icient knowledge of the intrinsic factors affecting the
results it becomes possible to infer from the appearance of
material its probable variation from the average. As yet too
little is known of the relation of structure and chemical
composition to the mechanical and physical properties to permit
more than general conclusions.

RATE OF GROWTH

To understand the effect of variations in the rate of growth it
is first necessary to know how wood is formed. A tree increases
in diameter by the formation, between the old wood and the inner
bark, of new woody layers which envelop the entire stem, living
branches, and roots. Under ordinary conditions one layer is
formed each year and in cross section as on the end of a log
they appear as rings--often spoken of as _annual rings_. These
growth layers are made up of wood cells of various kinds, but
for the most part fibrous. In timbers like pine, spruce,
hemlock, and other coniferous or softwood species the wood cells
are mostly of one kind, and as a result the material is much
more uniform in structure than that of most hardwoods. (See
Frontispiece.) There are no vessels or pores in coniferous wood
such as one sees so prominently in oak and ash, for example.
(See Fig. 22.)

[Illustration: FIG. 22.--Cross sections of a ring-porous
hardwood (white ash), a diffuse-porous hardwood (red gum), and a
non-porous or coniferous wood (eastern hemlock). X 30.
_Photomicrographs by the author._]

The structure of the hardwoods is more complex. They are more or
less filled with vessels, in some cases (oak, chestnut, ash)
quite large and distinct, in others (buckeye, poplar, gum) too
small to be seen plainly without a small hand lens. In
discussing such woods it is customary to divide them into two
large classes--_ring-porous_ and _diffuse-porous_. (See Fig.
22.) In ring-porous species, such as oak, chestnut, ash, black
locust, catalpa, mulberry, hickory, and elm, the larger vessels
or pores (as cross sections of vessels are called) become
localized in one part of the growth ring, thus forming a region
of more or less open and porous tissue. The rest of the ring is
made up of smaller vessels and a much greater proportion of wood
fibres. These fibres are the elements which give strength and
toughness to wood, while the vessels are a source of weakness.

In diffuse-porous woods the pores are scattered throughout the
growth ring instead of being collected in a band or row.
Examples