=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.

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.)]