A lily, showing long narrow leaves.
The dandelion, showing a whorled arrangement of long irregular leaves.
Effect of Light on Leaf Arrangement.—It is a matter of common knowledge that green leaves turn toward the light. Place growing pea seedlings, oxalis, or any other plants of rapid growth near a window which receives full sunlight. Within a short time the leaves are found to be in positions to receive the most sunlight possible. Careful observation of any plant growing outdoors shows us that in almost every case the leaves are so disposed as to get much sunlight. The ivy climbing up the wall, the morning-glory, the dandelion, and the burdock all show different arrangements of leaves, each presenting a large surface to the light. Leaves are often definitely arranged, fitting in between one another so as to present their upper surface to the sun. Such an arrangement is known as a leaf mosaic. In the case of the dandelion, a rosette or whorled cluster of leaves is found. In the horse-chestnut, where the leaves come out opposite each other, the older leaves have longer petioles than the young ones. In the mullein the entire plant forms a cone. The old leaves near the bottom have long stalks, and the little ones near the apex come out close to the main stalk. In every case each leaf receives a large amount of light. Other modifications of these forms may easily be found on any field trip.
Starch made by a Green Leaf.—If we examine the palisade layer of the leaf, we find cells which are almost cylindrical in form. In the protoplasm of such cells are found a number of little green-colored bodies, which are known as chloroplasts or chlorophyll bodies. If we place the leaf in wood alcohol, we find that the bodies still remain, but that the color is extracted, going into the alcohol and giving to it a beautiful green color. The chloroplasts are, indeed, simply part of the protoplasm of the cell colored green. These bodies are of the greatest importance directly to plants and indirectly to animals. The chloroplasts, by means of the energy received from the sun, manufacture starch out of certain raw materials. These raw materials are soil water, which is passed up through the bundles of tubes into the veins of the leaf from the roots, and carbon dioxide, which is taken in through the stomata or pores, which dot the under surface of the leaf. A plant with variegated leaves, as the coleus, makes starch only in the green part of the leaf, even though these raw materials reach all parts of the leaf.
An experiment to show the effect of excluding light (but not air) from the leaves of a green plant. The result of this experiment is seen in the next picture. (Experiment performed by C. Dobbins and A. Schwartz.)
Starchless area in a leaf caused by excluding sunlight by means of a strip of black cloth.