|----------------------------------+--------+--------+-------+-------+----------| | Number of tests averaged | 5 | 4 | 5 | 3 | 4 | | _r_ in bending | 3,002 | 3,362 | 6,458 | 8,400 | 10,170 | | _C_ in compression | 2,680 | 3,025 | 6,120 | 7,610 | 9,335 | | 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. This is particularly the case with thick-walled cells such as are found in late wood. [Footnote 6: Jaccard, P.: Étude anatomique des bois comprimés. Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band, 1. Heft. Zurich, 1910, p. 66.]