186 THE RELATIONS OF PLANTS TO ANIMALS 



animals. Man may not feed upon the leaves of plants, but he eats 

 fruits and seeds in one form or another. Even if he doeS not feed 

 directly upon plants, he eats the flesh of herbivorous animals, 



Carbon dioxide 

 (CO2) 



Carbon dioxide 

 A (CO2) 



Energy from sun* 



/ I \ 



Energy set free 

 as heat. 



The relations between green plants and animals. 



which in turn feed directly upon plants. And so it is the world 

 over; the plants are the food-makers and supply the animals. 

 Green plants also give a very considerable amount of oxygen to 

 the atmosphere every day, which the animals may make use of. 



The Nitrogen Cycle. — 



^AniTnal Life 



qsing Bacteria^ 



The animals in their turn 

 supply much of the carbon 

 dioxide that the plant uses 

 in starch-making. They 

 also supply most of the 

 nitrogenous matter used 

 by the plants, part being 

 given the plants from the 

 dead bodies of their own 

 relatives and part being 

 released 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 



Nitrites 

 "Nitric Bacteria 



The nitrogen cycle. 



