UNICELLULAR PLANTS, 11 
more generally, that the life-history of this humble organism 
can only be unfolded under certain well-defined conditions. 
Protococcus (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 color- 
ing matter (chlorophyl). It probably contains a nucleus, The 
cell is mostly globular in form, 
© Gs 
Fie. 7. 
Figs, 5 to 7 represent successive stages observed in the life-history of Protococci scraped from 
the bark of a tree. 
Fig. 5.—A group in the dried state, illustrating method of division. 
Fic. 6.—One of the above after two days’ immersion in water. ‘ 
Fic. 7.—Various phases in the later motile stage assumed by the above specimens. , The nu- 
cleus is denoted 4 ne ; the cell wall es ¢.w ; and the coloring-matter by the dark spot. 
On the left of Fig. 7 an idual may be seen that is devoid of a cell wall. 
ee 
ne 
indiv: 
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,), fix- 
ing the carbon and setting the oxygen free. It can flourish per- 
fectly 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 
