A red alga (Callithamnion), showing sporangium _A_, and the tetraspores discharged _B_. (After Thuret.)] [Illustration: Fig. 185. Gracilaria, portion of frond, showing position of cystocarps.] [Illustration: Fig. 186. Gracilaria, section of cystocarp showing spores.] =376. Gonidia in the red algæ.=—The common type of gonidium in the red algæ is found in the _tetraspores_. A single mother cell divides into four cells arranged usually in the form of tetrads within the _tetrasporangium_. In Callithamnion the tetrasporangium is exposed. In Polysiphonia, Rhabdonia, Gracilaria, etc., it is imbedded in the cortex. In Batrachospermum there are monosporangia, each monosporangium containing a single gonidium, while in Lemanea, and according to some also in Nemalion, gonidia are wanting. =377. Gracilaria.=—Gracilaria is one of the marine forms, and one species is illustrated in fig. 185. It measures 15-20_cm_ or more long, and is profusely branched in a palmate manner. The parts of the thallus are more or less flattened. The fruit is a cystocarp, which is characteristic of the Rhodophyceæ (Florideæ). In Gracilaria these fruit bodies occur scattered over the thallus. They are somewhat flask-shaped, are partly sunk in the thallus, and the conical end projects strongly above the surface. The carpospores are grouped in radiating threads within the oval cavity of the cystocarp. These cystocarps are developed as a result of fertilization. Other plants bear gonidia in groups of four, the so-called _tetraspores_. =378. Rhabdonia.=—This plant is about the same size as the gracilaria, though it possesses more filiform branches. The cystocarps form prominent elevations, while the carpospores lie in separated groups around the periphery of a sterile tissue within the cavity. (See figs. 187, 188.) Gonidia in the form of tetraspores are also developed in Rhabdonia. [Illustration: Fig. 187. Rhabdonia, branched portion of frond showing cystocarps.] [Illustration: Fig. 188. Section of cystocarp of rhabdonia, showing spores.] =379. Fertilization of the higher red algæ.=—The process of fertilization in most of the red algæ is very complicated, chiefly because the fertilized egg-cell (procarp) does not develop the spores directly, as in Nemalion, Lemanea, etc., but fuses directly, or by a short cell or long filament with one or more auxiliary cells before the sporocarp is finally formed. Examples are Rhabdonia, Polysiphonia, Callithamnion, Dudresnaya, etc. (fig. 189). The auxiliary cell then develops the sporocarp. See fig. 189 for conjugation of a filament from the fertilized procarp with an auxiliary cell. [Illustration: Fig. 189. Dudresnaya purpurifera. _tr_, trichogyne, with spermatozoids attached; _ct_, connecting-tube which grows out from below the base of the trichogyne, and comes in contact with the fertile branches _f_, _f_; _ct′_, young connecting-tube. (After Thuret and Bornet.)]