Fixation of Nitrogen. 199 



at last discovered that the ability of leguminous plants 

 to grow under these conditions was connected with the 

 presence of nodules on their roots and that in the ab- 

 sence of such structures, the legumes required as fertile 

 a soil in order to produce a good crop as did any other 

 plant. It was also found that where the nodules did not 

 develop the legume, instead of having a beneficial effect 

 on the soil, left it less fertile, the same as any other plant. 

 It was also found that when the nodules developed on 

 the roots, the beneficial effect was due to the fact that the 

 soil contained more nitrogen after the crop had been re- 

 moved than before, even though that portion of the crop 

 which was removed contained a considerable amount of 

 nitrogen. 



Cause of the nodules. The root hairs of the plant 

 which are exceedingly delicate and unprotected by any 

 thick membrane become infected with the legume bac- 

 teria which are in the soil. These cause an increased cell 

 growth at the point of entrance, thus forming the nodule 

 or tubercle which becomes filled with bacterial cells. In 

 some way not well understood, these organisms are able 

 to fix the nitrogen of the soil air and build it up into such 

 a form that the green plant can use it. 



Through these discoveries the peculiar effect of the 

 leguminous plants on the fertility of the soil and the 

 ability of the plant to grow in the absence of combined 

 nitrogen was explained. It is evident that the farmer 

 should be certain that any legume he grows should pos- 

 sess an abundance of nodules on the roots so that he will 

 receive the maximum benefit from the crop. 



Different kinds of legumes require different kinds of 

 bacteria, as for instance the organisms that normally in- 

 fect clover have no influence on alfalfa or vice versa.. 



