UNICELLULAR PLANTS 103 



animals and plants and show that, except for the function of 

 nutrition the fundamental biological principles for animals 

 and plants are the same. 



B. PLEUROCOCCUS PLUVIATILIS AND SPHAERELLA LACUSTRIS 



These two organisms are good types of the unicellular plants, 

 the former existing as quiescent non-motile cells, the latter 

 having two phases, one motile, the other not. As unicellular 

 forms they are allied to a large number of low types of plants 

 included in the group known as Algae in which some forms 

 are included which cannot be accurately determined as plants 

 or as animals. Some of these like the Peridiniales or Dino- 

 flagellata and the Volvocidae are included by botanists as 

 plants by zoologists as animals. 



Pleurococcus is widely distributed in damp places where 

 it exists as a green covering to stones, tree trunks, ground, etc., 

 Each cell is composed of protoplasm differentiated into cyto- 

 plasm and nucleus, and containing minute grains of starch. 

 Green coloring matter, chlorophyll, is uniformly distributed 

 throughout the cell, which, finally is covered by a transparent 

 coating of cellulose, a distinctly plant product similar in 

 chemical composition to starch but with a different arrange- 

 ment of molecules. Reproduction occurs by simple division, 

 the daughter cells separating when formed or remaining to- 

 gether in groups of two, three or more cells, sometimes eight or 

 nine forming a loosely arranged colony. The union is only 

 temporary however, for ultimately the cells separate and live 

 as independent units (Fig. 40). 



Sphaerella lacustris in its quiescent phase is similar to 

 Pleurococcus save for the presence of what are termed chloro- 

 plastids, by-products of the cell which are specialized for the 

 purpose of manufacturing chlorophyll, which is now confined 

 to these bodies. At times the green color is replaced by a dis- 

 tinct red haematochrome which is only a masked form of chloro- 

 phyll and is known to be a condition brought about by lack 

 of nitrogen. The flagellated phase of Sphaerella is quite 



