PART II 



FACTORS IN PLANT GROWTH 



CHAPTER V 



THE AIR 



Of all the factors which influence plant growth, there is 

 one over which the farmer has no control and vet one which 

 is absolutely necessary to the life of both plants and animals. 

 This factor is the air. It is important not only because it 

 supplies plants and animals with certain essential elements, 

 but also because its constituents and the changes in these 

 constituents cause variations in climate. Moreover, the air 

 and its constituents have a very considerable effect on the 

 formation and decomposition of soils. It is, in short, of 

 such vital importance to the farmer that a short discussion 

 of its properties and constituents is advisable at this point. 



108. Height of the Air. — If the air were of the same 

 density throughout it would extend away from the earth for 

 five or six miles, but since its density becomes less as the 

 distance from the earth increases, it has been estimated that 

 our planet is enclosed within a gaseous envelope about 200 

 miles thick. 



109. Pressure or Weight of the Air. — At sea level, and 

 at 0° C. the air exerts normally a pressure or weight of 1033 

 grams per square centimeter. This is 14.7 pounds per 

 square foot or 46,100 tons per acre. The pressure diminishes 

 with the altitude. At an elevation of about 18,000 feet the 

 pressure is one-half that at sea-level, and at 36,000 feet 

 about one-fourth. Since the average farm is not at sea- 

 level it would be reasonable, then, to say that the weight of 



