ABSORPTION OF FREE NITROGEN 



105 



which are in a state of very fine division due to the disin- 

 tegrating action of the organisms. The hot, dry, finely 

 divided material oxidizes so rapidly on contact with the air 

 that it ignites. 



A practical use of h^at production by bacteria is in the 

 making of "hot beds" for forcing vegetables (Fig. 73). 



ABSORPTION OF FREE NITROGEN. 



This is likewise one of the most important practical 

 activities of certain types of bacteria present in the soil. 



Fig. 74. — Root tubercles on soy bean. X j- 



The ability of plants of the legume family to enrich the 

 soil has been known and taken advantage of for centuries, 

 but it is only about thirty years since it was demonstrated 

 that this property is due to bacteria. These plants, and 

 several other kinds as well, have on their roots larger or smaller 

 nodules (Fig. 74) spoken of as "root tubercles" which are at 

 certain stages filled with bacteria. When conditions are 

 favorable, these bacteria live in symbiotic relationship with 



