ght and nourishment that periods of rapid and slow growth
may alternate. Some trees, such as southern oaks, maintain the
same width of ring for hundreds of years. Upon the whole,
however, as a tree gets larger in diameter the width of the
growth rings decreases.

It is evident that there may be decided differences in the grain
of heartwood and sapwood cut from a large tree, particularly one
that is overmature. The relationship between width of growth
rings and the mechanical properties of wood is discussed under
Rate of Growth. In this connection, however, it may be stated
that as a general rule the wood laid on late in the life of a
tree is softer, lighter, weaker, and more even-textured than
that produced earlier. It follows that in a large log the
sapwood, because of the time in the life of the tree when it was
grown, may be inferior in hardness, strength, and toughness to
equally sound heartwood from the same log.

After exhaustive tests on a number of different woods the U.S.
Forest Service concludes as follows: "Sapwood, except that from
old, overmature trees, is as strong as heartwood, other things
being equal, and so far as the mechanical properties go should
not be regarded as a defect."[22] Careful inspection of the
individual tests made in the investigation fails to reveal any
relation between the proportion of sapwood and the breaking
strength of timber.

[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]

In the study of the hickories the conclusion was: "There is an
unfounded prejudice against the heartwood. Specifications place
white hickory, or sapwood, in a higher grade than red hickory,
or heartwood, though there is no inherent difference in
strength. In fact, in the case of large and old hickory trees,
the sapwood nearest the bark is comparatively weak, and the best
wood is in the heart, though in young trees of thrifty growth
the best wood is in the sap."[23] The results of tests from
selected pieces lying side by side in the same tree, and also
the average values for heartwood and sapwood in shipments of the
commercial hickories without selection, show conclusively that
"the transformation of sapwood into heartwood does not affect
either the strength or toughness of the wood.... It is true,
however, that sapwood is usually more free from latent defects
than heartwood."[24]

[Footnote 23: Bul. 80: The commercial hickories, p. 50.]

[Footnote 24: _Loc. cit._]

Specifications for p