Mccliaiiisin of Nitrogen Fixation 



221 



Fig. 91. Scnbeaiis grown in sand culture. Left, no humus; right, abundant 

 suppK of luimus. Note how luinius has stimulated root and plant growth (from 



Blair). 



Mechanism of Nitrogen Fixation by Leguminous Plants 



The mechanism of nitrogen fixation by the leguminous plants has 

 long been, and still is, a subject of controversy. These plants fix 

 atmospheric nitrogen through their roots and not through their 

 leaves, as was first assumed for some plants. In the early stages of 

 growth, the roots contain the larger part of the nitrogen in the 

 plant; at the time of harvest, however, 74 per cent of the nitrogen is 

 found in the tops. The fixation of the nitrogen takes place in the 



