162 THE RELATIONS OF PLANTS TO ANIMALS 



directly ii])oii plants, lie eats the flesh of i)lant eating animals, 

 which in turn feed directly upon i)lants. And so it is the world 

 over; the j^lants are the food makers and supply the animals. 



Carbon dioxide 

 (COo) 



Carbon dioxide 

 A (COo) 



Water 



v(H«0) 



Water 

 (H2O) 



Ammonia 



(nhJ 



Ammonia 



/ t \ 



Energy from sun* 



ids etc 



imal 



/ I \ 



Energy set free 

 as heat. 



The relations between green plants and animals. 



Green plants also give a very considerable amount of oxygen lo 

 the atmosphere every day, which the animals may use. 



The Nitrogen Cycle. — The animals in their turn supply much 

 of the carbon dioxide that the plant uses in starch making. They 



also supply some of the 

 nitrogenous matter used by 

 the plants, part being given 

 the plants from the dead 

 bodies of their own rela- 

 tives and part being pre- 

 pared from the nitrogen of 

 the air through the agency 

 of bacteria, which live 

 upon the roots of certain 

 plants. These bacteria are 

 the only organisms that 

 can take nitrogen from 

 the air. Thus, in spite of all the nitrogen of the atmosphere, 

 plants and animals are limited in the amount available. And the 



Nitric Bacteria 



The nitrogen cycle. Trace the nitrogen from 

 its source in the air until it gets back again 

 into the air. 



