216 



Nitrogen Fixation— Symbiotic 



Fig. 90. Different t>iie.s of nodules of leguminous plants. 



Specific Differentiation 



Three groups of methods are usually employed for the specific 

 differentiation of the root-nodule bacteria: (1) plant inoculation, 

 (2) morphological and cultural studies, (3) serological and immuno- 

 logical reactions. 



Zipfel made agglutination tests and concluded that the various 

 root-nodule bacteria do not represent merely varieties of the same 



