192 FIXATION OR ASSIMILATION OF NITROGEN 



70 C. or over entirely checked the formation of nodules 

 suggested that living organisms were in some way 

 responsible for the appearance of these peculiar struc- 

 tures, and since the researches of Hellriegel and Wilfarth 



the soil bacteria which stimu- 

 late the production of root 

 nodules have been isolated and 

 extensively studied. 



The evidence obtained by 

 Hellriegel and Wilfarth of the 

 assimilation of free nitrogen 

 was indirect : they showed a 

 large gain of nitrogen in plants 

 grown under conditions in 

 which no other source than 

 that of the free nitrogen of 

 the air was available. In 

 1890, however, direct proof of 

 the removal of free nitrogen 

 from the air and its fixation 

 in the plant was obtained by 

 Schloesing and Laurent. They 

 grew leguminous plants in 

 closed vessels and made an- 

 alyses of the air at the be- 

 ginning and end of the 

 FIG. 33.-Root of runner bean (Phase*- experiment i the amount of 



nitrogen taken from the air 



was found to correspond very closely with the increase 

 of nitrogen in the crop. 



3. The form, number and arrangement of the root- 

 nodules upon the different species of leguminous plants 

 varies considerably. Those of lupins are large, com- 



