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taining nitrogen from the soil alone, and are unable to use the free 
nitrogen of the atmosphere. Putting all of these facts together, it 
has seemed to science that the nitrogen store upon which plants 
can draw is being used up and must be sometime exhausted, thus 
putting an end to vegetation. 
Practically our agriculturists have for some time experienced 
the difficulties arising from this source. Many soils under long 
cultivation have become largely exhausted of their nitrogen sup- 
ply, and the farmer appreciates more and more the necessity of 
nitrogen fertilizers. These are now brought from long distances. 
The nitrate beds of Chili and the guano beds of the South Pacific 
are the chief stores from which this valued food is obtained. But 
even the nitrogen beds have their limits, and as the need for ni- 
trogen on our cultivated soils becomes greater through exhaus- 
tion, the price of nitrogen must become greater also. One writer 
has said that the explosion of powder in a gun does more injury 
than the bullet. The latter only kills a man, the former aids in 
in using up the nitrogen store which cannot be replaced and is 
a lasting injury to mankind. It is very plain from all of this that 
there is a great need for.some means of obtaining nitrogen for 
our soils besides the store of nitrates‘in our nitrate beds. Curi- 
ously enough, the bacteriologists are to-day pointing out the me- 
thod by which this problem can be solved. 
As mentioned above, it was the conclusion of many experi- 
ments that the higher plants cannot make use of free nitrogen 
from the air. Up to 1880 all experiments seemed to point to the 
same direction. At about that period, however, experiments 
with certain of the legumes began to show an increase in the ni- 
trogen in plants beyond that which was fed to them in their food. 
The experiments were at first rather indefinite and strongly denied, 
but as the decade from 1880 to 1890 passed they became more 
numerous and conclusive until, finally, it was a definitely estab- 
lished fact that many of the legumes can in some way obtain ni- 
trogen from some source besides the soil. In 1888 Hellriegel and 
Wilfarth, in a series of careful experiments upon the subject, 
studied the relation of this nitrogen assimilation to the production 
of tubercles upon the roots of plants. Pea plants and other le- 
gumes have been for a long time known to develop small nodules 
