It is necessary, of course, that the split be radial, that is, in a plane passing through the axis of the log, and not tangentially.
In the latter case it is quite probable that the wood would split straight, the line of cleavage being between the growth rings.
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.