218 



Bacteria in Relation to Country Life 



pheric nitrogen when developing within the nodules. 

 The term virulence has been used to designate this 

 variable power of the bacteria. Organisms of a high 

 degree of virulence readily penetrate the legume roots, 

 and secure large quantities of nitrogen from the air. 



Organisms of a low de- 

 gree of virulence are 

 feeble in this respect. 



Under favorable soil 

 conditions the virulence 

 of the bacteria may be 

 increased; under un- 

 favorable soil condi- 

 tions their virulence may 

 be greatly diminished. 

 Hence, soils well pro- 

 vided with lime and 

 humus, soils in good 

 tilth, well aerated and 

 supplied with an abun- 

 dance of moisture, will 

 favor the fixation of 

 large quantities of at- 

 mospheric nitrogen. On 

 the other hand, ill-drained soils, or those deficient in 

 humus, will tend to diminish the virulence of the 

 nodule-bacteria, to the injury of the legume crops that 

 may be grown upon them. 



Balance. — The conception of resisting power of plants 

 on the one hand and that of the virulence of bacteria 

 on the other, introduce a third conception of a balance 



Fig. 36. Root tubercle? of unhealthy al- 

 falfa plants. The tubercles are large 

 and few in number. 



