106 PLANTS, THE FOOD OF ANIMALS 



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 activities. Surrounded by an impervious 

 membrane of cellulose there is no trace of a mouth opening and 

 solid matters or dissolved proteids 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 and in 

 this way the plant cells take in CO2, water, salts of various kinds, 

 and give out CO2 free oxygen, and waste matters, none of which 

 have much energy stored up in them. The carbon dioxide and 

 water are broken down into their constituent parts through the 

 energy of sunlight acting through the chlorophyll and the ele- 

 ments thus freed are recombined into sugars and starch from 

 which, by a series of changes which can best be described so far 

 as known in connection with a higher type of plant, they are 

 finally made into protoplasm, and this when life is extinct, be- 

 comes proteid, the food of animals. 



The higher animals, like Hydra, must likewise turn to the 

 plant world for food, but to trace back the connection in some 

 cases would involve a far more complicated chain of organisms 

 than in the case of the simple coelenterate. Man is almost 

 omnivorous, taking his food directly from the vegetable kingdom 

 in his green vegetables, cereals, etc., and from the animal in his 

 beef, mutton, fish, or fowl. Cattle, sheep and birds, in turn, are 

 herbivorous or graminivorous and get their main nourishment 

 from plants. Birds, indeed, eat worms and insects to a great 



