Middle--Actual appearances of pith ray, PP.
Bottom--Microscopic enlargement of a tangential section of White Oak showing large pith ray, pp.
PLATE 4. EXOGENOUS STRUCTURE IN WOOD.
(CROSS-SECTIONS, ENLARGED.)
Top--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.
Bottom--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]
Fig. 4.