Relations of Bacteria to Legumes 



217 



soon turn yellow and pass into a state of "nitrogen- 

 hunger." In this weakened state, they have but slight 

 power of resistance, and the nodule-bacteria find it 

 comparatively easy to enter their roots. On the other 

 hand, the legumes grow- 

 ing in soils well supplied 

 with available nitrogen 

 compounds remain vigor- 

 ous and retain a high 

 power of resistance, thus 

 precluding the entrance 

 of the bacteria. Further- 

 more, it is known that 

 after some tubercles are 

 formed on the legume 

 plants, the formation of 

 additional tubercles be- 

 comes more difficult. 

 Now, since plants already 

 possessing a few tuber- 

 cles are better supplied 

 with nitrogen, their vigor 

 is increased and they can 

 better resist the attacks of the bacteria. 



Virulence. Another conception in the relations of 

 legumes and their nodule bacteria should not be over- 

 looked. In considering the greater or slighter resist- 

 ing power of different plants under different con- 

 ditions, it should be remembered that the bacteria, also, 

 may display differences in their ability to penetrate 

 the legume roots, as well as in their ability to fix atmos- 



Fig. 35. Root tubercles of healthy al- 

 falfa plants. The tubercles are 

 small and numerous. 



