A STUDY OF FOODS AND DIETARY 



347 



381. The Carbon Cycle. Of the raw materials coming to 

 the green plant, water and mineral matter come to it from the 

 earth by way of the roots, while the carbon dioxide is absorbed 

 through the leaves. Out of the raw materials and with the 

 energy derived from the sun, the plant makes a simple sugar in 

 the leaves. Oxygen is liberated at the same time. This 

 escapes into the air. From the simple sugar, the plant, by 

 means of other materials, produces other foods. When the 

 plant or an animal consumes the food the plant has produced, 

 oxygen is taken from the air, the food is oxidized, its energy 

 is liberated, and carbon dioxide and water are produced. The 

 carbon dioxide is returned to the air, subsequently to be built 



FIG. 244. The carbon cycle. 



into a food again by some green plant. This round of changes 

 goes on continually. It is known as the " CARBON CYCLE" 

 (Fig. 244). All of the energy possessed by fuels is traceable 

 to the work of green plants. 



382. What is Food? 



DEFINITION. A food is anything from which the body may 

 obtain its substance and energy. 



Many of the substances we eat furnish both of these for 

 the body and are therefore rightly called foods. Some of the 

 other substances we eat and which are sometimes called foods, 

 are without the ability to supply the body with energy. They 

 ought not to be thought of as foods in the same sense as those 

 materials which furnish both for the body. Thus our meats 



