5 1 8 Dawson . — On the Economic 
soluble nitrogenous compounds are converted into more easily 
assimilated forms. Such a change in the nature of the food- 
supply would naturally lessen the plant’s need of root-nodules 
as nitrogen-collectors. In ordinary soils, on the other hand, 
the processes of nitrification and denitrification, so conti- 
nuously going on, produce frequent variations in the quantity 
and quality of the nitrogenous food available to higher plants ; 
so that in the struggle for existence it may well be a distinct 
advantage to the Leguminosae to have the power to draw upon 
additional supplies, provided through the agency of the root- 
nodules. 
Further, the action of the host in the probable absorption 
from the nodules of the products of the activity of the sym- 
biotic or parasitic organisms, doubtless in its turn aids in 
securing the continuance of their action upon the nitrogenous 
constituents of the soil or air. For it is quite conceivable 
that an accumulation of the products of their own metabolism 
within the nodules would in time result in the death of the 
organisms, or at least would entirely inhibit the exercise 
of their special metabolic functions. Such an accumulation 
of products in the artificial media of cultivation may perhaps 
be an important factor in the loss of virulence which seems to 
result from artificial cultivation, as compared with organisms 
derived direct from infected soils. From the strictly practical 
point of view, however, the problem is to determine the con- 
ditions under which a further supply of nodule-organisms — 
such as are now available — would be undoubtedly beneficial 
to the plants. I am inclined to think that a special study 
of these conditions is necessary for each type of Leguminous 
crop ; Wollny 1 has recently shown that on chalky soils 
yellow Lupins and Serradella do not react to infection of the 
soil with nodule-organisms, but that a considerable increase 
in their development is seen if they receive a supply of easily 
assimilable nitrogen compounds ; and as another example 
I may repeat my former statement that for peas grown upon 
1 Wollny, ‘ Versuche iiber die Wirkung des Nitragins.’ Centralbl. fUr Bakt. u. 
Parasitenkunde, 1899. 
