The medullary rays assume a direction more or less parallel to the lumen of the cells on which they border; the latter curve to the right or left to make room for the ray and then close again beyond it.

If the force acts parallel to the axis of growth, the tracheids are more likely to be displaced if the marginal cells of the medullary rays are provided with weak walls that are readily compressed.

This explains why on the radial surface of the test blocks the plane of rupture passes in a direction nearly following a medullary ray, whereas on the tangential surface the direction of the plane of rupture is oblique--but with an obliquity varying with the species and determined by the pitch of the spirals along which the medullary rays are distributed in the stem." See Jaccard, _op.

cit._, pp.

57 _et seq._] SHEARING STRENGTH Whenever forces act upon a body in such a way that one portion tends to slide upon another adjacent to it the action is called a ~shear~.[8] In wood this shearing action may be (1) ~along the grain~, or (2) ~across the grain~.