ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 97 
April; and the least in March. At Ida-Marienhiitte, 
Kuschen, and Regenwalde, in 1865-6, nearly half the 
year’s atmospheric nitrogen came down in summer; but 
at Insterburg only 30 per cent fell in summer, while 40 
per cent came down in winter. 
The nitrogen that is brought down in winter, or in 
spring and autumn, when the fields are fallow, cam!be! 
counted upon as of use to summer crops only so far as it 
remains in the soil in an assimilable form. It is well 
known that, in general, much more water evaporates from 
cultivated fields during the summer than falls upon them 
in the same period; while in winter, the water that falls 
is in excess of that which evaporates. But how much of 
the winter’s fall comes to supply the summer’s evaporation, 
is an element of the calculation likely to be very variable, 
and not as yet determined in any instance. 
We conclude, then, that the direct atmospheric supply 
of assimilable nitrogen, though not unimportant, is insuf- 
ficient for crops. 
We must, therefore, look to the soil to supply a large 
share of this element, as well as to be the medium through 
which the assimilable atmospheric nitrogen chiefly enters 
the plant. 
The Other Ingredients of the Atmosphere, so far as 
‘we now know, are of no direct significance in the nutri- 
tion of agricultural plants. Indirectly, atmospheric ozone 
has an influence on the supplies of nitric acid, a point we 
shall recur to in a full discussion of the question of the 
Supplies of Nitrogen to Vegetation, in a subsequent 
chapter. ' 
§ 11. 
ASSIMILATION OF ATMOSPHERIC FOOD. 
Boussingault has suggested the very probable view that 
the first process of assimilation in the chlorophyll cells of 
the leaf,—where, under the solar influence, carbonic acid 
5 
