The Smtrces of the Nitrogen of mir Leguminovs Crops. 663 
It may, in fact, we think, be concluded that, at any rate in 
the case of our gramineous, our cruciferous, our chenopodiaceous, 
and our solaneous crops, free nitrogen is not the source. Never- 
theless, we have long admitted that existing evidence was insuffi- 
cient to explain the source of the whole of the nitrogen of the 
Legwnmosce ; that there icas, in fact, a missing Iviik ! 
According to some even recent experimenters, however, 
gain of nitrogen is not limited to our leguminous crops ; 
and the modes of explanation of the gains which have been 
observed are extremely various. Thus it has been assumed : 
that combined nitrogen was absorbed from the air, either 
by the soil or by the plant ; that there is fixation of free 
nitrogen within the soil by the agency of porous and alkaline 
bodies ; that there is fixation by the plant itself ; that there is 
fixation within the soil by the agency of electricity ; and, finally, 
that there is fixation under the influence of micro-organisms 
within the soil, both with and without the accompanying growth 
of higher plants. 
Limiting our discussion here mainly to the question of the 
sources of the nitrogen of the Leguminosce, it may be said that 
this has been the subject of experiment and of controversy for 
about half a centuiy, and it is generally admitted that all the 
evidence that has been acquired, on lines of inquiry until re- 
cently followed, has failed to solve the problem. During the 
last few years, however, the discussion has assumed a somewhat 
I diflFerent aspect. The question still is, whether free nitrogen is 
an important source of the nitrogen of vegetation generally, but 
especially of the Leguminosce ; but whilst few now assume that 
the higher chlorophyllous plants directly assimilate free nitro- 
gen, it is nevertheless supposed to be brought under contribu- 
tion in various ways ; but especially by being brought into 
combination under the influence of micro-organisms, or of other 
low forms, either within the soil itself, or in symbiotic growth 
with a higher plant. 
Of all the recent results bearing upon the subject, those of 
Hellriegel and Wilfarth with certain leguminous plants seem 
to us to be by far the most definite and significant ; pointing to 
the conclusion that, although the higher chlorophyllous plants 
may not directly utilise free nitrogen, some of them, at any rate, 
may acquire nitrogen brought into combination under the influ- 
ence of lower organisms ; the development of which is, appar- 
ently, in some cases a coincident of the growth of the higher 
plant whose nutrition they are to serve. 
It was in the Agricultural Chemistry Section of the " Natur- 
forscher Versammlung held in Berlin, in 1886, when one of 
