ECOLOGY 



95 



is that which exists between bacteria and seed plants. Certain 

 bacteria live in the soil and have the power of assimilating the 

 free nitrogen of the air, of breaking up ammonia compounds, or 

 of oxidizing nitrites into nitrates and thus bringing the nitrogen 

 compounds into a form available for green plants. Such nitro- 

 gen bacteria accumulate in masses on the roots of leguminous 



Fig. 50. — The trap of Ihe Venus fly-trap, half closed. X2. 



plants, presumably finding there conditions favorable to their 

 own development and certainly enabling the associated seed 

 plant to thrive in soil which would otherwise be too poor in 

 available nitrogenous compounds to support the plant. 



Carnivorous Plants 



218. A considerable variety of unrelated plants have acquired 

 the power in one form or other of capturing small animals and 



