A considerable portion of every wood is made up of these rays, which for the most part have their cells lying in a radial direction instead of longitudinally.

(See Frontispiece.) In pine, over 15,000 of these occur on a square inch of a tangential section, and even in oak the very large rays which are readily visible to the eye as flakes on quarter-sawed material represent scarcely one per cent of the number which the microscope reveals.
A pith ray shrinks in height and width, that is, vertically and tangentially as applied to the position in a standing tree, but very little in length or radially.

The other elements of the wood shrink radially and tangentially, but almost none lengthwise or vertically as applied to the tree.

Here, then, we find the shrinkage of the rays tending to shorten a stick of wood, while the other cells resist it, and the tendency of a stick to get smaller in circumference is resisted by the endwise reaction or thrust of the rays.

Only in a tangential direction, or around the stick in direction of the annual rings of growth, do the two forces coincide.