=351. The Blue-Green Algæ=, or =Cyanophyceæ= form slimy looking thin mats on damp wood or the ground, or floating mats or scum on the water. The color is usually bluish green, but in some species it is purple, red or brown. All have chlorophyll, but it is not in distinct chloroplasts and is more or less completely guised by the presence of other pigments. Two orders and eight families are recognized. The following include some of our common forms: =352. ORDER COCCOGONALES (COCCOGONEÆ).=—Single-celled plants, occurring singly or in colonies, in some forms forming short threads. One of the two families is mentioned. =353. Family Chroococcaceæ.=—The plants multiply only through cell division. Chroococcus, forms rounded, blue-green cells enclosed in a thick gelatinous coat, in fresh water and in damp places; certain species form “lichen-gonidia” in some genera of lichens. Glœocapsa is similar to Chroococcus, but the colonies are surrounded by an additional common gelatinous envelope (fig. 168); on damp rocks, etc. [Illustration: Fig. 169. _A_, Oscillatoria princeps, _a_, terminal cell; _b_, _c_, portions from the middle of a filament. In _c_, a dead cell is shown between the living cells; _B_, Oscillatoria froelichii, _b_, with granules along the partition walls.] =354. ORDER HORMOGONALES (HORMOGONEÆ).=—Plants filamentous, simple celled or with false or true branching, usually several celled (Spirulina is single celled). Multiplication takes place through _hormogones_, short sections of the threads becoming free; also through resting cells. Two of the six families are mentioned. =355. Family Oscillatoriaceæ.=—This family is represented by the genus Oscillatoria, and by several other genera common and widely distributed. Oscillatoria contains many species. They are found on the damp ground or wood, or floating in mats in the water. They often form on the soil at the bottom of the pool, and as gas becomes entangled in the mat of threads, it is lifted from the bottom and floated to the surface of the water. The plant is thread-like, and divided up into many short cells. The threads often show an oscillating movement, whence the name _Oscillatoria_.