TRANSFORMATIONS OF CARBON AND NITROGEN. 



75 



kindred compounds. The direct oxidizing power of the air 

 may produce a destruction of part of the compounds thus 

 formed, but the microorganisms, including yeasts, fungi and 

 bacteria, complete the process of destruction, so that eventually 

 most of the carbonaceous material extracted from the air is 

 returned to it in its original form. 



That portion of the plant food that is built into fats and into 

 proteids has a future history of reduction which is practically 

 identical with that of the same substances in animals. All 

 these substances may, therefore, be best considered in connec- 

 tion with animals. 



By these various transformations the cycle of most of the 

 carbon is completed. The accompanying diagram (Fig. 11) 



FIG. ii. 



<7 



The carbon cycle. 



will illustrate this cycle, showing graphically the course of 

 transformation of the element from its origin in the air as CO 2 , 

 through the bodies of animals and plants, back again to the 

 atmosphere. 



