THE NITEOQEN CYCLE 459 



days. The entrance of the organism is effected through 

 a root-hair which it penetrates, and it may be seen as a 

 filament extending the entire length of the hair and into 

 the cells of tlie cortex of the root, where the growth of 

 the tubercle starts. 



Even where the causative bacteria occur in cultures 

 or in the soil, a leguminous plant may not secure any 

 atmospheric nitrogen, or perhaps only a small quantity, 

 if there is an abundant supply of readily available com- 

 bined nitrogen on which the plant may draw. The bac- 

 teria have the ability to utilize combined nitrogen as 

 well as uncombined nitrogen, and prefer to have it in 

 the former condition. On soils rich in nitrogen, legumes 

 may therefore add little or no nitrogen to the soil ; while 

 in properly inoculated soils deficient in nitrogen an impor- 

 tant gain of nitrogen results. 



While B, radicicola is considered the organism common 

 to all leguminous plants, it is now known that the organ- 

 isms from one species of legume are not equally well adapted 

 to the production of tubercles on each of the other species 

 of legumes. They show greater activity on some species 

 than on others, but do not develop so successfully on all 

 species as on the one from which the organisms were 

 taken. It was rather generally believed at one time that 

 the longer any species of legume is in contact with the 

 organisms from another species, the more active this 

 species becomes and the greater is the utilization of 

 atmospheric nitrogen. Considerable doubt has been cast 

 on this view in recent years, and it is now generally con- 

 ceded that the bacteria of certain legumes are not capable 

 of inoculating certain other species of legumes. 



379. Transfer of nitrogen to the plant, — It has been 

 shown by several investigators that bacteria from the 



