Sources of Nitrogen in Plants. 355 
to the view that nitrogen is fixed by organic substances in 
the soil during slow electric changes, and so on. 
The chief points to be summarised seem to be these. 
There is a general tendency to the view that the Legu- 
minosae at least take more nitrogen from the soil than can 
be accounted for if the only sources are (1) the combined 
nitrogen of the atmosphere washed down into the soil, and 
(2) the combined nitrogen of the soil found by analysis of 
samples. It is therefore surmised that the free nitrogen 
of the atmosphere is c fixed ’ under such conditions that 
it can combine with other elements, and so supplies the 
deficiencies. 
In favour of this are quoted the experiments of Berthelot, 
Frank, Hellriegel, and others. As a point against the ne- 
cessity of this — not as against the facts of such fixation — 
Messrs. Lawes and Gilbert especially remind us that sub-soils 
may and do contain large quantities of combined nitrogen, 
and it is still questionable how far these can be carried up 
into the soil, or reached by the roots of deep-rooted Legu- 
minosae. 
It should be noted that the water of the sub-soil (con- 
taining dissolved substances) may rise for long periods in 
dry summers, when the plants above are transpiring, by 
capillarity ; hence the adduced increase of nitrates in the 
upper parts of the soil during active vegetation is not in 
itself a proof of absorption from the air. Of course this 
does not apply to pot-plant experiments. 
Then comes the consensus as to nitrification by means of 
organisms in the soil. But it must not be overlooked that 
the usual case consists in the oxidation of nitrogenous com- 
pounds already present in the soil. 
The startling point in Hellriegel’s experiments-— more 
cautiously entertained by Frank and Vines — is that organisms 
co-operate in the fixation of free nitrogen under such con- 
ditions that it then enters into combination. That we are 
here face to face with a difficulty must be clear to every one. 
In conclusion, it seems that we cannot, as yet, clear up 
