Source of Carbon. This element exists in the soil in inor- 

 ganic compounds, such as carbonate of lime, carbonate of mag- 

 nesia, etc., but the plant cannot use the carbon in this form for 

 the manufacture of organic compounds. The carbon for these 

 functions is obtained from the air in the form of carbon dioxide. 

 Under ordinary conditions the atmosphere contains from .028 

 to .03 per cent, of carbon dioxide. We have no reason to believe 

 that this amount is materially changed from year to year, al- 

 though it has probably undergone very decided changes during 

 the long geological periods of the earth's history. The burning 

 of fuel, the disintegration of rock carbonates of the soil, the 

 breathing of animals and the decay of organic materials set free 

 great quantities of carbon dioxide which becomes available for 

 plant growth. The amount of CO 2 may be slightly greater in 

 the vicinity of cities and near the ground, but the movements 

 of the air currents tend to keep it very equally distributed. The 

 presence of carbon in the plant can be very readily demonstrated 

 by drying and burning the plant so as to form charcoal, which is 

 pure carbon plus the small amount of mineral ash to which we 

 have previously referred. 



Composition of the Air. The air contains approximately : 



79 per cent, of nitrogen. 



20 per cent, of oxygen. 



00.03 per cent, carbon dioxide 



The nitrogen of the air plays no direct part whatever in the 



growth of the plant (see page 203) ; both oxygen and carbon 

 192 



