Perhaps, after these statements, you may think there is danger that the carbon of the air may sometime become exhausted. The air of the whole world contains about 1,760,000,000,000 pounds of carbon. Moreover, this is continually being added to by our fires and by the breath of animals. When wood or coal is used for fuel the carbon of the burning substance is returned to the air in the form of gas. Some large factories burn great quantities of coal and thus turn much carbon back to the air. A single factory in Germany is estimated to give back to the air daily about 5,280,000 pounds of carbon. You see, then, that carbon is constantly being put back into the air to replace that which is used by growing plants. The carbon of the air can be used by none but green plants, and by them only in the sunlight. We may compare the green coloring matter of the leaf to a machine, and the sunlight to the power, or energy, which keeps the machine in motion. By means, then, of sunlight and the green coloring matter of the leaves, the plant secures carbon. The carbon passes into the plant and is there made into two foods very necessary to the plant; namely, starch and sugar. Sometimes the plant uses the starch and sugar immediately. At other times it stores both away, as it does in the Irish and the sweet potato and in beets, cabbage, peas, and beans. These plants are used as food by man because they contain so much nourishment; that is, starch and sugar which were stored away by the plant for its own future use. =EXERCISE= Examine some charcoal. Can you see the rings of growth? Slightly char paper, cloth, meat, sugar, starch, etc. What does the turning black prove? What per cent of these substances do you think is pure carbon? SECTION XIII. THE SAP CURRENT The root-hairs take nourishment from the soil. The leaves manufacture starch and sugar. These manufactured foods must be carried to all parts of the plant. There are two currents to carry them. One passes from the roots through the young wood to the leaves, and one, a downward current, passes through the bark, carrying needed food to the roots (see Fig. 28). If you should injure the roots, the water supply to the leaves would be cut off and the leaves would immediately wither. On the other hand, if you remove the bark, that is, girdle the tree, you in no way interfere with the water supply and the leaves do not wither. Girdling does, however, interfere with the downward food current through the bark. [Illustration: FIG. 28 MOVEMENT OF THE SAP CURRENT]