THE CHEMICAL CYCLE OF LIFE 



59 



C0<r Animal 



plants would be affected by the lack of carbon dioxid. Should 

 the amount of plant life decrease greatly, a limit to the growth 

 of animals would soon be reached for lack of food (Fig. 18). 



88. The oxygen cycle. In the matter of oxygen we can see 

 a similar relation between plants and animals. The amount of 

 oxygen in the atmosphere is very much greater than the amount 

 of carbon dioxid, but it is a limited amount, and all living things 

 are constantly draw- 

 ing upon it to en- 

 able them to set free 



the energy that they 

 use up in the course 

 of their activities. 

 After oxygen has 

 once been used in 

 the oxidation of or- 

 ganic material, it is 

 no longer available 

 for similar use. If 

 all green plants 

 should suddenly 

 stop their activities, 

 the amount of car- 

 bon dioxid in the 

 atmosphere would 

 steadily increase, 

 but the amount of 



oxygen would just as rapidly diminish. A point would soon be 

 reached at which the maintenance of animal life would be no 

 longer possible. Through photosynthesis oxygen is liberated, 

 thus becoming again available for the breathing of animals 

 and plants (Fig. 19). 



89. The nitrogen cycle. In the matter of nitrogen our de- 

 pendence upon living things of various orders is still more 

 marked. The plants growing in a given area take from it 



C0 2 



CO 2 



FIG. 19. The oxygen cycle 



Oxygen from the atmosphere is taken up by plants, by 



animals, and by fires. All of these return carbon dioxid 



to the atmosphere. The green plants take carbon dioxid 



from the atmosphere and return oxygen 



