162 THE ROTHAMSTED EXPERIMENTS. 



in rich soil, the explanation may simply be that, as in the 

 sand the infection was dependent on the additions of rich- 

 soil- extract only, the diffusion of the microbes would be only 

 limited, and the infection of the roots therefore only local or 

 accidental; whilst the much greater size of the individual 

 nodules may be due to the want of power in the more weakly 

 plant growing in nitrogen-free soil to resist the free develop- 

 ment of the parasite. On the other hand, in the mixture of 

 rich soil and sand, the microbes would probably be distributed 

 throughout it, and the roots accordingly exposed to infection 

 along their whole range. The much less development of the 

 individual but more numerous nodules in the rich soil may 

 be due to one of two very different causes. It may be that 

 although the more vigorous plants grown in the rich soil 

 could not resist the original infection, they were able to resist 

 the further development of the parasite. Or, it may be that 

 with the vigorous growth, the nodules were more rapidly ex- 

 hausted of their contents to feed the host. It will be obvious 

 that on the former supposition, some of the nitrogen of the 

 restrictedly developed individual nodules may have been ob- 

 tained from the nitrogenous matters of the plant itself, derived 

 from soil-nitrogen ; in which case the gain from fixation would 

 be less than would otherwise be indicated by the great num- 

 ber of the nodules produced ; and in favour of this supposi- 

 tion, which implies that in the early stages of the infection 

 the bacteria derive nitrogenous nutriment from the stores of 

 the higher plant itself, and only later from the fixation of free 

 nitrogen, is the fact of the observed " nitrogen hunger stage " 

 so characteristic of plants for some time after infection when 

 growing in nitrogen-free soil ; probably indicating that during 

 that period the limited stores of the plant are being drawn 

 upon. On the second supposition, on the other hand — namely, 

 that the smallness of the nodules was due to their rapid ex- 

 haustion by the host — it might be that more of the nitrogen 

 of the nodules would be due to fixation, and that hence a 

 larger proportion of the total nitrogen of the plant would be 

 gain attributable to that source. 



Obviously more evidence is needed before a decisive opinion 

 can be formed as to how far fixation of free nitrogen is an 

 essential coincident of nodule-development at all its stages of 

 accumulation, and how far, therefore, the amount of nodule- 

 formation may be taken as a fair measure of the fixation. 



It is to be supposed that when nodules develop abundantly 

 on the roots of leguminous plants growing in soil rich in 

 readily available combined nitrogen, the nitrogen assimilated 

 will be partly due to soil-supplies of combined nitrogen, and 

 partly to fixation. That there is gain when red clover, for 



