THE CULTURE OF FARM CROPS. 



CHAPTER XIII. 



'THE SOURCES OF THE CARBON OF PLANTS. HOW IT 



ENTERS INTO THEIR CIRCULATION 



AND SUBSTANCE. 



It is obvious to the cultivator of the soil that the various 

 plants of which his crops consist, are supported by the earth 

 and the air, both. It is necessary to the intelligent culture 

 of farm crops then to learn how much plants ow r e to each of 

 these ; and for which of their elements they are indebted to 

 the soil and for which to the air. As carbon contributes 

 the larger part of their substance to plants this element de- 

 mands the first consideration. 



Carbon is a solid substance and is therefore incapable of 

 entering directly into the structure of plants. It must then 

 present itself to the roots of plants in the soil, in solution in 

 water; and to the leaves in a gaseous form; for it is a law 

 of plant growth that no solid substance can enter into the 

 roots; and no liquid or solid substance can enter the leaves 

 or any other portion of the plant which is above the ground. 

 Therefore the sources of all vegetable carbon must be the 

 soil in which the roots of plants exist, and through which 

 they penetrate; and the air in which the stems and leaves 

 of plants are constantly bathed. There is always a large 

 quantity of vegetable matter in a decaying state in the soil, 

 and which is made up of the remains of previous vegetation ; 

 and the farmer is continually adding to this by the manure 

 which he applies to the land for the purpose of feeding his 

 crops. And it has been shown that about one-half of this 

 matter consists of carbon. The question then arises; is this 

 carbon of the soil, the source from which plants derive their 

 supply; and do they feed upon it by and through their 

 roots; or do they derive it from the air, in which we have 

 learned that a vast amount of carbonic acid exists in the 

 form of an evenly diffused mixture. It is not usual for far- 



