_Winter Study of Trees._
Many of the peculiarities of trees can be studied much better during the
winter and early spring than at any other time of the year. The plan of
branching, the position, number, size, form, color, and surface of buds,
as well as the arrangement of the leaves within the bud and the
peculiarities of the scales that cover them, are points for winter
investigation.
GENERAL PLAN OF BRANCHING.--There are two distinct and readily
recognized systems of branching. 1. The main stem is _excurrent_ (Fig.
3) when the trunk extends as an undivided stem throughout the tree to
the tip; this causes the spire-like or conical trees so common among
narrow-leaved evergreens. 2. The main stem is _deliquescent_ (Fig. 4)
when the trunk divides into many, more or less equal divisions, forming
the broad-topped, spreading trees. This plan is the usual one among
deciduous trees. A few species, however, such as the Sweet Gum and the
Sugar-maple, show the excurrent stem while young, yet even these have a
deliquescent stem later in life. The English Maple and the Apple both
have a deliquescent stem very early.
All the narrow-leaved evergreens, and many of the broad-leaved trees as
well, show what is called _definite_ annual growths; that is, a certain
amount of leaf and stem, packed up in the winter bud, spreads out and
hardens with woody tissue early in the year, and then, no matter how
long the season remains warm, no additional leaves or stem will grow.
The buds for the next year's growth then form and often become quite
large before autumn.
There are many examples among the smaller plants, but rarely one among
the trees, of _indefinite_ annual growth; that is, the plant puts forth
leaves and forms stems throughout the whole growing-season. The common
Locust, the Honey-locust, and the Sumacs are illustrations.
BUDS.--Buds are either undeveloped branches or undeveloped flowers. They
contain within the scales, which usually cover them, closely packed
leaves; these leaves are folded and wrinkled in a number of different
ways that will be defined at the end of this chapter.
[Illustration: Fig. 14.]
POSITION AND NUMBER.--While the axils of the leaves and the ends of the
stems are the ordinary places for the buds, there are many peculiarities
in regard to their exact position, number, etc., that render them very
interesting for winter study. Sometimes there are several to the single
leaf. In the Silver Maple there are buds on each side of the true
axillary one; these are flower-buds, and during the winter they are
larger than the one which produces the branch. The Butternut (Fig. 14)
and the Walnut have several above each other, the upper one being the
largest and at quite a distance from the true axil. In these cases the
uppermost is apt to grow, and then the branch is said to be
_extra-axillary_. In the Sycamore the bud does not show while the leaf
remains on the tree, as it is in the hollow of the leafstalk. In the
winter the bud has a ring-like scar entirely around it, instead of the
moon-shaped scar below as in most trees. The Common Locust has several
buds under the leafstalk and one above it in the axil. This axillary
bud may grow during the time the leaf remains on the tree, and afterward
the growth of the strongest one of the others may give the tree two
branches almost together.
Some plants form extra buds especially when they are bruised or injured;
those which have the greatest tendency to do so are the Willows,
Poplars, and Elms. Such buds and growths are called _adventitious_. By
cutting off the tops or _pollarding_ such trees, a very great number of
adventitious branches can be made to grow. In this way the Willow-twigs
used for baskets are formed. Adventitious buds form the clusters of
curious thorns on the Honey-locust and the tufts of whip-like branches
on the trunks and large limbs of the Elms.
In trees the terminal bud and certain axillary ones, differing according
to the species or variety of tree, are, during the winter, much larger
than the rest. These are the ones which naturally form the new growth,
and upon their arrangement the character of branching and thus the form
of the tree depend. Each species has some peculiarity in this regard,
and thus there are differences in the branching of all trees. In
opposite-leaved plants the terminal bud may be small and weak, while the
two buds at its side may be strong and apt to grow. This causes a
forking of the branches each year. This plan is not rare among shrubs,
the Lilac being a good example.
BUD-SCALES.--The coverings of buds are exceedingly varied, and are well
worthy of study and investigation. The large terminal buds of the
Horse-chestnut, with their numerous scales, gummy on the outside to keep
out the dampness, and hairy within to protect them from sudden changes
of temperature, represent one extreme of a long line; while the small,
naked, and partly buried buds of the Honey-locust or the Sumac represent
the other end.
The scales of many buds are merely extra parts formed for their
protection, and fall immediately after the bursting of the buds; while
other buds have the stipules of the leaves as bud-scales; these remain
on the twigs for a time in the Tulip-tree, and drop immediately in the
Magnolia.
FORMS OF BUDS.--The size of buds varies greatly, as before stated, but
this difference in size is no more marked than the difference in form.
There is no better way to recognize a Beech at any time of the year than
by its very long, slender, and sharp-pointed buds. The obovate and
almost stalked buds of the Alders are also very conspicuous and
peculiar. In the Balsam Poplar the buds are large, sharp-pointed, and
gummy; in the Ailanthus they cannot be seen.
[Illustration: Fig. 15.]
All the things that might be learned from a small winter twig cannot be
shown in an engraving, but the figures here given illustrate some of the
facts easily determined from such specimens. The first twig (Ash) had
opposite leaves and is 3 years old (the end of each year's growth is
marked by dotted lines on all the figures); the year before last it had
6 leaves on the middle portion; last year it had 8 leaves on the end
portion and 12 on the side shoots of the middle portion. The buds near
the end of the annual growth are strongest and are most apt to grow.
The specimen illustrated was probably taken from the end of a branch of
a rather young and luxuriantly growing tree. Thus the Ash must have
quite a regular growth and form a regularly outlined tree.
The second twig (Sweet Gum) shows 7 years' growth and is probably a side
shoot from more or less within the tree-top. It is stunted in its growth
by the want of light and room. The leaves were alternate.
The third twig (Sycamore) also had alternate leaves; the pointed buds
must have been under the leafstalks, as the leaf-scars show as rings
around the buds. The larger branch grew three years ago. From the
specimen one judges that the Sycamore is quite an irregularly formed
tree. The twig had 11 leaves last year.
The fourth twig (Silver Maple) shows that the plant had opposite leaves,
and supernumerary buds at the sides of the true axillary ones; the true
axillary buds are smaller than those at the sides. It would, in such
cases, be reasonable to suppose that the supernumerary buds were floral
ones, and that the plant blooms before the leaves expand. The annual
growths are quite extended; two years and a part of the third make up
the entire twig. If it was cut during the winter of 1891-92, it must
have had leaves on the lower part in 1889 and 12 leaves on the middle
portion in 1890, as well as probably 4 on the lower portion on the side
shoots. Last year it had 14 leaves on the end portion, two at least on
each side shoot below, making 24 in all.
_Folding of Leaves in the Bud._
There are some peculiarities in the arrangement of leaves in the bud
which can be investigated only in the early spring. The common plans
among trees are--_Inflexed_: blade folded crosswise, thus bringing it
upon the footstalk. Tulip-tree. _Conduplicate_: blade folded along the
midrib, bringing the two halves together. Peach. _Plicate_: folded
several times lengthwise, like a fan. Birch. _Convolute_: rolled
edgewise from one edge to the other. Plum. _Involute_: both edges rolled
in toward the midrib on the upper side. Apple. _Revolute_: both edges
rolled backward. Willow. _Obvolute_: folded together, but the opposite
leaves half inclosing each other. Dogwood.