The fertilisation of the Mosses cannot be effected without water. Rain and dew therefore play a very important part in this process, and for this end various modifications of structure are found. [Illustration: FIG. 190.--_Andreæa rupestris._ Longitudinal section through a sporangium at the time when the mother-cells of the spores are dividing: _p_ pseudopodium; _f_ foot; _v_ vaginula; _h_ neck; _c_ columella; _w_ wall of the sporangium; _e_ external row of cells; _s_ the spore-sac; _t_ the spore-mother-cells; _r_ the calyptra with the neck of archegonium (_z_).] [Illustration: FIG. 191.--_Andreæa rupestris._ Transverse section through a ripe sporangium. In the middle is seen the four-sided columella, surrounded by the numerous spores, drawn diagrammatically. Surrounding them is seen the wall of the sporangium, whose outer layer of cells is thickened and coloured. The layer of cells is unthickened in four places (_x_), indicating the position of the clefts (see Fig. 193).] Among the sexual organs, paraphyses--filamentous or club-shaped bodies--are to be found. =The asexual generation, the sporophyte= (Moss-fruit or sporogonium). As the result of fertilisation the oosphere surrounds itself with a cell-wall, and then commences to divide in accordance with definite laws.[17] The embryo (Fig. 189 _C_) produced by these divisions remains inside the wall _a-a_ of the archegonium (Figs. 190, 199 _D_, _E_), and developes into the _sporogonium_, which remains attached to the mother-plant, often nourished by it, as if the two were one organism. The lower extremity of the sporogonium, _the foot_ (Figs. 190 _f_; 199 _D_), very often forces its way deep down into the tissue of the mother-plant, but without an actual union taking place. The central portion of the sporogonium becomes a shorter or longer _stalk_ (_seta_), while the sporangium itself is developed at the summit. At a later stage, during the formation of the spores, the sporangium very often assumes the form of a _capsule_, and dehisces in several ways characteristic of the various genera (Figs. 192, 193, 194, 195, 200). The basal portion of the archegonium grows for a longer or shorter period, forming a sheath, the _calyptra_, in which the capsule is developed, but eventually it ceases to enlarge, and is then ruptured in different ways, but quite characteristically, in each group. Anatomically, the asexual generation is often more highly differentiated than the sexual; thus, for instance, stomata are present on the sporangia of the true Mosses, but are absent in the sexual generation.