THE STUDY OF TREES IN WINTER
Outside my window the trees in a little wood stand leafless. Everything
which made this wood a delight in June, the contrast of light and
shade among the leaves, the varying tones of green in broken sunlight,
the warmth and color and summer freshness, has gone, but the trees
themselves, in all their wealth of foliage were never so beautiful
as now. The massive moulding of their trunks, the graceful curves of
their branches, the fine tracery of their little bare twigs, now clear
against the sky and again lost in a tangled network of intersecting
branches,—the whole beauty of their symmetry, their poise, strength,
and delicacy is revealed as it is never revealed in summer.
Attracted first by the obvious grace of the forms of trees as we see
them from our windows in winter, we discover that a closer study of
the details of bare twigs and buds in the woods discloses unsuspected
beauty in texture, form, and color. Each tree has definite traits of
its own which distinguish it from every other tree, and by tracing
individual characteristics in the branches, trunk, stems, buds, and
leaf-scars we are able to identify every tree with certainty.
[Sidenote: Trunk and Branches]
By observing the shapes and general outlines of trees in winter we
are able to recognize them at a distance. This study of tree forms
adds much to the pleasure of a railroad journey or a winter’s drive
in the country, and accuracy is acquired by constant practice when we
walk in the woods and fields and can verify the name of each tree. In
this way we become familiar with the common trees, and learn to know
the predominating trees of the forests through which we pass, often
recognizing a rare species the distance of a field away.
[Illustration: TREE WITH A DELIQUESCENT TRUNK]
[Illustration: Cross section of a tree trunk, showing the rings of
annual growth, the medullary rays, the dark heartwood, the lighter
sapwood and the bark.]
There are two distinct plans of branching in trees. When the main trunk
extends upward to the tip, as it does in the larch and other conical
trees, it is said to be _excurrent_, and when the main stem divides
into many more or less equal divisions, as we find it in the American
elm and other spreading trees, it is said to be _deliquescent_,—the
latter form is the most common one among our deciduous trees.
[Illustration: TREE WITH AN EXCURRENT TRUNK]
The inner structure of these dicotyledonous trunks is seen when we
examine the cross section cut of a felled tree. In the centre is the
heartwood, the durable wood of commercial value, the cells of which
are hard and dry; next it the soft sapwood, the living part of the
tree containing cells filled with sap; then the cambium layer, the
zone of growing cells, and outside of this the bark. Each year new
cells are formed in the cambium layer, the inner ones making new wood,
the outer ones new bark, and by counting these annual rings of growth
the approximate age of the tree is found. In young trees there is
a conspicuous central portion of pith which remains after the tree
matures, as long as the heartwood is sound. The lines radiating from
the centre to the circumference are called medullary or pith rays
and form the “silver grain” of the wood. As the size of the trunk
increases, the bark unable to expand, cracks in fissures or peels in
layers, and is pushed off by the tremendous growing power from within.
The heartwood is not a living part of the structure and often trees
live for years without it,—hollow shells with a normal amount of
vitality so long as the roots, the cambium layer and the buds are not
injured.
Branches grow from the axillary or lateral buds on the stem, continuing
their growth every year by the development and unfolding of new buds,
both terminal and lateral. When the growth is carried on by the
terminal buds, the tree is more apt to be regular in outline than
when these are injured or killed and lateral buds develop the growth
instead. Branches vary in showing an upright, drooping, or horizontal
habit of growth, as we see them in the Lombardy poplar, weeping willow
and tupelo, and within these divisions there are other contrasts of
rigidity and flexibility, with differences of color and texture as well.
Apart from the general shape of the tree, the bark on the trunk and
branches is a constant help in identification. It is hard and smooth on
some trees, like that of the hornbeam and beech, fissured into ridges
like the sugar maple on others, it sometimes flakes off in rough plates
like those of the shagbark hickory, and again in thin, brittle strips
like those of the hop hornbeam, the bark peels off laterally as in
the canoe birch, and occasionally becomes ridged and corky as we find
it on the branches of the liquidamber and cork elm. Very often the
color of the bark is distinctive as is that of the green stems of the
sassafras and moosewood maple and the white, brown, pink, and yellow
trunks of different birches. The taste and odor of the bark are also
characteristic of certain species, as, for instance, the unpleasant,
bitter taste of the black cherry, the mucilaginous taste of the
slippery elm, and the aromatic fragrance of the stems of the mockernut
hickory. The little dots on young bark are called _lenticels_, they
are openings for admitting air to the inner tissues. Lenticels are
conspicuous in the bark of the birches.
