[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 181 



teria and small animals in the bottle. We might avoid many of these or- 

 ganisms by using sterilized water and pure chemical salts, but some of 

 them are also in the gelatinous walls of the algae, upon the surface of 

 the fish, and especially in its alimentary canal. 



Fig. 65. Diagrams of microcosms in sealed containers in which the organisms 

 indicated live for many months: A, algae; B, algae and a fish; C, Helxine 

 (Paddy's wool). 



We may ignore these small organisms for the moment and begin with 

 the fish and algae. The statement that "the fish and algae each give off 

 what the other needs," is both false and misleading. Of the many things 

 necessary for the existence and growth of the algae in the bottle, only 

 one of them comes in part from the fish, but the algae are not dependent 

 upon the fish or any other animal for even that one. To explain the 

 microcosm it is necessary to account for (1) the oxygen used by both 

 the fish and the algae in the process of respiration, ( 2 ) the food used by 

 botli fish and algae in respiration and assimilation, (3) the carbon di- 

 oxide used by the algae in photosynthesis, (4) the energy used by the 

 plant in photosynthesis, ( 5 ) the chemically bound energy used by both 

 fish and algae, and (6) the conditions under which respiration and 

 assimilation in both fish and algae are compensated by photosynthesis in 



