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.:  Etude anatomique des bois comprimes.  Mit. d.  Schw.  Centralanstalt f.d. forst.  Versuchswesen.  X. Band, 1.  Heft.  Zurich, 1910, p. 66.]

|------------------------------------------------------

-|
|                       TABLE VI                        |
|-------------------------------------------------------|
|  RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR  |
|         PIECES OF 40 WOODS IN GREEN CONDITION         |
|               (Forest Service Cir. 213)               |
|-------------------------------------------------------|
|                   |   Fibre   |          |  Modulus   |
|    COMMON NAME    | stress at | Crushing |     of     |
|    OF SPECIES     |  elastic  | strength | elasticity |
|                   |   limit   |          |            |
|-------------------+-----------+----------+------------|
|                   | Lbs. per  | Lbs. per |  Lbs. per  |
|                   | sq. inch  | sq. inch |  sq. inch  |
|                   |           |          |            |
|    Hardwoods      |           |          |            |
|                   |           |          |            |
| Ash, white        |   3,510   |   4,220  |  1,531,000 |
| Basswood          |     780   |   1,820  |  1,016,000 |
| Beech             |   2,770   |   3,480  |  1,412,000 |
| Birch, yellow     |   2,570   |   3,400  |  1,915,000 |
| Elm, slippery     |   3,410   |   3,990  |  1,453,000 |
| Hackberry         |   2,730   |   3,310  |  1,068,000 |
| Hickory,          |           |          |            |
|   big shellbark   |   3,570   |   4,520  |  1,658,000 |
|   bitternut       |   4,330   |   4,570  |  1,616,000 |
|   mockernut       |   3,990   |   4,320  |  1,359,000 |
|   nutmeg          |   3,620   |   3,980  |  1,411,000 |
|   pignut          |   3,520   |   4,820  |  1,980,000 |
|   shagbark        |   3,730   |   4,600  |  1,943,000 |