Longitudinal shrinkage is accordingly least at the centre and greater toward the outside, tending to become greatest in the sapwood.

When a round or a box-heart timber dries fast it splits radially, and as drying continues the cleft widens partly on account of the greater tangential shrinkage and also because the greater contraction of the outer fibres warps the sections apart.

If a small hardwood stem is split while green for a short distance at the end and placed where it can dry out rapidly, the sections will become bow-shaped with the concave sides out.

These various facts, taken together, explain why, for example, an oak tie, pole, or log may split open its entire length if drying proceeds rapidly and far enough.

Initial stresses in the living trees produce a similar effect when the log is sawn into boards.