Every leaf is a laboratory, in which, by the help of that great
magician, the sun, most wonderful changes and transformations are
wrought. By the aid of the sun the crude sap which is taken up from
the ground is converted by the leaves into a substance which goes to
build up every part of the tree and causes it to grow larger from year
to year; so that instead of the tree making the leaves, as we commonly
think, the leaves really make the tree.

Leaves, like other parts of the plant or tree, are composed of cells
and also of woody material. The ribs and veins of the leaves are the
woody part. By their stiffness they keep the leaves spread out so that
the sun can act upon them fully, and they prevent them also from being
broken and destroyed by the winds as they otherwise would be. They
serve also as ducts or conduits by which the crude sap is conveyed to
the leaves, and by which when it has there been made into plant food,
it is carried into all parts of the tree for its nourishment.
Protected and upheld by these expanded woody ribs, the body of the
leaf consists of a mass of pulpy cells arranged somewhat loosely, so
that there are spaces between them through which air can freely pass.
Over this mass of cells there is a skin, or epidermis as it is called,
the green surface of the leaf. In this there are multitudes of minute
openings, or breathing pores, through which air is admitted, and
through which also water or watery vapor passes out into the
surrounding atmosphere. In the leaf of the white lily there are as
many as 60,000 of these openings in every square inch of surface and
in the apple leaf not fewer than 24,000. These breathing pores, called
stomates, are mostly on the under side of the leaf, except in the case
of leaves which float upon the water. There is a beautiful contrivance
also in connection with these pores, by which they are closed when the
air around is dry and the evaporation of the water from the leaves
would be so rapid as to be harmful to the tree, and are opened when
the surrounding atmosphere is moist.

The green color of the leaves is owing to the presence in the cells of
minute green grains or granules, called chlorophyll, which means
leaf-green, and these granules are indispensable to the carrying on of
the important work which takes place in the leaves. They are more
numerous and also packed more closely together near the upper surface
of the leaf than they are near the lower. It is becaus