THE RELATIONS OF PLANTS TO ANIMALS 185 



to at least part of the animal life within the aquarium. The ani- 

 mals give off certain nitrogenous wastes of which we shall learn 

 more later. These materials, with other nitrogenous matter from 

 the dead parts of the plants or animals, form the part of the raw 

 material of the proteid food manufactured within the plant. The 

 animals eat the plants and give off organic waste, from which the 



A balanced aquarium. 



plants make their food and living matter. The plants give off 

 oxygen to the animals, and the animals give carbon dioxide to the 

 plants. Thus a balance exists between the plants and animals 

 in the aquarium. 



Relations between Green Plants and Animals. What goes 

 on in the aquarium is an example of the relation existing between 

 all green plants and all animals. Everywhere in the world green 

 plants are making food which becomes, sooner or later, the food of 



