: Fig. 328. Selaginella with three fruiting spikes. (Selaginella apus.)] [Illustration: Fig. 329. Fruiting spike showing large and small sporangia.] [Illustration: Fig. 330. Large sporangium.] [Illustration: 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. [Illustration: Fig. 332. Details of microspore and male prothallium of selaginella; 1st, microspore; 2d, wall removed to show small prothallial cell below; 3d, mature male prothallium still within the wall; 4th, small cell below is the prothallial cell, the remainder is antheridium with wall and four sperm cells within; 5th spermatozoid. After Beliaieff and Pfeffer.]