114 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



that a minute bacterium is responsible for this process. The bac- 

 terium in question was finally (1888) isolated in pure culture by the 

 Dutch bacteriologist Beijerinck, and was named Bacillus radicicola. 

 This organism provokes the development of certain growths, 



Fig. 52. — Influence of the legume organisms on growth of peas. All are 



growing in sterilized sand, but in the four pots placed alternately, which show 



good growth, extract of garden soil has been added (from HeUriegel and 



Wilfarth, after Russell). 



called nodules, on the roots, and in these newly formed biological 

 laboratories causes the fixation of nitrogen to take place. 



Since the organisms are not contained within the seeds of the 

 legumes they must enter the plant during its growth in order to 

 develop their effects. Infection takes place through the root hairs, 



