LIFE HISTORIES OF ALG.E 
253 
of a tiny green plant, called Pleurococcus, usually of the 
species Pleurococcus vulgaris. The individual plants are 
so small that /they may be seen only with the aid of the 
microscope (Fig. 183). An examination of the trees in 
any given locality will disclose the fact that Pleurococcus 
prefers one side of the tree to the other, and that the 
choice of sides has a direct relation to light, temperature, 
or moisture one or all. 
FIG. 184. Pleurococcus vulgaris. Sections of one-, two-, and four-celled 
plants, showing the nuclei and the large chlorophyll bodies (chb) to which 
the green color of the plants is due. In D, the larger chloroplast is shown 
in perspective. (Camera lucida drawings from a microscopic preparation 
by E. W. Olive.) 
243. Structure. No plant structure could be much 
simpler than that of Pleurococcus, for the plant body is a 
single cell, the simplest organic unit capable of independent 
existence. The protoplast possesses a well-defined nucleus 
and a chloroplast, and is surrounded by a cellulose cell- 
