426 THE PRINCIPLES OF SOIL MANAGEMENT 



from the nodules of legumes are able to fix atmos- 

 pheric nitrogen even when not associated with legumi- 

 nous plants. There would seem to be no doubt, there- 

 fore, that the fixation of nitrogen in the tubercles of 

 legumes is accomplished directly by this organism, 

 and not by the plant itself, or through any combina- 

 tion of the plant and organism, both of which hy- 

 potheses have been advanced. The part which the 

 plant plays is doubtless to furnish the carbohydrates 

 required in large quantities by all nitrogen-fixing 

 organisms and which the legumes are able to supply in 

 large amounts. The utilization of large quantities 

 of carbohydrates by the nitrogen-fixing bacteria in 

 the tubercles may also account for the small proportion 

 of non-nitrogenous organic matter in the plants. 



Kow the plant absorbs this nitrogen after it has 

 been secured by the bacteria is less well understood. 

 Early in the growth of the tubercle, a mucilaginous 

 substance is produced which permeates the tissues 

 of the plant in the form of long, slender threads, and 

 which contain the bacteria. These threads develop 

 by branching or budding, and form what have been 

 called Y and T forms known as bacteroids, which are 

 peculiar to these bacteria, and not produced by them 

 when grown in the media of the laboratory. The threads 

 finally disappear, and the bacteria diffuse themselves 

 more or less, through the tissues of the root. What 

 part the bacteroids play in the transfer of nitrogen is 

 not known. It has been suggested that in this form 

 the nitrogen is absorbed by the tissues of the plant. 

 It seems quite likely that the nitrogen compounds 



