440 PLANTS AND THEIR RELATION TO MAN 



in life. As soon as the stored food is exhausted, the plant 

 must work for itself and secure its own food. The only sources 

 from which a plant can secure food are the soil and the air. 

 The mighty forest trees with their enormous trunks, massive 

 branches, and numerous leaves, as well as the tender young 

 blades of grass, owe their existence to soil food and air food. 



When we examine plants carefully, we find that they contain 

 carbon. The framework of the plant, the gum of the stem, the 

 sugar of the trunk, the starch of the grains, the oil of the nuts, 

 the fibers and spices, etc., are largely carbon. We know that 

 the soil does not furnish the carbon found in the plant, hence 

 we conclude that the air furnishes it. The air does not furnish 

 pure carbon to the plant, but it does furnish carbon dioxide. 

 The plant takes the carbon dioxide from the air and separates 

 it into carbon and oxygen. It combines the carbon obtained 

 from the carbon dioxide with the water absorbed through the 

 roots, and from the combination makes starch and sugar. 



The foods manufactured by the plant from the substances 

 gathered from the air and soil dissolve in cell sap and are 

 carried by it to all parts of the plant. Sometimes more food is 

 manufactured than is needed for immediate use. The surplus 

 food then accumulates in roots and stems for the future use of 

 the plant. A surplus of sugar is stored in the beet and sweet 

 potato root. A surplus of starch is stored in underground 

 swellings of the white potato. Asparagus, carrots, rhubarb, 

 all contain a good supply of surplus food. Because plants 

 accumulate surplus food, man is able to utilize them. 



How the plant secures carbon dioxide. An acre of fast- 

 growing banana plants requires about eight tons of carbon each 

 year, and an acre of slow-growing beech trees requires about one 

 ton of carbon each year. When we consider the multitude of 

 plants which are scattered throughout the land, the large forest 

 trees, the luxurious tropical ferns, the waving prairie grains, 



