While the stalk of wheat or corn grows for one season, exhausts
itself in seed production, and then dies, the tree continues to grow
from season to season, in length as well as in thickness. The growth
in length of shaft and branches proceeds from buds, made up of cell
tissues, which can subdivide and lengthen into shoots, as well as make
leaves. These buds are formed during summer, and when winter begins
contain embryo leaves, more or less developed, under the protecting
cover of scales (fig. 3). When spring stimulates the young plant to
new activity, the buds swell, shed their scales, distend their cells,
increasing their number by subdivision, and thus the leaves expand, and
the bud lengthens into a shoot and twig. During the season new buds are
formed, and the whole process repeats itself from year to year, giving
rise to the ramification and height growth of the tree. The end buds
being mostly stronger and better developed, the main axis of tree or
branch increases more rapidly than the rest. All these buds originate
from the youngest, central part of the shoot, the pith, and hence
when the tree grows in thickness, enveloping the base of the limbs,
their connection with the pith can always be traced. This is the usual
manner of bud formation; in addition, so-called "adventitious" buds
maybe formed from the young living wood in later life, which are not
connected with the pith. Such buds are those which develop into sprouts
from the stump when the tree is cut; also those which give rise to
what are known as "water sprouts." Many buds, although formed, are,
however, not developed at once, and perhaps not at all, especially as
the tree grows older; these either die or remain "dormant," often for a
hundred years, to spring into life when necessary (fig. 4).
[Illustration: Fig. 3.--Buds of maple. _A_, longitudinal section
through tip of a maple twig; _g_, end bud; _s_, lateral buds; _l_,
scars of leaves of last season. _B_, cross-section through end bud,
showing folded leaves in center and scales surrounding them.]
The fact that each ordinary limb starts as a bud from the pith is an
important one to the timber grower; it explains knotty timber and gives
him the hint that in order to obtain clear timber the branches first
formed must be soon removed, either by the knife or by proper shading,
which kills the branches and thus "clears" the shaft.
[Illustration: Fig. 4.--Dormant bud, _K_, on a 12-year old branch of
beech. The bud is still capable of development and is connected with
the pith, _mm_, of the stem by a line trace of pith, _S_.]
[Illustration: Fig. 5.--Section through a 12-year old stem of beech,
showing manner of bud and limb formation, _a_, dormant buds; _b_, their
trace of pith extending to the pith of the stem; _c_, a limb which
started two years ago from a dormant bud; _d_, normal limb; _e_, a limb
dead for four years; _f_, adventitious buds.]
The planter has it also in his power to influence the form development
of the tree by removing some of the buds, giving thereby better chance
to the remaining ones. This pruning of buds is, where practicable,
often better practice than the pruning of limbs.
Since the tree does not grow in length except by its buds it is evident
that a limb which started to grow at the height of 6 feet has its base
always 6 feet from the ground, and if allowed to grow to size, must be
surrounded by the wood which accumulates on the main stem or trunk. If
a limb is killed and broken off early, only a slender stub composed
entirely of rapidly decaying sapwood, is left, occasioning, therefore,
only a small defect in the heart of the tree; but if left to grow to
considerable age, the base of the limb is encased by the wood of the
stem, which, when the tree is cut into lumber, appears as a knot. The
longer the limb has been allowed to grow, the farther out is the timber
knotty and the thicker is the knot. If the limb remained alive, the
knot is "sound," closely grown together with the fibers of the tree.
If the limb died off, the remaining stub may behave in different ways.
In pines it will be largely composed of heartwood, very resinous and
durable; separated from the fibers of the overgrowing wood, it forms a
"loose" knot, which is apt to fall out of a board, leaving a hole.
In broad-leaved trees, where no resin assists in the process of
healing, the stub is apt to decay, and this decay, caused by the
growth of fungi, is apt to penetrate into the tree (fig. 6). In parks
and orchards, pruning is resorted to, and the cuts are painted or
tarred to avoid the decay. In well-managed forests and dense woods
in general, the light is cut off, the limb is killed when young and
breaks away, the shaft "clears itself," and the sound trunk furnishes
a good grade of material. The difference in development of the branch
system, whether in full enjoyment of light, in open stand, or with the
side light cut off, in dense position, is shown in the accompanying
illustration (fig. 7).
[Illustration: Fig. 6.--Section through partly decayed knot in
oak wood. _a_, wood of knot; _b_ and _c_, wood callus of the stem
covering the wound; shaded portion, decayed wood, black part, a cavity
remaining.]
Both trees start alike; the one retains its branches, the other loses
them gradually, the stubs being in time overgrown; finally the second
has a clear shaft, with a crown concentrated at the top, while the
first is beset with branches and branch stubs for its whole length
(fig. 8).
When ripped open lengthwise, the interior exhibits the condition shown
in figure 9, the dead parts of the knot being indicated in heavier
shading. Since the brandies grow in more or less regular whorls,
several knots, stumps, or limbs are met every 6 to 24 inches through
the entire stem.
Hence, in forest planting, trees are placed and kept for some time
close together, in order to decrease the branching in the lower part of
the tree and thus produce a clean bole and clear lumber.