NATURE OF THE BACTERIA 



119 



the nodules may appear in abundance close to the main root 

 near the surface of the soil. Where the moisture content of 

 the soil is low, as in the semi-arid regions, nodulation may be 

 prominent at a considerable distance from the surface of the soil. 

 The abundance and size of the nodules are good indexes of the 

 extent of fixation of nitrogen. Where the nodules are large and 

 numerous, nitrogen fixation proceeds rapidly; where the nodules 

 are smaller and more scattered, fixation is much slower. 



Fig. 57. — Nodulation on roots of peas (from Wilson and Leland). 



Nature of the Bacteria and Classification of Legumi- 

 nous Plants. — The bacteria can be readily isolated from the nod- 

 ules, and grown upon artificial culture media and studied carefully. 

 It has been found that not all leguminous plants can be equally 

 infected by the same microbe. In fact, the bacteria infecting 

 legumes include a number of different species or strains, each of 

 which can inoculate only one or more leguminous plants. At 

 least twelve species or strains of the legume bacteria have been 

 established, each of which will inoculate legumes belonging to only 

 one of the groups listed below. Consequently the number of 



