Cross Section of a Red Oak:
 The
Trunk and its Branches
The cross section of a tree shows the well
defined features in succession from the
outside to the center: (1) bark and
cambium layer; (2) wood, which in most
species is clearly differentiated into
sapwood and heartwood; and (3) pith, the
small central core. The pith and bark are
excluded from finished lumber. Most
branches originate at the pith, and their
bases are intergrown with the wood of the
trunk as long as they are alive. These
living branch bases constitute ingrown or
tight knots. After the branches die, their
bases continue to be surrounded by the
wood of the growing trunk and thus loose
or encased knots are formed. After the
dead branches fall off, the stubs become
overgrown and subsequently clear wood is
formed. FAS and Selects grades of
hardwoods are cut from the outside part of
the log. Therefore, FAS and Selects grades
will have more sapwood than the common
grades. All growth in thickness takes
place in the cambium layer by cell
division. No growth in either diameter or
length takes place in wood already formed;
new growth is purely the addition of
cells, not the further development of
existing cells. The cambium layer is the
only growing part of the tree.
Annual Rings
Most species grown in temperate climates
(having four seasons yearly) produce well
defined annual growth rings, which are
formed by the difference in density and
color between wood formed early and wood
formed late in the growing season. The
inner part of the growth ring formed first
is called “spring wood” and the outer part
formed later in the growing season is
called “summer wood.” Spring wood is
characterized by cells having relatively
large cavities and thin walls. Summer wood
cells have smaller cavities and thicker
walls, and consequently are more dense
than spring wood. The growth rings, when
exposed by conventional methods of sawing,
provide the grain or characteristic
pattern of the wood. The distinguishing
features of the various species are
thereby enhanced by the differences in
growth ring formation. By counting the
growth rings you can determine a trees age
as one ring is formed each year.
Sapwood
Sapwood contains living cells and performs
an active role in the life process of the
tree. It is located next to the cambium
and functions in sap conduction and
storage of food. Sapwood size varies by
species and where the tree is growing.
Heartwood
Heartwood consists of inactive cells
formed by changes in the living cells of
the inner sapwood rings, presumably after
their use for sap conduction and other
life processes of the tree have largely
ceased. The cell cavities of heartwood may
also contain deposits of various materials
that frequently provide a much darker
color. All heartwood, however, is not
darker. The infiltrations of material
deposited in the cells of heartwood
usually make lumber cut from heartwood
more durable when exposed to weather.
Sapwood is not weather resistant in any
species.
Medullary Rays
Medullary rays extend radially from the
pith of the log toward the circumference.
The rays serve primarily to store food and
transport it horizontally. They vary in
height from a few cells in some species to
four or more inches in the oaks, and
produce the flake effect common to the
quarter sawn lumber in these species.
Softwoods and Hardwoods
Native species of trees and the wood
produced by these trees are divided into
two botanical classes-hardwoods, which
have broad leaves, and softwood, which
have needle like or scale-like leaves.
This botanical classification is sometimes
confusing, because there is no direct
correlation between it and the hardness or
softness of the wood. Generally hardwoods
are more dense than softwoods, but some
hardwoods are softer than many softwoods.
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