| Per cent _r_ | | | | | |
| is in excess of _C_ | 12.0 | 11.1 | 5.5 | 10.4 | 9.0 |
|-------------------------------------------------------------------------------|

When a short column is compressed until it breaks, the manner of
failure depends partly upon the anatomical structure and partly
upon the degree of humidity of the wood. The fibres (tracheids
in conifers) act as hollow tubes bound closely together, and in
giving way they either (1) buckle, or (2) bend.[5]

[Footnote 5: See Bulletin 70, _op. cit._, p. 129.]

The first is typical of any dry thin-walled cells, as is usually
the case in seasoned white pine and spruce, and in the early
wood of hard pines, hemlock, and other species with decided
contrast between the two portions of the growth ring. As a rule
buckling of a tracheid begins at the bordered pits which form
places of least resistance in the walls. In hardwoods such as
oak, chestnut, ash, etc., buckling occurs only in the
thinnest-walled elements, such as the vessels, and not in the
true fibres.

According to Jaccard[6] the folding of the cells is accompanied
by characteristic alterations of their walls which seem to split
them into extremely thin layers. When greatly magnified, these
layers appear in longitudinal sections as delicate threads
without any definite arrangements, while on cross section they
appear as numerous concentric strata. This may be explained on
the ground that the growth of a fibre is by successive layers
which, under the influence of compression, are sheared apart.