The presence of thorns on the trunk and branches of certain trees helps
us to distinguish them from others, and the clusters of dry fruit which
remain hanging on some trees through the winter are another means of
identification.
[Sidenote: Stems and Twigs]
Stems and twigs vary from the finest, lightest sprays to the most
coarsely moulded ones,—from the delicate twigs of the hop hornbeam to
the stout shoots of the horsechestnut;—like larger branches their tips
either ascend, droop, or grow at right angles from the stem, and may be
smooth, downy, or rough to the touch.
The pith in cross sections of twigs shows different forms and is a
means in itself of distinguishing some trees. It is usually circular,
but in some species it takes the form of a pentagon or a star. In
a vertical section we sometimes find it in horizontal plates, like
the chambered pith of the walnuts. The color is usually white, but
sometimes we find it pink, yellowish, green, red, and brown.
It is interesting to find that the history of a tree for several
years past can be told by studying the scars along the bare stems.
The annual growth each year is marked by a circle of scars around the
stem, which was left by the scales of the buds when they opened in the
spring, and these scars mark each season’s growth for successive years
along the stem.
[Sidenote: Leaf-scars]
Besides these circles of scars, there are scars on each side of the
stem which were left by the leaves when they fell in the autumn. These
leaf-scars differ distinctly in various species and may be round,
narrow, triangular, oval, heart-shaped, or horseshoe-shaped according
to the species of the tree. They are either flat upon the stem or on a
projection, they are sometimes concave and again convex. They may be
opposite each other on the stem, as those of the horsechestnut, maples,
and ashes, or the arrangement may be alternate, as that of hickories,
walnuts and oaks. The places on the stem where the leaf-scars appear
are called _nodes_, and the spaces between the nodes are called
_internodes_.
Occasionally stipule scars are found on the stems,—inconspicuous scars
left by stipules, the leaf-like bodies found at the base of leafstalks
on some trees,—and sometimes we find the scars of fruit stalks.
[Sidenote: Bundle-scars]
[Illustration: BUNDLE-SCARS]
Bundle-scars are the scars of the little fibres, the vascular bundles
which fastened the leaves to the stems in summer. They are found on
the leaf-scars and usually take their shape more or less. On the large
leaf-scars they can be seen clearly, but on delicate twigs where the
leaf-scars are small it is well to use a magnifying glass.
[Sidenote: Buds]
In our climate the buds of trees are formed in the summer during
the season’s growth. The bud at the tip of the stem is called the
_terminal_ bud, the buds in the axils of the leaf-scars are called
the _axillary_ or _lateral_ buds. Buds contain complete branches
in miniature which develop in the spring into a new crop of twigs.
By opening a bud in winter the little leaves can be seen and often
a cluster of flowers, packed away from the cold in marvellous warm
wrappings.
As a rule the terminal bud carries on the growth of the tree and the
lateral buds furnish the side branches. Flowers are found in both
terminal and lateral buds, but sometimes they are enclosed in buds by
themselves which open before the leaves come out in the spring, like
those of the red maple and American elm,—these are called _flower_
buds. Occasionally we find two or three lateral buds together called
_accessory_ buds,—_superposed_, if placed one above another as they are
in the butternut; _collateral_, if side by side as in the red maple.
When several buds are crowded together one bud usually remains latent.
Latent buds are sometimes caught in the growing bark of the tree and
remain undeveloped for years, breaking out at length perhaps up and
down the sides of the trunk as we see them in “feathered elms.” These
abnormal and irregular buds are called _adventitious_ buds.
The winter buds of trees may be large or small, they may be slender,
flat, oval, pointed or round, hidden or exposed, they may be smooth,
downy, sticky, or rough, covered with scales or naked, and they may
differ in color from pale yellow to an inky black.
From the great outlines of the trees against the sky to the little
scales of the buds on the stems we marvel to find here as in all
nature, order, law, consistency out of infinite variety.