DIVISION I. 
THE ATMOSPHERE AS RELATED TO 
VEGETATION. 
CHAPTER I. 
ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 
§ 1. 
CHEMICAL COMPOSITION OF THE ATMOSPHERE. 
A multitude of observations has demonstrated that 
from ninety-five to ninety-nine per cent of the entire mass 
(weight) of agricultural plants is derived directly or indi- 
rectly from the atmosphere. 
The general composition of the Atmosphere is familiar 
to all. It is chiefly made up of the two elementary gases, 
Oxygen and Nitrogen, which have been described in “ How 
Crops Grow,” pp. 33-39.* These two bodies are present 
in the atmosphere in very nearly, though not altogether, 
invariable proportions. Disregarding its other ingredients, 
the atmosphere contains in 100 parts 
By weight. By volume. 
Oxygen.........+65 7 ee re 20.95 
Nitrogen.......... 16.83 keane owen 79.05 
100.00 100.00 
Besides the above elements, several other substances oc- 
* In our frequent references to this book we shall employ the abbreviation 
H.C. G. 
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