[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.)]
[Illustration: FIG. 64. To show that chlorophyl is necessary for starch formation in the leaf. 10 is a variegated leaf from a silver-leaved geranium; the center is an irregular patch of green, with an irregular border of white. 11, after taking out the green. 12, after iodine treatment, the leaf turns purple only where it was originally green, showing that no starch forms in the white border. (Drawings by M.E. Feltham.)]
Some leaves on the branch are larger than others because in the struggle for light and air they have had a better chance than the others or they have had more of the food which has come up from the root.
Some of the leaves have developed longer stems than others in their effort to reach out after light and air.
Most leaves have the little mouths through which air is taken in and water and oxygen given out on the rough side, and that side is turned down toward the earth probably so that rain and dust will not choke the little pores.
The leaves of the lower branches tend to spread out in a broad, flat plane because in the effort to get light no leaf will grow directly under and in the shadow of another, while on those branches which grow straight up from the top of the tree the leaves can get light from all sides and so arrange themselves around the stem.
Is it of any value to the plant grower to know these facts about leaves? It is, for knowing these things he can better understand the necessity of caring for the leaves of his growing plants to see that their work is not interfered with.
HOW THE WORK OF SOME LEAVES IS INTERFERED WITH
Many people who grow house plants have trouble in keeping them well clothed with leaves, for instance, the geranium and the rubber plant. The leaves are constantly turning yellow and dropping off or drying up. This sometimes occurs from over-watering or not sufficiently watering the soil in the pot or box. If the watering is all right the trouble may occur in this way: The air of the house is quite dry, especially in winter. As a result transpiration from the leaf may be excessive. More water is transpired than is necessary, consequently more is pumped by the roots and with it more food is sent to the leaf than it can take care of. As the excess of water is transpired the excess of food is left in the leaf. The tendency is to clog its pores and therefore interfere with its work, and gradually weaken and finally kill it. The remedy for this is to spray the leaves frequently so as to keep the air about them moist and so check transpiration. Keeping a vessel of water near them helps also as this tends to keep the air moist. Dust sometimes chokes the leaves. Washing or spraying remedies this.