460 SOILS: PBOPEBTIES AND MANAGEMENT 



nodules of legumes are able to fix atmospheric nitrogen 

 even when not associated with leguminous plants. There 

 would seem to be no doubt, therefore, that the fixation of 

 nitrogen in the tubercles of legumes is accomplished di- 

 rectly by this organism, not by the plant itself nor through 

 any combination of the plant and the organism — though 

 both of these hypotheses have been advanced. The part 

 played by the plant is doubtless to furnish the carbohydrates 

 which are 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 tuber- 

 cles may also account for the small proportion of non- 

 nitrogenous organic matter in the plants. 



How 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 containing the bacteria. 

 These threads develop by branching or budding, and form 

 what have been called Y and T forms, known as bac- 

 teroids, which are peculiar to these bacteria. 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 produced within the bacteria 

 cells are diffused through the cell wall and absorbed by 

 the plant. 



380. Soil inoctilatioii for legumes. — Immediately fol- 

 lowing the discovery of the nitrogen-fixing bacteria, the 

 possibility was conceived of securing a better growth of 



