The _sporangia_ are in all cases _capsule-like_, and burst open when ripe to eject the spores. They are nearly always situated on the leaves (in _Lycopodiaceæ_, in the axils of the leaves, or above these, on the stems themselves). In some forms (LEPTOSPORANGIATÆ), the sporangia are developed from a single epidermal cell; in others (EUSPORANGIATÆ), from a group of epidermal cells, or from cells which lie beneath the epidermis. In the first group a primitive mother-cell (archesporium) is formed, which divides commonly into sixteen special mother-cells. In the latter group, on the other hand, a number of primitive spore-mother-cells are developed. In each sporangium three different tissues are generally developed; an innermost _sporogenous_ one (_s_ in Fig. 204 _A_), which arises from the archesporangium; an outermost one, which forms the _wall_ (_a_), and may be one or, more rarely, several layers in thickness; and an intermediate one, the _tapetum_ (Fig. 204 _A_, _B_, _b t_), which is rich in protoplasm, and whose cells are dissolved so that the spores float freely in the fluid thus provided. The spores arise as in the Mosses (in tetrads), by the cross-division of the special mother-cells, and according to the manner in which they are arranged in the mother-cell have either a tetrahedral form, with a large base resembling a segment of a ball, or are oblong (bilateral spores). Their construction is the same as in the Mosses (p. 187). [Illustration: FIG. 204.--_Selaginella inæqualifolia. A_ A young sporangium, which may develope either into a macro-, or a microsporangium. _B_ A microsporangium.] The spore-formation in its earliest commencement takes place in the same way in the Isosporous and the Heterosporous Vascular Cryptogams; but from a certain point, after the tetrahedral division, a difference occurs with regard to the macrosporangia. All the spores formed in the microsporangium may complete their development; but those which are formed in the macrosporangium are generally aborted, with the exception of one or four, and these consequently attain a much larger size (see Fig. 239.--The series to the left are microsporangia; those to the right, macrosporangia).