Arbor Day Leaves | Page 4

fifty thousand or more known species of trees, the
leaves of each have a characteristic shape. The leaves of no two species
are precisely alike in form. More than this is also true. No two leaves
upon the same tree are in this respect alike. While there is a close
resemblance among the leaves of a given tree, so that one familiar with
trees would not be in doubt of their belonging to the same tree, though
he should see them only when detached, yet there is more or less
variation, some subtle difference in the notching or curving of the
leaf-edge perhaps, so that each leaf has a form of its own. These
differences of shape in the leaves are a constant source of beauty.
What a variety of size also have the leaves, from those of the birches
and willows to those of the sycamores, the catalpas and the paulownias.
On the same tree also the leaves vary in size, those nearest the ground
and nearest the trunk being usually larger than those more remote. How
different as to beauty would the trees be if their leaves were all of the
same size; how much less pleasing to the sight.
Then what a wide difference is there in the position of the leaves on the
trees and their relative adjustment to each other? Sometimes they grow
singly, sometimes in pairs, sometimes in whirls or clusters. Some droop,
others spread horizontally, while others still are more or less erect. The
leaves of some trees cling close to the branches, others are connected
with the branches by stems of various length and so are capable of
greater or less movement. The leaves of poplars and aspens have a

peculiarly flattened stem, by reason of which the slightest breath of
wind puts them in motion.
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, 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