UNICELLULAR PLANTS 109 



Sphaerella lacustris in its quiescent phase is similar to Pleuro- 

 coccus, save for the presence of what are termed chloroplastids, 

 products of the cell, which are specialized for the purpose 

 of manufacturing chlorophyll, now confined to these bodies. 

 At times the green color is replaced by a distinct red haema- 

 tochrome, which is only a masked form of chlorophyll and is 

 known to be a condition brought about by lack of nitrogen. 

 The flagellated phase of Sphaerella is quite different in appear- 

 ance from the resting form. The chief structural difference 

 is the presence of two definite flagella, originating from basal 

 granules in the cell protoplasm and extending through the 

 coating of cellulose to the outside, where their undulations 

 in the water lead to energetic and jerky movements of the 

 entire organism. A red so-called " eye-spot " is also present, 

 and represents a more sensitive bit of protoplasm, especially 

 in respect to light (Fig. 41). 



Like Pleurococcus, Sphaerella reproduces by simple division, 

 but the cells do not form colonies or remain connected after 

 division. At times, furthermore, the protoplasm of the cell, 

 protected by the firm cell membrane, divides repeatedly until 

 from thirty-two to sixty-four minute cells are formed. These 

 ultimately break out of the cyst and swim about by means of 

 two flagella. Two of these small products, upon meeting, 

 fuse, lose their flagella, and settle down as a resting cell. This 

 process of conjugation is a primitive type of sexual repro- 

 duction, and the minute cells may be called gametes. 



The chief interest of Pleurococcus and Sphaerella lies in 

 their physiological activites. Surrounded by a membrane of 

 cellulose and an even more resistant cell membrane, solid 

 matters cannot enter the cell. Salts, however, dissolved in 

 water can be absorbed by osmosis through the body wall, and 

 gases can diffuse through the cellulose. In this way the plant 

 cells take in CO 2 , water, salts of various kinds, and give out CO 2 , 

 free oxygen, and waste matters, none of which has much 

 chemical energy. The carbon dioxide and water are broken 

 down into their constituent parts through the energy of sun- 

 light acting through the chlorophyll, and the elements thus 



