FIXATION OF FREE NITROGEN. 153 



How is the Fixation of Nitrogen to he explained ? 



Eeviewing the whole of the results which have been brought Assimiia- 

 forward, there can be no doubt that the fact of the fixation of ^^ 

 free nitrogen in the growth of Leguminosse under the influence from the 

 of suitable microbe-infection of the soil, and of the resulting gl^Med. 

 nodule -formation on the roots, may be considered as fully 

 established. How, then, is it to be explained ? Unfortu- How is it 

 nately there is much yet to learn before a satisfactory answer w^^"; 

 can be given. Obviously we must know more of the nature 

 and mode of life of the organisms which, in symbiosis with 

 the leguminous plant, bring about the fixation of free nitrogen, 

 before the nature of the action can be understood. As to the 

 mode of life of these bodies, we owe much to the investigations 

 of Marshall Ward, Prazmowski, Beyerinck, and others ; and 

 some of their results have been discussed in our papers. But 

 the facts which they have established so far are insufficient to 

 afford an adequate explanation of the phenomena involved. 

 Nobbe, also, has recently published results on the subject. 



It has, indeed, been assumed that the activity of the process One as- 

 depends on the quantity of the nitrogenous compounds at the sum P twn - 

 disposal of the roots — a supposition which implies that the 

 source of nitrogen of the bacteria is the combined nitrogen in 

 the soil. The experimental results which have been described 

 clearly show, however, that the nodules may develop very 

 plentifully in a nitrogen-free soil, and that there may, under 

 such conditions, be great gain of nitrogen if only the soil be 

 suitably infected ; nor would there be any such actual gain of 

 nitrogen in nitrogen-free soils as there undoubtedly is, if the 

 source of the nitrogen, either of the parasite or of the host, 

 were essentially the supplies of combined nitrogen within the 

 soil. 



Further, one assumption is, that the organisms become dis- other 

 tributed in the soil, both during the life of the host and after- theories - 

 wards, and that the fixation takes place under their agency 

 within the soil itself rather than in the course of the develop- 

 ment of the organisms in symbiosis with the higher plant. 

 Another is, that the fixation takes place in the soil itself 

 under the influence of microbes existing within it, and that 

 the higher plant assimilates the resulting combined nitrogen. 

 As bearing upon these points, it may be observed that in the 

 experiments with peas in 1888 there was practically no gain 

 of nitrogen within the soil itself, which it may be supposed 

 there would have been if the fixation had taken place within 

 it, and the host had acquired its gain from the compounds 

 there produced. Indeed, the evidence at present at command 

 certainly does not point to the conclusion that the gain of 



