178 



THE NITROGEN CYCLE 



roots in soil and sand under controlled conditions. Their 

 results may be summarised as follows: — 



In normal soil, normal growth occurs and nodules are formed 

 on the roots ; analysis shows that an increase in the nitrogen- 

 content of the system (soil -f plant) has occurred during 

 growth. If the peas are sown into sterile sand from which 

 all fixed-nitrogen has been washed away, then no growth 

 can occur and no nitrogen-fixation takes place. If the 

 sterile washed sand is treated with fixed-nitrogen in the 

 form of nitrate, etc., then growth occurs, but no formation 

 of nodules takes place, and analysis shows that the nitrogen- 

 content of the system (soil + plant) has remained constant, 

 so that nitrogen-fixation has not occurred. On the other hand, 

 if the soil has been freed from fixed-nitrogen but not sterilised, 

 then normal growth takes place, nodules appear on the roots, 

 and the entire nitrogen-content of the plant is obtained by 

 fixation of atmospheric nitrogen. These experiments demon- 

 strate that (1) nitrogen-fixation occurs only in the presence 

 of root-nodules, and (2) nodules never form on the roots if 

 growth takes place in sterile soil. 



The formation of nodules on the roots of these plants is 

 due to the action of bacteria called Rhizobia, which exist in 

 soil and are able to penetrate the root-hairs of certain plants 

 with which they come into contact. When penetration has 

 taken place the root-cells in the vicinity of the invading 

 bacteria are stimulated to division and the increased growth 

 so produced gives rise to the nodule. The Rhizobia in contact 

 with the plant within the nodule are now able to carry out 



