s occur between two roots
or between the collars of two roots, (_d_) They are most
frequent in old, stout-rooted, broad-crowned trees; in younger
stands it is always the stoutest members that are found with
frost splits, while in quite young stands they are altogether
absent, (_e_) Trees on wet sites are most liable to splits, due
to difference in wood structure, just as difference in wood
structure makes different species vary in this regard. (_f_)
Frost splits are most numerous less than three feet above the
ground.

[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
Centralb., XXXII, 2, 1910, pp. 74-81.]

When a tree is swayed by the wind the roots are counteracting
forces, and the wood fibres are tested in tension and
compression by the opposing forces; where the roots exercise
tension stresses most effectively the effect of compression
stresses is at a minimum; only where the pressure is in excess
of the tension, _i.e._, between the roots, can a separation of
the fibre result. Hence, when by frost a tension on the entire
periphery is established, and the wind localizes additional
strains, failure occurs. The stronger the compression and
tension, the severer the strains and the oftener failures occur.
The occurrence of reports of frost splits on wind-still days is
believed by Busse to be due to the opening of old frost splits
where the tension produced by the frost alone is sufficient.

Frost splits may heal over temporarily, but usually open up
again during the following winter. The presence of old splits is
often indicated by a ridge of callous, the result of the
cambium's effort to occlude the wound. Frost splits not only
affect the value of lumber, but also afford an entrance into the
living tree for disease and decay.

SHAKES, GALLS, PITCH POCKETS

_Heart shake_ occurs in nearly all overmature timber, being more
frequent in hardwoods (especially oak) than in conifers. In
typical heart shake the centre of the hole shows indications of
becoming hollow and radial clefts of varying size extend outward
from the pith, being widest inward. It frequently affects only
the butt log, but may extend to the entire hole and even the
larger branches. It usually results from a shrinkage of the
heartwood due probably to chemical changes in the wood.

When it consists of a single cleft extending across the pith it
is termed _simple heart shake_. Shake of this character in
straight-grained trees aff