The young seedling and the young shoot of the older tree much resemble
in interior structure that of any herbaceous plant, being composed of a
large amount of pith, loose squarish cells, and a few bundles of long
fibers symmetrically distributed about the center, the whole covered
with a thin skin or epidermis. Each strand or bundle of fibers, called
fibro-vascular (fiber-vessel) bundles, consists of two kinds, namely,
wood fibers on the inner side and bast fibers of different structure
on the outer side. Between these two sets of fibers, the bast and the
wood, there is a row of cells which form the really active, growing
part of the plantlet, the cambium. The cambium cells are actively
subdividing and expanding, giving off wood cells to the interior and
bast cells to the exterior, and extending at the same time side-wise,
until at the end of the season not only are the wood and bast portions
increased in lines radiating from the center, but the cambium layer,
the wood cells, and the bast cells of all the bundles (scattered at
the beginning) join at the sides to form a complete ring, or rather
hollow cylinder, around the central pith. Only here and there the pith
cells remain, interrupting the wood cylinder and giving rise to the
system of cells known as medullary rays. The cross-section now shows a
comparatively small amount of pith and bast or bark and a larger body
of strong wood fibers. The new shoot at the end, to be sure, has the
same appearance and arrangement as the young plantlet had, the pith
preponderating, and the continuous cylinder of cambium, bast, and wood
being separated into strands or bundles.
[Illustration: Fig. 7.--Development in and out of the forest. _A_,
young tree alike. In both cases; _B_ and _C_, successive stages of tree
grown in the open; _B´_ and _C´_, corresponding stages of the tree
grown in the forest. Numbers refer to annual growth in height.]
During the season, through the activity of the cambial part of the
bundles, the same changes take place in the new shoot as did the
previous year in the young seedling, while at the same time the cambium
in the yearling part also actively subdivides, forming new wood and
bast cells, and thus a second ring, or rather cylinder, is formed. The
cambium of the young shoot is always a continuation of that of the ring
or cylinder formed the year before, and this cambium cylinder always
keeps moving outward, so that at the end of the season, when activity
ceases, it is always the last minute layer of cells on the outside of
the wood, between wood proper and bark. It is here, therefore, that
the life of the tree lies, and any injury to the cambium must interfere
with the growth and life of the tree.
[Illustration: Fig. 8.--Tree in and out of the forest. _D_, tree grown
in the open; _D´_, tree grown in the forest.]
The first wood cells which the cambium forms in the spring are usually
or always of a more open structure, thin-walled, and with a large
opening or "lumen," comparable to a blown-up paper bag; so large, in
fact, sometimes, is the "lumen" that the width of the cells can be
seen on a cross-section with the naked eye, as, for instance, in oak,
ash, elm, the so-called "pores" are this open wood formed in spring.
The cells, which are formed later in summer, have mostly thick walls,
are closely crowded and compressed, and show a very small opening or
"lumen," being comparable, perhaps, to a very thick wooden box. They
appear in the cross-section not only denser but of a deeper color, on
account of their crowded, compressed condition and thicker walls. Since
at the beginning of the next season again thin-walled cells with wide
openings or lumina are formed, this difference in the appearance of
"spring wood" and "summer wood" enables us to distinguish the layer of
wood formed each year. This "annual ring" is more conspicuous in Some
kinds than in others. In the so-called "ring porous" woods, like oak,
ash, elm, the rings are easily distinguished by the open spring wood;
in the conifers, especially pines, by the dark-colored summer wood;
while in maple, birch, tulip, etc., only a thin line of flattened,
hence darker and regularly aligned, summer cells, often hardly
recognizable, distinguishes The rings from each other. Cutting through
a tree, therefore, we can not only ascertain its age by counting its
annual layers in the cross-section, but also determine how much wood is
formed each year (fig. 10). We can, in fact, retrace the history of
its growth, the vicissitudes through which it has passed, by the record
preserved in its ring growth.
To ascertain the age of a tree correctly, however, we must cut so near
to the ground as to include the growth of the first year's little
plantlet; any section higher up shows as many years too few as it took
the tree to reach that height.
[Illustration: Fig. 9.--Sections of logs showing the relative
development of knots. _E_, from tree grown in the open; _E´_, from tree
grown in a dense forest; _a_ and _c_, whorls of knots; _b_, dead limb;
_sk_, "sound knot;" _dk_, "dead knot."]
This annual-ring formation is the rule in all countries which have
distinct seasons of summer and winter and temporary cessation of
growth. Only exceptionally a tree may fail to make its growth
throughout its whole length on account of loss of foliage or other
causes; and occasionally, when its growth has been disturbed during
the season, a "secondary" ring, resembling the annual ring, and
distinguishable only by the expert, may appear and mar the record.
To the forest planter this chapter on ring growth is of great
importance, because not only does this feature of tree life afford the
means of watching the progress of his crop, calculating the amount of
wood formed, and therefrom determining when it is most profitable for
him to harvest (namely, when the annual or periodic wood growth falls
below a certain amount), but since the proportion of summer wood and
spring wood determines largely the quality of the timber, and since he
has it in his power to influence the preponderance of the one or other
by adaptation of species to soils and by their management, ring growth
furnishes an index for regulating the quality of his crop.