BACTERIA OF THE SOIL 251 



The microorganisms capable of fixing atmospheric nitro- 

 gen are in part symbiotic, that is, living upon or in the roots 

 of higher plants, and nonsymbiotic, that is, living free in 

 the soil. Under each of these headings bacteria and fungi 

 are to be considered. 



Nitrogen Fixation ~by Symbiotic Bacteria. The bacteria 

 which live in the roots of plants and are capable of fixing 

 atmospheric nitrogen belong to the genus Rhizobium. 

 Whether or not more than one species should be recognized 

 is somewhat uncertain. There is some evidence that there 



FIG. 57. RHIZOBIUM LEQUMINOSARUM. A. Peritrichous type. B. Mo- 

 notrichous type. 1. Motile rods. 2. Non-motile or involution 

 forms. 



are several distinct species on the roots of distantly related 

 leguminous plants. For the present discussion, however, 

 all may be regarded as belonging to a single species termed 

 Rhizobium leguminosarum. 1 



This organism is apparently widely distributed in soil. 

 Whether or not it multiplies actively in soil is not very 

 definitely proved. It is usually found associated with the 

 roots of leguminous plants, causing the formation of 

 nodules, sometimes termed "tubercles," upon them. No 

 species of leguminous plant is known which, under its native 

 conditions, does not have these nodules appearing upon the 



1 This organism is variously known in the literature as Bacillus 

 radicicola and Pseudomonas radicicola. 



