TUBERCLE BACTERIA 425 



hair, and into the cells of the cortex of the root, 

 where the growth of the tubercle starts. 



Even where the causative bacteria occur in cultures 

 or in the soil, leguminous plants may not secure any 

 atmospheric nitrogen, or perhaps only a small quantity, 

 if there is an abundant supply of readily available 

 combined nitrogen upon which the plant may draw. 

 The bacteria 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 nitro- 

 gen legumes may, therefore, add little or no nitrogen 

 to the soil, while in properly inoculated soils deficient 

 in nitrogen an important gain of nitrogen results. 



While P. radicicola has been considered the organism 

 common to all leguminous plants, it is now known 

 that the organisms 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 any species as on 

 the one from which the organisms were taken. It was 

 quite generally believed at one time that the longer any 

 species of legume is in contact with the organisms 

 from another species the more active they become, 

 and the greater the utilization of atmospheric nitrogen. 

 Considerable doubt has been cast upon this view in 

 recent years, and it is now generally conceded that 

 the bacteria of certain legumes are not capable of 

 inoculating certain other species of legumes. 



289. Transfer of nitrogen to the plant. It has 

 been shown by several investigators that bacteria 



