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). [Illustration: 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. [Illustration: PLATE 3. EXOGENOUS STRUCTURE IN WOOD. MEDULLARY OR PITH RAY. Actual appearances of pith ray, _PP_. Microscopic enlargement of a tangential section of White Oak showing large pith ray, _pp_.] [Illustration: PLATE 4. EXOGENOUS STRUCTURE IN WOOD. (CROSS-SECTIONS, ENLARGED.) Two yearly layers of White Oak. The larger pores were formed in spring, the smaller ones in summer. The small circles are ends of fibres. A knife pressed upon this surface would crush or cut into the cells. The mass would not split or separate evenly. Two yearly layers of White Pine. The space in the outer or darker one is a resin-duct; the circles are ends of fibres (tracheids); pith rays are noticeable. A knife pressed upon this surface would find easy passage. The mass would separate easily.] Logs are sometimes sawn into quarters and then into pieces crossing and exposing the yearly rings. (See Fig. 4.) These "quarter-sawn" surfaces are structurally stronger and better, but are, by reason of waste or small pieces, more costly than others. The pith-rays of some woods, such as oaks, are very prominent when split as they are in "quarter-sawing," and the appearance of such woods is consequently improved.[10] [Illustration: FIG. 4.]