ELEMENTARY BIOLOGY 



Oxygen 



dioxid. This carbon dioxid then becomes available as a source 

 of raw material for food in the leaves of plants. Now we must 



remember that the 

 carbon dioxid from 

 fires and from ani- 

 mals is limited in 

 amount by the work 

 of plants, for the 

 only burnable mate- 

 rial that is available 

 is the organic ma- 

 terial manufactured 

 in the first instance 

 by the green plants. 

 Whichever way 

 we go at it, we see 

 that our lives are 

 dependent upon the 

 activities of the 

 green plants ; and 

 on the other hand, 

 the continued exist- 

 ence of new green 

 plants is made pos- 

 sible by the oxida- 

 tion of their organic 

 substances in the 

 bodies of animals 



Carbon 

 Dioxid 



in 

 Atmosphere 



' t x 



FIG. 18. The carbon cycle 



Fires and all kinds of living things are constantly throw- 

 ing carbon dioxid into the air. Green plants, represented 

 by the tree in the diagram, withdraw carbon dioxid from 

 the atmosphere and return oxygen. The material of the 

 green plant is made up in part of the carbon derived 

 from the carbon dioxid. This material serves as food for 



animals and as fuel for fires. The animals oxidize this 

 material ; or they are eaten by other animals. Finally, the 

 carbon of larger plants and animals is oxidized by simple 

 organisms, such as bacteria and fungi, and is returned 

 to the atmosphere 



or in fires. There 

 is, then, a certain 

 balance, or limited 

 relation, 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, there would come a time when the growth of 



