The young wood of exogenous trees is porous. It permits the passage of sap and is known as sapwood (Alburnum). As a layer is enclosed by others and retreats from the surface of the tree, it becomes denser, its canals are filled with gums or tannin, color changes, and the result is heartwood (Duramen). This change goes forward rapidly in some trees, such as locusts, so that their sections appear to be almost wholly heartwood; other species require longer time, and sapwood then predominates.
Heartwood gives stability to the tree, but is not needed in its physiological processes. It is tougher, heavier, stronger, and more valued in construction. Sapwood is vitally essential to the life of the tree, but is lighter, weaker, less durable, and less valued in construction. Sapwood is pliable, and the sapwoods of several trees are valued for this reason.
Wood-making varies as it takes place in the springtime and in the summer. Consequent differences in the densities [p008] of the deposits serve to mark the limits of the yearly rings. Some species, as the oaks and hickories, show pores throughout their spring woods which thus contrast with denser summer growths. Others, as Southern pines, change sharply, and their spring and summer growths appear as solid bands. In even climates, where seasons are not pronounced, growth is more regular and layers correspondingly less definite.[9]
The cellular structure of wood is principally vertical, a fact that explains the ease with which wood is split up and down. Beside the vertical, there are horizontal cells, that cross the tree, strengthen and bind the vertical cells, and assist in the life-processes of the tree. These horizontal cells form what are known as medullary or pith-rays and appear as simple lines or glistening plates according to the way in which the wood is cut. Woods differ in the size and number of these rays, which are by no means always visible to the eye (see plate 3).
Fig. 3.
Woods are easy or difficult to work in proportion as their fibres are arranged in a simple or a complicated manner. This is shown in the figures on plate 4. A knife pressed upon the oak must crush or cut into the fibres themselves, whereas with the pine it finds some natural passage between the cells. Wood may be cut so as to develop cross-sections (C, Fig. 3), radial sections (R), or tangential sections (T). The respective [p009] markings are in a general way indicated on the boards in the figure.
PLATE 3. EXOGENOUS STRUCTURE IN WOOD.
Medullary or Pith Ray.
