RI-X :i..MMIN(i LOST NITKOi.LN. 145 



A I.MILATION OF NITROGEN. 



A still more difficult problem to settle is the relation of the 

 whole process to the assimilation of free nitrogen. That the 

 increase of nitrogen in the legume which develops the nodule 

 is an assimilation of free nitrogen from the air is abundantly 

 demonstrated, though whether the nitrogen is absorbed 

 through the leaves or through the roots is not so easily 

 answered. It has been recently asserted that the nitrogen 

 does not come directly from the air but from the nitrogen dis- 

 solved in the soil water and absorbed through the root hairs. 

 That the ultimate source is the air is certain. 



There are several possibilities suggested as to the relation 

 of the microorganism to the legume. In the first place it is 

 perfectly clear that in the ordinary soil the association of the 

 legume with the microorganism is necessary to the fixation of 

 nitrogen in the form of a nitrogen compound. But whether it 

 is the legume or the bacterium which fixes the nitrogen, or 

 whether it is a phenomenon occurring only as the result of the 

 two organisms combined together in a united growth, is not so 

 clear. It has been claimed, on the one hand, that the bacte- 

 rium itself fixes the nitrogen, and that the root of the legume 

 simply furnishes a convenient and appropriate medium for its 

 growth. This would seem to be rendered possible by the fact 

 already mentioned, that some soil bacteria certainly do fix free 

 nitrogen. If this is the case, the tubercle bacterium should be 

 able to fix nitrogen, when growing vigorously under proper 

 conditions, even in the absence of the leguminous plant. One 

 of the more recent experimenters ( Maze ) has claimed to have 

 demonstrated this fact. Upon cultivating the B. radicicola 

 in an especially favorable medium, and supplying them with an 

 abundance of atmospheric oxygen, he has found an increase of 

 nitrogen in the culture media, which increase he attributed to 

 the assimilating action of the bacteria. If this result be con- 



