METABOLISM OF COLORLESS PLANTS 



49 



nitrogen in the form of ammonia or free nitrogen escapes 

 to the atmosphere. The greatest loss, however, is brought 

 about by a group of DENITRIFYING BACTERIA whose activities 

 are largely spent in changing nitrates into gaseous nitrogen 

 which escapes into the air, and so putting it beyond the reach 

 of green plants and animals. Fortunately there is also a 

 special group of NITROGEN-FIXING BACTERIA which rescue 



.Animal, 

 Proteins 



Proteins 

 of Green Plants 



itrite Ammonia 

 itrites^i^7 



Denitrifying 

 Bacteria 



FIG. 16. The Nitrogen Cycle. A schematic representation of the circula- 

 tion of nitrogen in nature. 



the nitrogen from the atmosphere and return it to the cycle 

 of elements in living nature. These organisms inhabit 

 the soil or little nodules which they produce on the 

 rootlets of leguminous plants, such as beans, clover, and 

 alfalfa; and this accounts for the fact, long known but not 

 understood, that these plants when plowed under are par- 

 ticularly efficient in enriching the soil. In brief, there is a 

 cycle of the elements in nature through green plants and 

 animals and back again to the inorganic world through the 



