Furthermore, structure underlies nearly all the technical
properties of this important product, and furnishes an explanation why
one piece differs in these properties from another. Structure explains
why oak is heavier, stronger, and tougher than pine; why it is harder
to saw and plane, and why it is so much more difficult to season
without injury. From its less porous structure alone it is evident
that a piece of young and thrifty oak is stronger than the porous wood
of an old or stunted tree, or that a Georgia or long-leaf pine excels
white pine in weight and strength.

Keeping especially in mind the arrangement and direction of the fibres
of wood, it is clear at once why knots and "cross-grain" interfere
with the strength of timber. It is due to the structural peculiarities
that "honeycombing" occurs in rapid seasoning, that checks or cracks
extend radially and follow pith rays, that tangent or "bastard" cut
stock shrinks and warps more than that which is quarter-sawn. These
same peculiarities enable oak to take a better finish than basswood or
coarse-grained pine.

Structure of Wood

The softwoods are made up chiefly of tracheids, or vertical cells
closed at the ends, and of the relatively short parenchyma cells of
the medullary rays which extend radially from the heart of the tree.
The course of the tracheids and the rays are at right angles to each
other. Although the tracheids have their permeable portions or pits in
their walls, liquids cannot pass through them with the greatest ease.
The softwoods do not contain "pores" or vessels and are therefore
called "non-porous" woods.

The hardwoods are not so simple in structure as softwoods. They
contain not only rays, and in many cases tracheids, but also
thick-walled cells called fibres and wood parenchyma for the storage
of such foods as starches and sugars. The principal structural
features of the hardwoods are the pores or vessels. These are long
tubes, the segments of which are made up of cells which have lost
their end walls and joined end to end, forming continuous "pipe lines"
from the roots to the leaves in the tree. Since they possess pores or
vessels, the hardwoods are called "porous" woods.

Red oak is an excellent example of a porous wood. In white oak the
vessels of the heartwood especially are closed, very generally by
ingrowths called tyloses. This probably explains why red oak dries
more easily and rapidly than white