150 BIOLOGY OF PLANTS. 



The other sources, from which plants derive their carbon, 

 have already been pointed out. The humus of the soil is 

 composed mostly, as we have seen, of humic or geic acid, 

 crenic and apocrenic acids. These substances contain large 

 quantities of carbon, generally in the form of salts, which are 

 soluble in water. They must enter the roots with that liquid, 

 and yield their carbon to the plant. 



TTie proportion of carbon derived from the atmosphere, 

 and from other sources, will depend upon the nature and 

 age of the plant; the quantity of food in the soil or the 

 air; climate; quantity of light, and similar circumstances. 



We know, by the observations of every day, that fields 

 which are constantly covered with vegetation, such as pastures 

 and wood lands, increase in carbon. They must not only 

 take from the air nearly the whole which enters into their 

 substance, but they must also add to the quantity in the soil, 

 This certainly must be the case in peat swamps, where the 

 vegetable matter accumulates to a depth of several feet. But 

 tillage crops, as appears both from observation and experi- 

 ment, take more from the soil than they return to it. By the 

 carefully conducted experiments* of Boussingault, the quan- 

 tity of carbon, which plants derive from the atmosphere dur- 

 ing five years' rotation, was about two-thirds of what they con- 

 tained ; but it is evident, that a considerable quantity of car- 

 bon in the soil, must pass into the atmosphere in the form of 

 carbonic acid ; and hence the quantity obtained from the air, 

 must exceed two-thirds of the whole.t 



* The principle, upon which the experiments were conducted, was, 

 to examine for a series of years, the quantity of carbon in the soil be- 

 fore the crop, the quantity in the crop itself, the quantity in the soil 

 after the crop was removed, and the quantity added in manure. 



t On the supposition, that two-thirds of the carbon is derived from 

 the atmosphere, and if we allow one ton and a half to be tlie average 

 quantity of dried produce on an acre of surface, the quantity of car- 

 bon would be about 1100 pounds. 



