424 



THE PRINCIPLES OF SOIL MANAGEMENT 



the crop produced is less in amount and the percentage 

 of nitrogen in the crop is less. 



It has for some years been the belief that the or- 

 ganism which produces the nodules and utilizes the 

 uncombined nitrogen is the Pseudomonas radicicola, 



but this has very lately 

 been called in ques- 

 tion. 



The nodules are not 

 normally a part of 

 leguminous plants but 

 are evidently caused 

 by some irritation of 

 the root surface, much 

 as a gall is caused to 

 develop on a leaf or 

 branch of a tree by an 

 insect. In a culture 

 containing the proper 

 bacteria, the prick of 

 a needle on the root 

 surface will cause a 

 nodule to form in the 

 course of a few days. 

 The entrance of the 

 bacteria is effected 

 through a root -hair 

 which it penetrates, 

 and may be seen as 



Fig. 114. Nodules on the roots of an cl , 



alfalfa plant Bacteria live in these nodules, a Ulament extending 



or tubercles, and have the power of utilizing . i , i ,1 < , 1 



the free nitrogen of the air in their growth. the entire length 01 the 



