These are rows of cells which give oak some of its figure and are known as wood rays . Rays serve to transport substance laterally within the tree for food storage and vary in size among species. In cottonwood, for example, they are not usually visible to the naked eye. Rays also appear more numerous in some kinds of wood than in others. Therefore both the size and number of rays are used in wood identification.
Rays are large and easily seen in oak beech and sycamore.
Rays in tuliptree, sweetgum and basswood appear much less distinct to the naked eye. The rays may also vary in size in any one species. White oak, for example, has some wide and very conspicuous rays and others, in between, that are narrow and indistinct. In black walnut the rays are quite uniform in width and indistinct without a hand lens. Color, Hardness, and Figure Color, degree of hardness, and figure are also used to assist in identification of woods.