FIXATION OF ATMOSPHERIC NITROGEN. 



279 



direct action on the plants as well as on the bacteria in the soil. Similarly, 

 the tubercle bacteria are favorably affected in their survival and multi- 

 plication by an abundant supply of organic matter. On the other hand, ( 

 acid soils or those deficient in humus and inadequately aerated are but ill 

 suited to the activities of Ps. radicicola. 



FIG. 70. These two pea plants were grown in clean quartz sand to which had been 

 added small quantities of all the necessary elements of plant food except nitrogen. The 

 conditions were exactly identical except that plant A was without root nodules (see 

 Fig. 71) and plant B had numerous nodules well developed (see Fig. 72). (Mich. 

 Exp. Station.) 



