333. Family Volvocaceæ.—These are all motile, during the vegetative stage. The individuals are single or form more or less globose colonies.
334. The “red snow” plant (Sphærella nivalis).—This is often found in arctic and alpine regions forming a red covering over more or less large areas of snow or ice. For this reason it is called the “red snow plant.”
335. Sphærella lacustris, a closely related species, is very widely distributed in temperate regions along streams or on the borders of lakes and ponds. Here in dry weather it is often found closely adhering to the dry rock surface, and giving it a reddish color as if the rock were painted. This is especially the case in the shallow basins formed over the uneven surface of the rock near the water’s edge. These places during heavy rains or in high water are provided with sufficient water to fill the basins. During such times the red snow plant grows and multiplies, loses its red color and becomes green, and, being motile, is free swimming. It is a single-celled plant, oval in form, surrounded by a gelatinous sheath and with two cilia or flagella at the smaller end, by the vibration of which it moves ([fig. 162).] The single cell multiplies by dividing into two cells. When the water dries out of the basin, the motile plant comes to rest, and many of the cells assume the red color. To obtain the plant for study, scrape some of the red covering from these rock basins and place it in fresh spring water, and in a day or so the swarmers are likely to be found. Under certain conditions small microzoids are formed.
Fig. 162.
Sphærella lacustris (Girod.) Wittrock. A, mature free swimming individual with central red spot. B, division of mother individual to form two. C, division of a red one to form four. D, division into eight. E, a typical resting cell, red. F, same beginning to divide. G, one of four daughter zoospores after swimming around for a time losing its red color and becoming green. (After Hazen.)
336. Chlamydomonas is a very interesting genus of motile one-celled green algæ, because the species are closely related to the Flagellates among the lower animals. The plant is oval, with a single chloroplast and surrounded by a gelatinous envelope through which the two cilia or flagella extend. One-celled organisms of this kind are sometimes called monads, i.e., a one-celled being. This one has a gelatinous cloak and is, therefore, a cloaked monad (Chlamydomonas). The species often are found as a very thin green film on fresh water. C. pulvisculus is shown in [fig. 163]. When it multiplies the single cell divides into two, as shown in B. Sometimes a non-motile palmella stage is formed, as shown in C and D. Reproduction takes place by gametes which are of unequal size, the smaller one representing the sperm and the larger one the egg, as in E and F. These conjugate as in G and H, the protoplasm of the smaller one passing over into the larger one, and a zygospore is thus formed.
Fig. 163.
Chlamydomonas pulvisculus (Müll.) Ehrb. A, an old motile individual; n, nucleus; p, pyrenoid; s, red eye spot; v, contractile vacuole; B, motile individual has drawn in its cilia and divided into two; C, mother plant has drawn in its cilia and divided into four non-motile cells; D, pamella stage; E, female gamete—egg; F, male gamete—sperm; G, early stage of conjugation; H, zygospore with conjugating tube and empty male cell attached. (After Wille.)