Specific DifFercntiation 217 



species but arc distinct species. He rec(),<2;iii/.ctl six groups of organ- 

 isms capable of iufccting (1) lAipinus, (2') Trifolium, (3) Medicago, 



(4) Pistim, (5) Falxi, and (6) PIkiscoIiis. Otlier investigators rec- 

 ognized nine groups of Icgiune liactcria on the basis of serological 

 investigations: (1) Ijipintis and Ornithopiis, (2) McIHotiis, Medi- 

 cago, and Trigoiu'lla, (3) Vicia (V. sativa), (4) PisiiDi, (5) Vicia 

 faha, (6) TrifoUum pratense, (7) Pliascohis, (8) Glycine (Soja), 

 and (9) Onobrycltis saliva. 



Bergey placed the root-nodule organism in a separate genus, 

 Rliizol)ium, and divided the different forms into two species: (1) 

 RJi. leguminosaniin Frank, inoculating Pistim, Vicia, Lathyrus, etc., 

 and (2) Rh. radicicola Beij., producing nodules on TrifoUum, Pliasc- 

 ohis, and others. 



Fred, Baldwin, and McCoy classified these bacteria into seven 

 groups: (1) alfalfa group, RJi. meliJoti; (2) clover group, Rh. trifolii; 

 (3) pea group, Rh. legiiminosarum; (4) bean group, Rh. phaseoli; 



(5) lupine group, Rh. hipini; (6) soybean group, Rh. japoniciim; 

 (7) cowpea group, Rliizobium sp. 



Within each species, there are various strains, which differ pri- 

 marily in their effectiveness, or ability to fix free nitrogen in associa- 

 tion with the proper host plant. Various explanations have been 

 suggested for the specificity of the root-nodule organisms, based on 

 soil reaction and climate. It was at first believed that this is a case 

 of specific enzymes produced by the bacteria or of differences in the 

 root sap. The various members of each cross-inoculation group are 

 closely related with respect to protein characteristics of their seeds. 



It was at first believed that some plants will interact with several 

 strains of Rhizobium, whereas other plants are limited to particular 

 strains. Cross-pollinating plants were said to be inoculated by more 

 bacterial strains than are self-pollinating plants. The application of 

 serological reactions brought out the fact that various strains of bac- 

 teria may form nodules on the same host plant, but only one type 

 is found in the same nodule. That not all strains of the organism 

 are capable of inoculating one type of plant suggests the existence 

 of various biotypes even for the same plant. Two types of the 

 organism can form nodules on the soybean plant. Both are iden- 

 tical morphologically, but they are different physiologically and 

 serologically. 



