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 atmosj^here. 



The general composition of the Atmosphei-e 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 7veight. By volume. 



Oxygen 23. 17 20. 95 



Nitrogen 76.83 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 abbreviatioa 

 H. C. G. 



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