In discussing such woods it is customary to divide them into two large classes--_ring-porous_ and _diffuse-porous_.

(See Fig.
22.) In ring-porous species, such as oak, chestnut, ash, black locust, catalpa, mulberry, hickory, and elm, the larger vessels or pores (as cross sections of vessels are called) become localized in one part of the growth ring, thus forming a region of more or less open and porous tissue.

The rest of the ring is made up of smaller vessels and a much greater proportion of wood fibres.

These fibres are the elements which give strength and toughness to wood, while the vessels are a source of weakness.
In diffuse-porous woods the pores are scattered throughout the growth ring instead of being collected in a band or row.
Examples of this kind of wood are gum, yellow poplar, birch, maple, cottonwood, basswood, buckeye, and willow.

Some species, such as walnut and cherry, are on the border between the two classes, forming a sort of intermediate group.
If one examines the smoothly cut end of a stick of almost any kind of wood, he will note that each growth ring is made up of two more or less well-defined parts.