_Stems and Branches._
The stem is the distinguishing characteristic of trees, separating them
from all other groups of plants. Although in the region covered by this
book the trees include all the very large plants, size alone does not
make a tree.
A plant with a single trunk of woody structure that does not branch for
some distance above the ground, is called a _tree_. Woody plants that
branch directly above the soil, even though they grow to the height of
twenty feet or more, are called _shrubs_, or, in popular language,
_bushes_. Many plants which have a tendency to grow into the form of
shrubs may, by pruning, be forced to grow tree-like; some that are
shrubs in the northern States are trees further south.
All the trees that grow wild, or can be cultivated out of doors, in the
northern States belong to one class, the stems having a separable bark
on the outside, a minute stem of pith in the center, and, between these,
wood in annual layers. Such a stem is called _exogenous_
(outside-growing), because a new layer forms on the outside of the wood
each year.
Another kind of tree-stem is found abundantly in the tropics; one, the
Palmetto, grows from South Carolina to Florida. While in our region
there are no trees of this character, there are plants having this kind
of stem, the best illustration being the corn-stalk. In this case there
is no separable bark, and the woody substance is in threads within the
pithy material. In the corn-stalk the woody threads are not very
numerous, and the pith is very abundant; in most of the tropical trees
belonging to this group the threads of wood are so numerous as to make
the material very durable and fit for furniture. A stem of this kind is
called _endogenous_ (inside-growing). Fig. 1 represents a longitudinal
and a cross section of an exogenous stem, and Fig. 2 of an endogenous
one.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
Since all the stems with which we have to deal are exogens, a particular
description of that class will here be given. Fig. 1 shows the
appearance of a section of an Ash stem six years old. The central
portion, which is about as thick as wrapping-twine, is the _pith_; from
this outward toward the bark can be seen the six annual layers of the
_wood_; and then comes the _bark_, consisting of two portions. First
there is an inside layer of greenish material, the fresh-growing
portion, and lastly the outer or dead matter. This outer portion must
crack open, peel off, or in some way give a chance for the constant
growth of the trunk. The different kinds of trees are readily known by
the appearance of the bark of the trunk, due to the many varieties of
surface caused by the allowance for growth. None of the characteristics
of trees afford a better opportunity for careful observation and study
than the outer bark.
The Birches have bark that peels off in thin horizontal layers--the
color, thinness, and toughness differing in the different species; the
Ashes have bark which opens in many irregular, netted cracks moderately
near each other; the bark of the Chestnut opens in large longitudinal
cracks quite distant from one another. The color of the bark and the
character of the scales are quite different in the White and the Black
Oaks.
In the woody portion radiating lines may be seen; these are the _silver
grain_; they are called by the botanist _medullary rays_.
The central portion of the wood of many large stems is darker in color
than the rest. This darker portion is dead wood, and is called
_heart-wood_; the outer portion, called _sap-wood_, is used in carrying
the sap during the growing season. The heart-wood of the Walnut-tree is
very dark brown; that of the Cherry, light red; and that of the Holly,
white and ivory-like. The heart-wood is the valuable part for lumber.
If examined under a magnifying glass, the _annual layers_ will be seen
to consist of minute tubes or cells. In most trees these tubes are much
larger in the portion that grew early in the season, while the wood
seems almost solid near the close of the annual layer; this is
especially true in the Ashes and the Chestnut; some trees, however, show
but little change in the size of the cells, the Beech being a good
example. In a cross-section, the age of such trees as the Chestnut can
readily be estimated, while in the Beech it is quite difficult to do
this. Boxwood, changing least in the character of its structure, is the
one always used for first-grade wood-engravings.
When wood is cut in the direction of the silver grain, or cut
"quartering" as it is called by the lumbermen, the surface shows this
cellular material spread out in strange blotches characteristic of the
different kinds of wood. Fig. 16 shows an Oak where the blotches of
medullary rays are large. In the Beech the blotches are smaller; in the
Elm quite small. Lumber cut carefully in this way is said to be
"quartered," and with most species its beauty is thereby much increased.
Any one who studies the matter carefully can become acquainted with all
the useful and ornamental woods used in a region; the differences in the
color of the heart-wood, the character of the annual layers, and the
size and the distribution of the medullary rays, afford enough
peculiarities to distinguish any one from all others.
BRANCHING.--The regular place from which a branch grows is the _axil_ of
a leaf, from what is called an _axillary bud_; but branches cannot grow
in the axils of all leaves. A tree with opposite leaves occasionally has
opposite branches; while a tree with alternate leaves has all its
branches alternate.
Most branches continue their growth year after year by the development
of a bud at the end, called a _terminal bud_. Many trees form this bud
for the next year's growth so early in the year that it is seldom or
never killed by the winter weather; such trees grow very regularly and
are symmetrical in form. Most evergreens are good examples. Fig. 3
represents a good specimen. The age of such trees, if not too great, can
be readily ascertained by the regularity of each year's growth. The tree
represented is sixteen years old. The branches that started the fifth
year, about the age at which regular growth begins, are shown by their
scars on the trunk.
[Illustration: Fig. 3.]
The terminal buds of many trees are frequently killed by the frosts of
winter; such trees continue their growth by the development of axillary
buds; but as growth from an axillary bud instead of a terminal one will
make a branch crooked, such trees are irregular in their branching and
outline. Just which axillary buds are most apt to grow depends upon the
kind of tree, but trees of the same variety are nearly uniform in this
respect. Most trees are therefore readily recognized by the form of
outline and the characteristic branching. A good example of a tree of
very irregular growth is the Catalpa (Indian Bean), shown in Fig. 4. The
tendency to grow irregularly usually increases with age. The Buttonwood,
for example, grows quite regularly until it reaches the age of thirty to
forty years; then its new branches grow in peculiarly irregular ways.
The twigs of a very old and a young Apple-tree illustrate this change
which age produces.
[Illustration: Fig. 4.]
There are great differences in the color and surface of the bark of the
twigs of different species of trees; some are green (Sassafras), some
red (Peach, on the sunny side), some purple (Cherry). Some are smooth
and dotless, some marked with dots (Birch), some roughened with corky
ridges (Sweet Gum), etc.
The taste and odor of the bark are characteristics worthy of notice: the
strong, fragrant odor of the Spice-bush; the fetid odor of the Papaw;
the aromatic taste of the Sweet Birch; the bitter taste of the Peach;
the mucilaginous Slippery Elm; the strong-scented, resinous, aromatic
Walnut, etc.
The branches of trees vary greatly in the thickness of their tips and in
their tendency to grow erect, horizontal, or drooping. Thus the delicate
spray of the Birches contrasted with the stout twigs of the Ailanthus,
or the drooping twigs of the Weeping Willow with the erect growth of the
Lombardy Poplar, give contrasts of the strongest character. In the same
way, the directions the main branches take in their growth from the
trunk form another distinctive feature. Thus the upward sloping branches
of the Elm form a striking contrast to the horizontal or downward
sloping branches of the Sour Gum, or, better still, to certain varieties
of Oaks.
When the main trunk of a tree extends upward through the head to the
tip, as in Fig. 3, it is said to be _excurrent_. When it is soon lost in
the division, as in Fig. 4, it is said to be _deliquescent_.