Page:A Text-book of Animal Physiology.djvu/41

 more generally, that the life-history of this humble organism can only he unfolded under certain well-defined conditions.

(Protococcus pluvialis).

The study of this one-celled plant will afford instructive comparison between the ordinary green plant and the colorless plants or fungi.

Like Torula it is selected because of its simple nature, its abundance, and the ease with which it may be obtained, for it abounds in water-barrels, standing pools, drinking-troughs, etc.

Morphological.—Protococcus consists of a structureless wall and viscid granular contents, i.e., of cellulose and protoplasm.

The protoplasm may contain starch and a red or green coloring matter (chlorophyl). It probably contains a nucleus. The cell is mostly globular in form.

Physiological.—It reproduces by division of the original cell (fission) into similar individuals, and by a process of budding and constriction (gemmation) which is much rarer. Under the influence of sunlight it decomposes carbon dioxide (CO$2$), fixing the carbon and setting the oxygen free. It can flourish perfectly in rain-water, which contains only carbon dioxide, salts of ammonium, and minute quantities of other soluble salts that may as dust have been blown into it.

There is a motile form of this unicellular plant, and in this stage it moves through the fluid in which it lives by means of