Order 2. Tetrasporaceæ reproduce both by vegetative divisions and swarmspores, some have also gamete-conjugation. The principal genera are: Tetraspora, Apiocystis, Dactylococcus, Dictyosphærium, Chlorangium.
Order 3. Chlorosphæraceæ. Chlorosphæra.
Order 4. Pleurococcaceæ. In this order the swarm-stages and sexual reproduction are entirely absent. Vegetative reproduction by division. The principal genera are: Pleurococcus (Fig. [47]), Scenedesmus (Fig. [48]), Raphidium, Oocystis, Schizochlamys, Crucigenia, Selenastrum.—Pleurococcus vulgaris (Fig. [47]) is one of the most common Algæ throughout the world, occurring as green coverings on tree-stems, and damp walls, and it is one of the most common lichen-gonidia.
Fig. 47.—Pleurococcus vulgaris.
Fig. 48.—Scenedesmus quadricauda.
Order 5. Protococcaceæ. The cells are motionless, free or affixed on a stalk (e.g. Characium, Fig. [49]), either separate or loosely bound to one another; they never form multicellular individuals. Multiplication by division is nearly always wanting. Reproduction takes place by swarmspores, which have 1 or 2 cilia, and sexual reproduction in some by gamete-conjugation. The principal genera are: Chlorococcum, Chlorochytrium, Chlorocystis, Scotinosphæra, Endosphæra, Phyllobium, Characium, Ophiocytium, Sciadium.
Fig. 49.—Characium strictum. A The cell-contents have divided into many swarmspores. B Swarmspores escaping.