within the nodules fix atmospheric nitrogen and thus furnish ni- 

 trates for the growth of the plant. Because of its economic im- 

 portance the genus has been studied in great detail. Morpholog- 

 ically the genus may be divided into two subgroups, mainly on 

 the basis of flagellation, which correlates fairly well with physio- 

 logical and cultural characteristics. The "phytopathogenicity" of 

 the organisms is very specific and the genus may be separated 

 into a number of so-called inoculation groups, each group infect- 

 ing a specific genus or group of genera of legumes. The bacteria 

 in each inoculation group generally have similar morphology and 

 cultural characteristics as shown in Table IV. 



The flagellation of eighty-two strains from thirty-seven plant 

 species was studied. All of these were furnished by Dr. L. W. 

 Erdman of the U.S.D.A., Beltsville, Maryland. 



Flagellar Characteristics 



Most strains of Rhizobium are rather poorly flagellated. Var- 

 ious media and cultural conditions were tried to improve the 

 flagellation but with rather limited success. Best results were 

 usually obtained with a peptone-mannitol broth incubated at 20° C. 

 In a few instances an agar medium of same composition gave as 

 good or better results. With a few cultures stains could be made 

 after 1-2 days incubation but most cultures required 3 or more 

 days to produce sufficient growth for staining. The flagella stained 

 readily and none of the cultures were completely devoid of flagel- 

 lated individuals. 



Rhizobium strains show two main types of flagellation, a non- 

 polar or peritrichous type and a subpolar monotrichous type. 

 The subpolar monotrichous flagellum usually emerges quite close 

 to the somatic pole and at a right angle to the long axis of 

 the soma. This is quite distinct from the situation in polar flagel- 

 lation where the flagellum emerges in a direction parallel to the 

 long axis of the soma. The subpolar rhizobia are almost invariably 

 monotrichous. Two flagella at the same pole are rarely observed 

 and one flagellum at each pole has never been observed. Of all 

 the strains of rhizobia studied the best flagellation is generally 

 found among the subpolar types. 



The other main type of flagellation may be called peritrichous. 

 Most of the strains studied with this type of flagellation were 

 poorly flagellated with most of the flagellated individuals having 

 only one flagellum, rarely several. The flagella, whether single or 



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