=Experiment.=--The escape of this oxygen gas may be seen by taking some water weed from either fresh or salt water and placing it in a glass jar of the kind of water from which it came, then set the jar in the sunlight. After a time bubbles of gas will be seen collecting and rising to the surface. If a mass of weed like the green scum of fresh water ponds or green sea lettuce be used, the bubbles of gas will become entangled in the mass and will cause it to rise to the surface of the water. At the same time prepare another jar of the weed and place it somewhere out of the sun; very few bubbles will be seen to rise and the weed will settle to the bottom of the jar (Fig. 65). All of the food of the plant, whether taken from the air or from the soil is digested in the leaves, and sunlight and air are necessary for this work. Another function of leaves then is to digest food for the plant. Important functions of leaves then are: To transpire moisture sent up by the roots. To manufacture starch by combining some of the water sent up by the roots with carbon taken from the air. To digest the starch and food sent up by the roots. To do these things well leaves must be connected with a strong, healthy root system and must have plenty of light and air. We are now ready to give reasons for the facts about leaves mentioned in the first part of the chapter (see page 109). Leaves are green because the green coloring matter is necessary for the leaf to do its work. Leaves are flat and thin and broad in order that they may present a large surface to the air and sunlight. [Illustration: FIG. 61. To show that growing leaves contain starch. 1. Represents a green cotton leaf as picked from the plant. 2. Is the same leaf after taking out the green coloring matter; the leaf is white. 3. The same leaf after treatment with weak iodine turned to a dark purple, showing the presence of starch. (Drawings by M.E. Feltham.)] [Illustration: FIG. 62. To show that starch disappears from the leaf when the plant is placed in the dark. The plant from which was taken the leaf represented in Fig 61, was immediately placed in a dark closet for 24 hours. Then leaf 4 was taken from it; 5 represents this leaf after the chlorophyl was taken from it: it is white; 6 is the same after treatment with iodine. The leaf remains white, showing no starch. (Drawings by M.E. Feltham.)] [Illustration: FIG. 63. To show that sunlight is necessary for starch-making by leaves. Leaf 7 had a paper label stuck to its upper surface a couple of hours while the plant was exposed to sunlight; 8 is the same leaf after the chlorophyl was taken out, and 9 represents it after treatment with iodine. The leaf turned purple in all parts except the part that was shaded by the label. Starch was removed from the portion under the label, but was not renewed because the label kept out the necessary sunlight. (Drawings by M.E. Feltham.)]