Experiment.—If a potted plant was used for the last experiment, set it away in a dark closet after taking the leaves for the experiment. A day or two after, take leaves from it before removing it from the closet. Boil these leaves and treat them with alcohol as in the previous experiment. Then wash them and test them with iodine as before. No starch will be found in the leaves ([Fig. 62]). The starch that was in them when placed in the closet has disappeared. Now paste some thick paper labels on some of the leaves of a plant exposed to the sunlight. After a few hours remove the leaves that have the labels on them, boil, treat with alcohol and test with the iodine. In this case starch will be found in all parts of the leaf except the part over which the label was pasted ([Fig. 63]). If the sunlight is intense and the label thin, some starch will appear under it.
According to these last experiments, leaves contain starch at certain times, and this starch seems to appear when the leaf is in the sunlight and to disappear when the light is cut off. The fact is that the leaves manufacture starch for the plant and sunlight is necessary for this work. The starch is then changed to sugar which is carried by the sap to other parts of the plant where it is again changed to starch to be built into the plant structure or stored for future use.
Experiment.—Take leaves from a plant of silver-leaf geranium growing in the sunlight. If this plant cannot be had, the leaves from some other variegated white and green leaved plant will do. Boil these leaves, treat with alcohol, wash and test with iodine ([Fig. 64]). Starch will be found in the leaf wherever there was green coloring matter in it, while the parts that were white will show no starch. The green coloring matter seems to have something to do with the starch making, in fact starch is manufactured only where it is present. This coloring matter is called chlorophyl or leaf green.
We are told by the chemists that this starch is made from carbon and water. There exists in the air a gas called carbonic acid gas; this gas is composed of carbon and oxygen. It is breathed out of the lungs of animals and is produced by the burning and decay of organic matter. The under side of the leaf contains hundreds of little pores or mouths called stomata. This gas mixed with air enters these mouths. The green part of the leaf aided by the sun takes hold of the gas and separates the carbon from the oxygen. The oxygen is allowed to go free, but the carbon is made to unite with water and form starch.
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.