134 PRINCIPLES OF SOIL MICROBIOLOGY 



The nodule bacteria can be modified in their ability to grow under 

 unfavorable conditions; a character, such as tolerance to dyes, may 

 be modified relatively quickly (Burke and Burkey). 151 However, 

 the character which has been lost as a result of cultivation on artifi- 

 cial media is quickly regained when the culture is returned to the 

 soil. 



Specific differentiation. Three groups of methods are usually em- 

 ployed for the specific differentiation of the nodule bacteria: (1) plant 

 inoculation, (2) morphological and cultural studies, (3) serological 

 and immunological reactions. Although Nobbe, Hiltner and Schmid 152 

 came to the conclusion that the bacteria in the nodules of all legumes 

 are strains of the same organism, the fact was soon brought to light 

 that not all the bacteria obtained from the nodules of various plants 

 can cross-inoculate and produce nodules on the roots of other leguminous 

 plants. These plants could readily be divided into several closely re- 

 lated groups, the plants belonging to each group having their own 

 specific organism, with cross inoculation taking place only by the mem- 

 bers of each group. 



Hiltner and Stdrmer 153 came to recognize, on the basis of morphological 

 and cultural studies, two groups of nodule bacteria: (1) Bad. radicicola 

 on Pisum, Vicia, Lathyrus, Phaseolus, Trifolium, etc., and (2) Bact. 

 beijerinckii on Lupinus, Ornithopus, Glycine. The former grows well 

 on certain gelatin media and readily produces branching forms, while 

 the latter grows poorly on gelatin media. It was soon found that a 

 further subdivision would have to be made, Pisum, Trifolium, Medicago 

 and Lupinus bacteria being taken as representative types. 



Zipfel 154,155 made use of agglutination tests and concluded that nodule 

 bacteria were not varieties of the same species, but that distinct species 

 existed. Six groups were thus distinguished: (1) Lupinus, (2) Trifolium, 

 (3) Medicago, (4) Pisum, (5) Faba, and (6) Phaseolus. 



151 Burke, V., and Burkey, L. Modifying Rhizobium radicicolum. Soil Sci. 

 20: 143-146. 1925. 



162 Nobbe, F., Hiltner, L., and Schmid, E. Versuche iiber die Biologie der 

 Knollchenbakterien der Leguminosen, insbesondere liber die Frage dei Arteinheit 

 derselben. Landw. Vers. Sta. 45: 1-27. 1895. 



153 Hiltner, L., and Stormer, K. Neue Untersuchungen iiber die Wurzelknoll- 

 chen der Leguminosen und deren Erreger. Arb. k. Gesundhtsamt., Biol. Abt. 3: 

 151-307. 1903. 



154 Zipfel, 1912 (p. 131). 



165 Vogel and Zipfel, 1921 (p. 130). 



