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.