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: FI