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TEXTBOOK OF BOTANY 



contain millions of bacteria, which become somewhat enlarged and 

 branched (Fig. 232). 



^.1>^'^' 





^., ^ 









Fig. 232. Nitrogen-fixing bacteria: A, section of a nodule of bean root in which 

 bacteria were present in many cells; B, one of the cells containing bacteria much 

 enlarged; C-E, forms of nitrogen-fixing bacteria from nodules of different host 

 plants: C, from soybean, D, from alfalfa, E, from common bean. From E. B. 

 Fred, I. L. Baldwin, and E. McCoy, Univ. of Wisconsin, 1932. 



The bacteria are dependent on the carbohydrates in the legume roots. 

 Unless there is an abundance of sugar in the roots of the host plant, few 

 or no nodules develop, because the bacteria do not invade the root hairs. 



Although se\eral species of bacteria infect legumes, there mav be 

 considerable specificity in their relations to the host. Some inxestigators 

 have classified legumes on the basis of the bacteria to which the^' are 

 most susceptible. The red, white, and alsike clovers, for example, consti- 

 tute one group; alfalfa, sweet clover, and yellow trefoil another group; 

 certain peas a third group; and cowpeas, peanuts, lima beans, and Japan 

 clover still another group. Recent experiments indicate that the specificity 

 of the nitrogen-fixing bacteria may not be as pronounced as earlier ex- 

 periments seemed to show. 



Although the series of events in "fixing" nitrogen are not well known, 

 the important process is the combining of free nitrogen with other 



