The leaf of the tree is its visiting card. We shall learn to know trees
by their leaves, as easily as if the name were written across the face of
the leaf. Some leaves have a single blade of green, and for this reason
the botanist calls them _simple_ leaves. This blade has a stem that
unites it with the twig. A _compound_ leaf is one whose stem bears more
than one blade. These small blades are called _leaflets_. There are two
types of compound leaves, one feather-like, having a main stem with
leaflets arranged in two rows on opposite sides of this stem. Such a leaf
is feather-like. The other type has a leaf stem with all the leaflets
attached at one end. The horse chestnut is the best example of this type.
The leaves spread from the end of the stalk somewhat as the fingers rise
from the palm of your hand.
The biggest leaves with single blades to be found in our forests grow on
trees of the magnolia family. The silver-lined leaves of the large-leaved
cucumber tree are over a foot in length, sometimes two and one-half feet,
down South. These great leaves are about one-fourth as wide as long, and
at the base each one broadens and extends backward into two rounded
ear-like lobes. This gives the tree the name, ear-leaved magnolia. The
whole leaf flaps in the wind, like the ear of an elephant, and, of
course, the wind lashes it into strings and soon robs it of its beauty.
The Northern cucumber tree is another magnolia whose leaves are
tropical-looking. This is the hardiest of the magnolia family, and its
heart-shaped leaves are six to ten inches long. They are not large for a
magnolia of the South, but they look larger because they grow among the
small-leaved trees of the Northern states.
The tulip tree has a large leaf of peculiar form. It is broad like a
maple leaf at the base, but at the tip it is cut off square as if with a
pair of shears, forming a right angle with its straight sides. Sometimes
the leaf is notched, as if a V-shaped piece were cut out of the square
tip. These leaves are long-stemmed, their blades polished, and they
flutter on the twigs with the lightness of a poplar leaf. Once we have in
mind the form of the leaf of the tulip tree, we shall never forget it,
for it is different from all other leaves.
The catalpa tree, which lifts its great blossom clusters above the
foliage in late June, is another of the few large-leaved trees of the
North. The single blade is heart-shaped, six to eight inches long, and
more than half as broad. These leaves usually have plain margins, but
sometimes they are wavy and notched near the base so as to produce faint
side lobes. The blades hang on long, stout stems.
Among the feather-leaved trees, the walnuts and butternuts, the sumachs,
and the ailanthus, furnish examples. A black walnut leaf is often two
feet long, with a dozen or more leaflets on the longest ones. These
leaflets are always set opposite in pairs, with an odd one on the tip of
the leaf stem. Butternut leaves have the same form, but the leaves are
longer. They range from fifteen to thirty inches, and have from ten to
twenty leaflets, but always an odd number. The peculiar gummy feeling of
these hairy leaves, and their pungent butternut odour when bruised, make
it easy to know the tree wherever we meet it, through the long summer.
The hickories are cousins of the walnuts, but their leaves, though of the
feather form, have larger and fewer leaflets than any walnut tree. A
shagbark hickory leaf has one or two pairs of little leaflets on the
stem, and above them three of larger size. The pignut has the same habit
of clustering its three largest leaves at the tip of the leaf stem, and
tapering off at the base with one or two pairs of decreasing size.
The largest of all the compound leaves have branched stems to which
leaflets are attached. The main leaf stem’s side branches may yet branch
again, forming a twice-branched framework that is set with leaflets, not
large, but so numerous as to make the whole leaf surprisingly large. The
greatest of these twice-compound leaves is borne by that astonishing,
spiny-stemmed Hercules’ club. A single leaf is often four feet long, and
nearly a yard wide. There are no leaflets on the main stem; they are on
the side branches.
How shall we tell a leaf stem from a twig? Leaf stems do not look like
the twigs of the tree. A little practice in looking closely and comparing
these leaf stems and twigs will obviate any confusion of the two. The
leaf has a bud at its base, and it breaks off easily at this joint.
[Illustration: The sugar maple trees are tapped in February; they bloom
in May after the leaves come out; they ripen their keys in October, when
the foliage turns to red and yellow.]
