A red alga (Nemalion). _A_, sexual branches, showing antheridia (_a_); carpogonium or procarp (_o_) with its trichogyne (_t_), to which are attached two spermatia (_s_); _B_, beginning of a cystocarp (_o_), the trichogyne (_t_) still showing; _C_, an almost mature cystocarp (_o_), with the disorganizing trichogyne (_t_). (After Vines.)] =373. Batrachospermum.=—This genus occurs in fresh water, and the species are found in slow-running water of shallow streams or ditches. There is a central slender strand which is more or less branched, and on these branches are whorls of densely crowded slender branches occurring at regular intervals. The plants are usually very slippery. Gonidia are formed on the ends of some of these branches in globose sporangia, called monosporangia, since but a single spore or gonidium is developed in each. Other branches often terminate in long slender hyaline setæ. =374. Lemanea.=—This genus also occurs in fresh water. The species develop only during the cold winter months in rapids of streams or where the water from falls strikes the rocks and is thoroughly aerated. They form tufts of greenish threads, cylindrical or whiplike, which in the summer are usually much broken down. The threads are hollow and have a firm cortex. These are the sexual shoots, and they arise as branches from a sterile filamentous-branched, Chantransia-like form. =375. Fertilization in the lower red algæ.=—The sexual organs in the red algæ consist of antheridia and carpogonia. The antheridia are usually borne in crowded clusters, or surfaces, and bear terminally the small non-motile sperm cells. The carpogonium is a branch of one or several cells, the terminal cell (procarp) extending into a long slender process, the trichogyne. The sperm cell comes in contact with the trichogyne, and in the case of Nemalion and some others the nucleus has been found to pass down the inside and fuse with the nucleus of the procarp. [Illustration: Fig. 183. _A_, part of a shoot showing whorls of branches with clusters of carpospores. _B_, carpogonic branch or procarp. _c_, procarp cell; _tr_, trichogyne. _C_, same with sperm (_sp_) uniting with trichogyne. _D_, same with carpospores developing from procarp cell. _E_, male branch with one-celled antheridia. _F_, same with some of antheridia empty. (After Schmitz.)] From this point in the lower red algæ like Nemalion, Batrachospermum and Lemanea the formation of the spores is very simple. The procarp is stimulated to growth, and buds in different directions, producing branched chains of spores (carpospores). The carpospores form a rather compact cluster called the sporocarp, which means spore-fruit or spore-fruit body. In Batrachospermum it is seen as a compact tuft in the loose branching, in Nemalion it lies in the surface of the cortex, while in Lemanea the sporocarps lie at different positions in the hollow tube of the sexual shoot. [Illustration: Fig. 184.