=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. [Illustration: 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.)] [Illustration: Fig. 171. Bacteria. _A_, Bacillus subtilis. Spores in threads, unstained rods, and stained rods showing cilia; _B_, Bacillus tetani, the tetanus or lockjaw bacillus, found in garden soil and on old rusty nails. Spores in club-shaped ends. _C_, Micrococcus; _D_, Sarcina; _E_, Streptococcus; _F_, Spirillum. (After Migula.)] Class Schizomycetes. =357. Bacteriales.=—The bacteria are sometimes classified with the Cyanophyceæ, under the name Schizophyta, and represent the subdivision Schizomycetes, or fission fungi, because many of them multiply by a division of the cells just as the blue-green algæ do. For example, Bacillus forms rods which increase in length and divide into two rods, or it may grow into a long thread of many short rods. Micrococcus consists of single rounded cells. Streptococcus forms chains of rounded cells, Sarcina forms irregular cubes of rounded cells, while others like Spirillum are spiral in form. Bacillus subtilis may be obtained by making an infusion from hay and allowing it to stand for several days. Bacillus tetani occurs in the soil, on old rusty nails, etc. It is called the tetanus bacillus because it causes a permanent spasm of certain muscles, as in “lockjaw.” This bacillus grows and produces this result on the muscles when it occurs in deep and closed wounds such as are caused by stepping on an old nail or other object which pierces the flesh deeply. In such a deep wound oxygen is deficient, and in this condition the bacillus is virulent. Opening the wounds to admit oxygen and washing them out with a solution of bichloride of mercury prevents the tetanus. Many bacteria are of great importance in bringing about the decay of dead animal and plant matter, returning it to a condition for plant food. (See also nitrate and nitrite bacteria, Chapter IX.) While most bacteria are harmless