The Balance of Powers 219 



between the two. The resistance of the plants must not 

 be too great to exclude the bacteria; the virulence of 

 the bacteria must not be great enough to prevent the 

 plant from securing the nitrogen fixed by them. It is 

 only when the true balance is established that a con- 

 dition of symbiosis is created. It is conceivable that 

 the virulence of the bacteria may be augmented to such 

 an extent as to enable them to retain all of the nitrogen 

 fixed by them for their own purposes. In such a case, 

 the bacteria would contribute nothing to the growth of 

 the host-plant, but would rather constitute a drain on 

 its starches, sugars and mineral salts. In other words, 

 they would become parasites. It is further believed by 

 some bacteriologists that vigorous bacteria need not 

 necessarily be also virulent bacteria. According to them, 

 vigorous strains of the nodule-bacteria that possess 

 but a slight power of fixing atmospheric nitrogen may 

 be produced. Such organisms would act as true para- 

 sites within the nodules. 



The final explanation. — These theoretical considera- 

 tions may help to account for the beneficial effects ob- 

 served at times from light top-dressings of nitrate on new 

 fields of alfalfa. By increasing the vigor, and, hence, the 

 resisting power of the young plants, the nitrate ex- 

 cludes from the roots all but the most virulent bacteria 

 and leads, thereby, to a greater ultimate fixation of 

 nitrogen. Heavier applications of nitrate would prove 

 objectionable, since they would exclude the bacteria to 

 too great an extent. Feeble plants, on the other hand, 

 would not only allow the entrance of bacteria of a low 

 degree of virulence; but would be miable to force tho 



