The Relation to Nitrogen 239 



Parasites generally make little or no return to the hosts 

 in which or upon which they live. These nodule bacteria, 

 Pseudomonas radicicola, are exceptions to this rule. The 

 tubercles are, in fact, root colonies of the micro-organisms. 

 The bacteria get their carbon, minerals, and water from 

 the host, yet ultimately they give to the host in return 

 combined nitrogen which has been acquired by the fixa- 

 tion of the free nitrogen of the air. 



In the earliest days of historic agriculture, it was known 

 that leguminous plants benefit the land for succeeding 

 crops. The methods by which benefit results were, of 

 course, unknown. When it was ascertained that the chief 

 benefit is concerned with the accumulation of nitrogen, 

 it was assumed that these legumes and other plants might 

 themselves be able to assimilate atmospheric nitrogen. 

 Boussingault's experiments clearly demonstrated the incor- 

 rectness of this view. His work was convincing, and 

 finally attention was directed to the tubercles of legumi- 

 nous plants as the cause of nitrogen accumulation. 



Hellriegel and Wilfarth demonstrated that on sterile 

 soil no tubercles are present and no nitrogen is fixed. 

 The complete chemical and biological studies which sub- 

 sequently followed have led to a full confirmation of the 

 work of Hellriegel and his associate. It is now a simple 

 matter to determine that leguminous plants growing in 

 the absence of the bacteria are wholly dependent upon the 

 soil supply of combined nitrogen, whereas, in the presence 

 of the proper bacteria, such plants are able to reach normal 

 development with a deficient soil supply of nitrogen, and 

 even to give to such deficient soil, through root decay, an 

 increased nitrogen content. 



