FROM THE AIR BY THE LEGUMINOS^ 39 



" Secondly, there are others which may enter the root 

 hairs, but are then at once engulfed by the cell nuclei, with 

 the result that the nodules are of small size or even com- 

 pletely lacking. 



" Thirdly, the bacteria are fairly virulent in their action 

 and entail the formation of nodules of varying size, secretion 

 of viscous matter, and fixation of nitrogen ; once the plant 

 is invaded, it can successfully resist the attacks of all less 

 virulent species — it has been rendered immune. 



The nodules formed on the lateral roots contain bacteria 

 of greater virulence than those in the primary root. This 

 may be explained by supposing a progressive intensification 

 of the microbe's activity. 



Finally, we mav admit that nodules are formed when 

 bacteria penetrate the root hairs; the bacteria are nourished 

 by carbohydrates and they secrete a viscous matter rich in 

 nitrogen and only perform their functions after conversion 

 into bacteroids. The quantity of bacteroids varies accord- 

 ing to the amount of sustenance in the particular medium. 



Mechanism of Free Xitrogex Assimilation. 



What is the mechanism by which free nitrogen is 

 assimilated ? 



According to M. Kayser, this arrangement is founded 

 upon the exchanges which take place between bacterium 

 and plant. The former receives nourishment from the plant 

 in the form of carbohvdrates, and in exchange returns it 

 the nitrogenous substance, through the intermediarv of 

 the nitrogen of the air. The plant thus profits only through 

 the products of bacterial katabolism. The bacteria are 

 capable of replacing the nitrogen required by the plants 

 only after their conversion into bacteroids; and this is the 

 reason why the mere presence of nodules does not enable 

 us to say that the association is necessarily a profitable one 

 for the plant. 



