But this amount is very small indeed when we consider what is being used 

 up by plants from hour to hour. There is, however, still another source. 



We have seen (see page 84) that all living things, while using oxygen 

 from the air, are at the same time throwing off carbon dioxide. Moreover, 

 every fire discharges quantities of carbon dioxide. This carbon dioxide in 

 the air then becomes raw material for food in green plants. However, the 

 amount of carbon dioxide that fires and animals can yield is limited by the 

 quantity of plant life. For the only fuel available is the organic material 

 which green plants manufactured in the first place. 



We see, then, that our lives depend upon the green plants, and that, 

 on the other hand, the growth of green plants depends upon the oxidation 

 of organic substances in the bodies of animals or in fires. There is, thus, a 

 certain balance between the total quantity of plant life in the world and the 

 total quantity of animal life. If the amount of animal life should diminish 

 very greatly, the growth of plants would in time be slowed or stopped by 

 the lack of carbon dioxide. Should the amount of plant life decrease 

 greatly, the growth of animals would soon reach a limit for lack of food 

 (see illustration, p. 150). 



The Oxygen Cycle Oxygen is the most abundant of the elements in 

 the earth's crust; and the amount of oxygen in the atmosphere is very 

 much greater than the amount of carbon dioxide. But it is a limited 

 amount. Now all living things are constantly drawing upon this oxygen, 

 for living includes the release of energy by the oxidation of food sub- 

 stances. After oxygen has taken part in the oxidation of organic material, 

 it is no longer available for similar action. Through photosynthesis, oxy- 

 gen is liberated, and thus becomes again available for the breathing of 

 animals and plants. If all green plants should suddenly stop their activ- 

 ities, the amount of oxygen would as rapidly diminish. In a short time 

 animal life would cease (see illustration, p. 150). 



The Nitrogen Problem In the bodies of plants and animals proteins 

 break down into simpler compounds of nitrogen. Plants can use some of 

 these in making new proteins, but others disappear in the air, and so nitrogen 

 is lost from the cycle of life. But of all the common elements, nitrogen 

 seems to be the one that does not come back into the life cycle by an auto- 

 matic process. 



The dead bodies of plants and animals on the ground and in the 

 ground contain vast quantities of nitrogen compounds, as well as of fats 

 and carbohydrates. These bodies are devoured by smaller organisms, 

 down to the decay action of bacteria and fungi, and the material is finally 

 returned to the soil and the earth. Particles of nitrogen at any moment 

 present in a living thing, as well as the particles of other elements, are thus 

 on their way out — in a constant process of circulating through the air and 



149 



