sides, due to the fact that
the vessels and other water-carriers are cut across, allowing
ready entrance of drying air and outlet for the water vapor.
Water does not flow out of boards and timbers of its own accord,
but must be evaporated, though it may be forced out of very
sappy specimens by heat. In drying a log or pole with the bark
on, most of the water must be evaporated through the ends, but
in the case of peeled timbers and sawn boards the loss is
greatest from the surface because the area exposed is so much
greater.

The more rapid drying of the ends causes local shrinkage, and
were the material sufficiently plastic the ends would become
bluntly tapering. The rigidity of the wood substance prevents
this and the fibres are split apart. Later, as the remainder of
the stick dries many of the checks will come together, though
some of the largest will remain and even increase in size as the
drying proceeds. (See Fig. 27.)

[Illustration: FIG. 27.--Excessive season checking. _Photo by U.
S. Forest Service._]

A wood cell shrinks very little lengthwise. A dry wood cell is,
therefore, practically of the same length as it was in a green
or saturated condition, but is smaller in cross section, has
thinner walls, and a larger cavity. It is at once evident that
this fact makes shrinkage more irregular, for wherever cells
cross each other at a decided angle they will tend to pull apart
upon drying. This occurs wherever pith rays and wood fibres
meet. A considerable portion of every wood is made up of these
rays, which for the most part have their cells lying in a radial
direction instead of longitudinally. (See Frontispiece.) In
pine, over 15,000 of these occur on a square inch of a
tangential section, and even in oak the very large rays which
are readily visible to the eye as flakes on quarter-sawed
material represent scarcely one per cent of the number which the
microscope reveals.

A pith ray shrinks in height and width, that is, vertically and
tangentially as applied to the position in a standing tree, but
very little in length or radially. The other elements of the
wood shrink radially and tangentially, but almost none
lengthwise or vertically as applied to the tree. Here, then, we
find the shrinkage of the rays tending to shorten a stick of
wood, while the other cells resist it, and the tendency of a
stick to get smaller in circumference is resisted by the endwise
reaction or thrust of the rays. Only in a tan