690 The Sources of the Nitrogen of mr Jjegvminous Crops; 
of suitable microbe infection of the soil, and of the resulting 
nodule-formation on the roots, may be considered as fully 
established. 
Admitting, then, the fact of fixation under the conditions 
described, the question still remains : How is it to be explained ? 
Unfortunately, there is much yet to learn before a satisfactory 
answer 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 ; but the facts which they have established so far are in- 
sufficient to afford an adequate explanation of the phenomena 
involved. 
It is a point of importance that it should be established, as 
it appears to be, that in the development of the parasite the 
cortex of the root of the host is penetrated, and so an intimate 
connection between the two — indeed, a symbiosis — is set up. 
Then there is abundant evidence that the nodules are very rich 
in nitrogen. Indeed, in certain stages of their development, their 
dry substance may contain a much higher percentage of nitrogen 
than that of any part of the growing plant itself ; and, in some 
cases at any rate, even higher than in that of the highly nitro- 
genous leguminous seed. 
Whence comes this nitrogen ? The views of those who 
have studied the histology and biology of the subject, without 
reference to quantitative chemical data, do not seem to be very 
clear or definite on the point. Thus, it has been assumed that 
the bacteria acquire their nutriment, including their nitrogen, 
from the protoplasmic cell-contents of the higher plant ; and 
that, on the other hand, the contents of the bacteroid cells are 
resorbed. In other words, the plant utilises the substance of 
the bacteroids. It is obvious, however, that, so far as the nitrogen 
of the bacteria is derived from the plant itself, the latter is not a 
gainer in a quantitative sense by its resorption. > 
It has further been assumed that the activity of the process 
depends on the quantity of the nitrogenous compounds at the 
disposal of the roots, the tubercles developing unhindered, and 
becoming lai'ge and typical, in a soil rich in nitrogen, but attain- 
ing no great size in poorer soils. The source of the nitrogen of 
the bacteria is here supposed to be combined nitrogen in the 
soil. The experimental results which have been described 
clearly show, however, that the nodules may develop very plenti- 
fully in a nitrogen-free soil, and that there may, under such 
