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.
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.
356. Family Nostocaceæ.—This family is represented by Nostoc, which forms rounded, slimy, blue-green masses on wet rocks. The individual plants in the slimy ball resemble strings of beads, each cell being rounded, and several of these arranged in chains as shown in [fig. 170]. Here and there are often found larger cells (heterocysts) in the chain. Nostoc punctiforme lives in the intercellular spaces of the roots of cycads (often found in greenhouses), and in the stems of Gunnera. N. sphæricum lives in the spaces between the cells in many species of liverworts (in the genera Anthoceros, Blasia, Pellia, Aneura, Riccia, etc.), and in the perforated cells of Sphagnum acutifolium. Anabæna is another common and widely distributed genus. The species occur in fresh or salt water, singly or in slimy masses. Anabæna azollæ lives endophytically in the leaves of the water fern, Azolla.
Fig. 170.
Nostoc linckii. A, filament with two heterocysts (h), and a large number of spores (sp); B, isolated spore beginning to germinate; C, young filament developed from spore. (After Bornet.)