of this kind of wood are gum, yellow poplar, birch,
maple, cottonwood, basswood, buckeye, and willow. Some species,
such as walnut and cherry, are on the border between the two
classes, forming a sort of intermediate group.

If one examines the smoothly cut end of a stick of almost any
kind of wood, he will note that each growth ring is made up of
two more or less well-defined parts. That originally nearest the
centre of the tree is more open textured and almost invariably
lighter in color than that near the outer portion of the ring.
The inner portion was formed early in the season, when growth
was comparatively rapid and is known as _early wood_ (also
spring wood); the outer portion is the _late wood_, being
produced in the summer or early fall. In soft pines there is not
much contrast in the different parts of the ring, and as a
result the wood is very uniform in texture and is easy to work.
In hard pine, on the other hand, the late wood is very dense and
is deep-colored, presenting a very decided contrast to the soft,
straw-colored early wood. (See Fig. 23.) In ring-porous woods
each season's growth is always well defined, because the large
pores of the spring abut on the denser tissue of the fall
before. In the diffuse-porous, the demarcation between rings is
not always so clear and in not a few cases is almost, if not
entirely, invisible to the unaided eye. (See Fig. 22.)

[Illustration: FIG. 23.--Cross section of longleaf pine showing
several growth rings with variations in the width of the
dark-colored late wood. Seven resin ducts are visible. X 33.
_Photomicrograph by U.S. Forest Service_]

If one compares a heavy piece of pine with a light specimen it
will be seen at once that the heavier one contains a larger
proportion of late wood than the other, and is therefore
considerably darker. The late wood of all species is denser than
that formed early in the season, hence the greater the
proportion of late wood the greater the density and strength.
When examined under a microscope the cells of the late wood are
seen to be very thick-walled and with very small cavities, while
those formed first in the season have thin walls and large
cavities. The strength is in the walls, not the cavities. In
choosing a piece of pine where strength or stiffness is the
important consideration, the principal thing to observe is the
comparative amounts of early and late wood. The width of ring,
that is, the number per inch, is