[Illustration: Leaves of black willow have frills at their bases. Twigs
of pussy-willows, cut and brought indoors, can be forced into bloom in
midwinter]
Among the fine, feathery leaves that are so beautiful and light that they
give great beauty to the tree tops are those of the honey locust. These
leaves are of the feather type, the slender stems, with double rows of
tiny leaflets. Very often we find among the single feather forms, leaves
of greater size, which have branched stems. This branching multiplies the
number of leaflets, and gives us, on the same trees, what the botanists
call _once compound_, and _twice compound_ leaves. The simple feather and
the branched feather forms add greatly to the beauty and luxuriance of
the foliage of the honey locust.
The common black locust of the roadside has single leaf stems with oblong
leaflets set in opposite rows upon it. Ash trees have the same feather
type of leaves, the leaflets usually pointed and oval, and always an odd
one at the tip. They are all larger than leaves of the locusts.
In the maple family there is a broad, simple blade, about as wide as it
is long. It is a family trait to have three main veins running out from
the end of the leaf stem, into the blade. Each of these veins has side
branches, and they are connected with a network of smaller veins. Between
the tips of these three main veins the leaf is usually notched, so as to
divide it into thirds. In the red maple these notches are shallow V’s cut
out, leaving triangular points. In the silver maple the leaves are cut by
deeper clefts, which reach more than half-way to the leaf stalk. The
three lobes are cut with jagged points into an uneven margin. The sugar
maple has its three lobes separated by wide, deep clefts, and its margins
are irregularly wavy. The box elder, which is a maple, is cleft so deeply
that the blade is split into three distinct leaflets, each with its own
short stem. This makes a compound leaf of it. It is the only maple with a
leaf of more than one blade.
The tree which shows the greatest difference in the form of its leaves is
the sassafras, whose oval leaves grow on the same stem with mittens and
double mittens—a mitten pattern with a thumb on each side. The hawthorns
have small oval leaves with variously cleft borders. There are over a
hundred kinds of hawthorns in our woods, and each kind has a leaf
different from all the rest; yet a single tree will often show leaves
that differ so much from the others in form that we might easily suspect,
if some one brought them to us, that each grew on a different tree from
all the rest.
Many oak trees have the same habit of leaf variation, so that even a
forester has to examine many leaves with care, and with them the buds and
the acorns, to make sure that he has called the oak by its right name.
The behaviour of the leaves of a tree depends largely on the length and
flexibility of their stems. If they are long, and slender, and supple,
the tree-top is in a continual flutter when the wind blows. If they are
thick and stiff, they do not catch the breeze as readily, and their
blades lie comparatively still when other trees near by may be twinkling
and trembling. Leaves with deeply cut borders, like some oaks and maples,
flutter much more than leaves like the basswood, whose borders are
unbroken. Oak leaves that are deeply cut will rarely lie down flat. The
curving bays in its borders cause the leaf to curl, so that no matter
what face is presented, the wind gets under and strikes some surface, and
sets the leaf to dancing.
The flat leaf stems of the trembling aspen, one of the poplar family, are
very flexible, and they are flattened at right angles to the blades of
the leaves. When a breeze comes by, it may strike the edge of the leaf,
but if so, it catches the flat leaf stem broadside. If it comes from any
other direction the leaf trembles, because one of the blades is sure to
receive the force of the wind. So the tree top is in one constant tremor,
even when the breeze is scarcely sufficient to disturb broad-leaved trees
which are near neighbours of the aspens.
Whatever the form and size and shape of its leaf, the tree depends upon
its foliage mass for all the life it enjoys, and for all the growth it
makes. The soil and the air feed the tree. The leaves and the sun do the
work of digesting the food. In the porous wood and bark are the channels
through which sap mounts upward to the leaves, and another set of
channels which carry the prepared food back, leaving it wherever needed,
along the way from tip of twig to tip of root. Whatever is not needed is
stored away, to be dissolved as needed and carried to the points where
the need is. In spring it is the growing buds that chiefly need this
stored food. Its presence explains the miracle of the bursting of
blossoms and leaves when spring comes.
One by one the trees of your own yard may be learned by name this summer.
The leaves are your sure guide. Trees stay where they are. Once we
recognise their leaves and call them by name, we may depend upon finding
them still standing the next day we pass them, and their leaves are still
held out as the sign of recognition. Every time we pass yonder red maple
let us glance at its three-pointed leaf, and fix its shape indelibly in
the mind. When we have done this a dozen times, I am sure that we shall
be able to pick out all the red maples in town; and if we journey far
from home we may find and recognise the same kind of trees by the same
sign. More and more as we grow older, we find out that half the pleasure
of travelling is the occasional meeting with old friends, be they people
or trees.