[Illustration:  FIG. 21.—­Cleavage of highly elastic wood.  The cleft runs far ahead of the wedge.]

This property of wood is very important in certain uses such as firewood, fence rails, billets, and squares.  Resistance to splitting or low cleavability is desirable where wood must hold nails or screws, as in box-making.  Wood usually splits more readily along the radius than parallel to the growth rings though exceptions occur, as in the case of cross grain.

Splitting involves transverse tension, but only a portion of the fibres are under stress at a time.  A wood of little stiffness and strong cohesion across the grain is difficult to split, while one with great stiffness, such as longleaf pine, is easily split.  The form of the grain and the presence of knots greatly affect this quality.

|---------------------------------------------|
|                 TABLE XIII                  |
|---------------------------------------------|
| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF  |
|         32 WOODS IN GREEN CONDITION         |
|          (Forest Service Cir. 213)          |
|---------------------------------------------|
|                   |    When    |    When    |
|   COMMON NAME     | surface of | surface of |
|   OF SPECIES      | failure is | failure is |
|                   |   radial   | tangential |
|-------------------+------------+------------|
|                   |  Lbs. per  |  Lbs. per  |
|                   |  sq. inch  |  sq. inch  |
|                   |            |            |
|    Hardwoods      |            |            |
|                   |            |            |
| Ash, black        |     275    |     260    |
|   white           |     333    |     346    |
| Bashwood          |     130    |     168    |
| Beech             |     339    |     527    |
| Birch, yellow     |     294    |     287    |
| Elm, slippery     |     401    |     424    |
|   white           |     210    |     270    |
| Hackberry         |     422    |     436    |
| Locust, honey     |     552    |     610    |
| Maple, red        |     297    |     330    |
|   sugar           |     376    |     513    |
| Oak, post         |     354    |     487    |
|   red             |     380    |     470    |
|   swamp white     |     428    |     536    |
|   white           |     382    |     457    |
|   yellow          |     379    |     470    |
| Sycamore          |     265    |     425    |
| Tupelo            |     277    |     380    |
|                   |            |            |
|    Conifers       |            |            |
|                   |            |            |
| Arborvitae         |     148    |     139    |
| Cypress, bald     |     167    |     154    |
| Fir, alpine       |     130    |     133    |
|   Douglas         |     139    |     127    |
|   white           |     145    |     187    |
| Hemlock           |     168    |     151    |
| Pine, lodgepole   |     142    |     140    |