38 THEORIES Ox\ THE ABSORPTION OF NITROGEN 



The microbe acts as a parasite so long as the tubercle 

 is incomplete, for the bacterium can only fix atmospheric 

 nitrogen after its transformation into bacteroids, i.e., after 

 dividing to form branching filaments. Consequently during 

 this transformation the plant must suffer, the bacterium 

 growing at its expense. In proportion as the nodule in- 

 creases in size so its richness in nitrogen increases; thus, 

 generally speaking, we find the greatest nitrogen content 

 towards the period of flowering and before the formation 

 of the fruit. 



We have had the opportunity of observing, says M. 

 Kayser, that Leguminos^e growing under certain conditions 

 do not produce nodules. The explanation of this can be 

 found in conclusions enumerated in certain works showing 

 that the plant does not form nodules when it has another 

 source of nitrogen at its disposal, or when the bacterium 

 present is not adapted to the particular leguminous plant. 

 This further explains how the plant which has another 

 source of nitrogen is able to resist infection. 



Woltmann asserts that humus, lime, &c., favour the 

 development of the root tubercles. Perrotti attributes their 

 growth and the increased crop to manganese, nickel, and 

 cobalt. 



Nobbe and Hiltner have drawn attention to another 

 factor, the relative virulence of the microbe. This virulence 

 varies in intensity and depends on the relative ease with 

 which the microbe is able to invade the root and to form its 

 colonies. It is on this that the proportion of nitrogen fixed 

 depends, for every kind of Leguminos^ offers more or less 

 resistance to the invasion of bacteria. 



There are, however, other possibilities, and M. Kayser, 

 in his "Agricultural Microbiology," writes as follows: — 



" Firstly, there are some bacteria which are incapable of 

 penetrating the root hairs through their inability to secrete 

 the diastase necessary to convert the hair membrane into 

 viscous matter. 



