ATMOSPIIEIIIC AIU AS THE FOOD OF PLANTS. 97 



April; and the least in March. At Td.i-^Maiienliiitte, 

 Kiischen, and Regenwaldc, in 1865-6, nearly half the 

 year's atmosplieric nitrogen came down in summer ; but 

 at Insterburg only 30 per cent fell in summer, while 40 

 •per cent came doAvn in winter. 



The nitrogen that is brought down in winter, or in 

 spring and autumn, when the fields are fallow, can 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 s'.mmer than falls upon them 

 in the same period ; while v.) Avinter, 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, th:;t tlie direct atmospheric supply 

 of assimilable nitrogen, though not miimportant, is insuf- 

 ficient for crops. 



We must, therefore, look to the soil to supjdy 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 Injfiedieiits of the Atmosphere, so far as 



we now know, are of no direct significance in the nutri- 

 tion of agricultural i)lants. Indirectly, atmos])iieric 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 ATMOSPIIEPvIC FOOD. 



Boussingault lias 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 



