In inspection, the elements to examine are the rays.  In the case of oak and certain other hardwoods these rays are so large that they are readily seen not only on a radial surface, but on the tangential as well.  On the former they appear as flakes, on the latter as short lines.  Since these rays are between the fibres it naturally follows that they will be vertical or inclined according as the tree is straight-grained or spiral-grained.  While they are not conspicuous in the softwoods, they can be seen upon close scrutiny, and particularly so if a small hand magnifier is used.

When wood has begun to dry and check it is very easy to see whether or not it is straight- or spiral-grained, since the checks will for the most part follow along the rays.  If one examines a row of telephone poles, for example, he will probably find that most of them have checks running spirally around them.  If boards were sawed from such a pole after it was badly checked they would fall to pieces of their own weight.  The only way to get straight material would be to split it out.

It is for this reason that split billets and squares are stronger than most sawed material.  The presence of the spiral grain has little, if any, effect on the timber when it is used in the round, but in sawed material the greater the pitch of the spiral the greater is the defect.

KNOTS

Knots are portions of branches included in the wood of the stem or larger branch.  Branches originate as a rule from the central axis of a stem, and while living increase in size by the addition of annual woody layers which are a continuation of those of the stem.  The included portion is irregularly conical in shape with the tip at the pith.  The direction of the fibre is at right angles or oblique to the grain of the stem, thus producing local cross grain.

During the development of a tree most of the limbs, especially the lower ones, die, but persist for a time—­often for years.  Subsequent layers of growth of the stem are no longer intimately joined with the dead limb, but are laid around it.  Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber.  In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place.[32]

[Footnote 32:  See Standard classification of structural timber.  Yearbook Am.  Soc. for Testing Materials, 1913, pp. 300-303.  Contains three plates showing standard defects.]