698 The Sources of the Nitrogen of our Leguminous Crops, 
much fewer nodules on tlie roots grown in sand, developed to a 
much greater size ; the much larger number iu the soil were 
very much smaller. 
As to the greater number of nodules formed in rich soil 
than in sand, 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 m.ore weakly plant gi'owing in nitrogeh- 
free soil to resist the free development of the parasite. On the 
other hand, in the mixture of I'ich 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 thd more vigorous gro'wth the nodules were 
more rapidly exhausted of their contents to feed the host, ti 
will be obvious that, on the former supposition, some of the 
nitrogen of the restrictedly developed individual nodules may 
have been obtained from the nitrogenous matters of the plaiit 
itself derived from soil-nitrogen, in which case the gain frofli 
fixation would be less than would otherwise be indicated 
the great number of nodules produced ; and in favour of this 
supposition, which implies that in the early stages of the infec- 
tion 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 chai-acteristic 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 — that the small- 
ness of the nodules was due to their rajDid exhaustion by the 
liost — it might be tliat more of the nitrogen of the nodules would 
be due to fixation, and that hence a larger proportion of the I 
total nitrogen of the plant would be gain attributable to thAti 
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. 
