These are some of the most obvious characteristics of the leaves, and
by which they are made the source of so much of the beauty of the
world in which we live. It will be a source of much pleasure to anyone
who will begin now, in the season of swelling buds and opening leaves,
to watch the leaves as they unfold and notice their various forms and
colors and compare them one with another. There is no better way of
gaining valuable knowledge of trees than this, for the trees are known
by their leaves.

But let us turn now from their outward appearance and consider what is
done by them, for the leaves are among the great workers of the world,
or, if we may not speak of them as workers, a most important work is
done in or by means of them, a work upon which our own life depends
and that of all the living tribes around us.

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,