Relations oj 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 plant.s 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. 



