Hardwoods are used in construction and repair
work because of their strength, durability, and ability
to resist warpage. They are used to make furniture,
dowels, and some patterns. Hardwoods include ash,
birch, beech, white oak, poplar, walnut, and maple.
HTs prefer softwoods for most patterns because
they work easily. Softwoods are also used as
structural lumber, boat planking, and for shoring.
Softwoods include white cedar, cypress, Douglas fir,
white pine, yellow pine, and redwood.
TREE GROWTH AND STRUCTURE
Wood consists of small cells. The size and
Figure 3-2.--Structure of wood.
arrangement of these cells determine the grain of
the wood and many of its properties. Look at a
and the bark (fig. 3-1). Sapwood is lighter in color
freshly cut tree stump. You will see thousands of
large and small cells arranged in circular rings
around the pith or center of the tree. The large
The cambium layer (fig. 3-1) is the boundary
cells have thin walls, and the smaller cells have thick
between the sapwood and the bark. New sapwood
walls (figs. 3-1 and 3-2). Rings form because of a
cells form in this thin layer.
difference in the growth rate during various seasons
of the year. In spring, a tree grows rapidly and
Medullary rays (fig. 3-1) are radial lines of wood
builds up a layer of soft, large cells. These cells
cells. They are highly visible. Their function is to
appear in the cross section of the trunk as the
move cell liquids horizontally in the tree trunk.
light-colored rings (spring rings).
When speaking of medullary rays, we use thickness
to refer to the horizontal dimension, and width to
As the weather gets hotter during early summer,
refer to the vertical dimension.
the growth rate slows. The summer cells form
closer together and become dark rings (summer
When a tree is sawed lengthwise, the annual
rings). The age of a tree can be determined very
rings form a pattern called the grain. Several terms
accurately by counting these dark rings. Some trees,
describe wood grain.
such as oak and walnut, have very distinctive rings.
White pine is so uniform that you can barely see the
If the wood cells that form the grain are
closely packed and small, the wood is
The sapwood of a tree is the outer section of the
fine-grained or close-grained. Maple and
tree between the heartwood (darker center wood)
birch are good examples.
If the cells are large and porous, the wood
is coarse-grained or open-grained. Oak,
walnut, and mahogany are examples of
When the wood cells are straight and
parallel to the trunk of the tree, the wood is
If the grain is crooked, slanting, or twisted,
the wood is cross-grained.
Figure 3-1.--Cross section of a tree.