SYMBIOTIC NITROGEN FIXATION 



159 



This fuel is silently passed down to those tiny dynamos, the bac- 

 teria, which fill the nodules on all the roots. Here the carbohy- 

 drates are burned, and the resulting energy used to cause the lazy 

 nitrogen to join hands 

 with hydrogen and oxy- 

 gen. Some of the 

 resulting protein is used 

 in the construction of 

 more bacterial cells, but 

 most of it is passed on 

 to the plant and then be- 

 comes a part of its tissues 

 and later its seeds. The 

 bacteria probably receive 

 this fuel in the form of 

 a soluble carbohydrate, 

 but we do not under- 

 stand the medium of ex- 

 change used in payment. 

 Is it a nitrite, a nitrate, 

 an amino-acid, or a pro- 

 tein? We only know 

 it is a nitrogen-carrying 

 compound which can be 

 utilized by the plant in 

 all of its constructive 

 metabolism. 



The Nodules. — These small wart-like protuberances which 

 occur only on the roots of infected legumes are the home of 

 bacteria-like microorganisms called bacteroids. These get into 

 the roots of the young, partly starved plants. As the tip of the 

 root hairs of the legumes pushes itself out into the soil, it chances 

 to come into intimate contact with the microbe. Some scientists 

 believe that the organism is attracted to the plant by chemotaxis. 

 They believe that the plant excretes a substance, probably a car- 

 bohydrate, which passes into the soil solution and attracts the mo- 



FiG. 33. — Nodules on soy beans. 

 , sell and Hasting.) 



(After Rus- 



