the one hundred and 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.