[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 



289 



Fig. 109. Non-porous woods of arbor vitae (above) and of long leaf yellow 

 pine (below). The large openings in the latter are cross sections of resin ducts. 

 Photos from Forest Products Laboratory. 



porous. If the vessels are of about the same size and are scattered uni- 

 fomily in the annual ring (Fig. 108B), the wood is said to be diffttse- 

 poroits. In conifer stems, where no vessels occur, tlie wood is non-porous 

 (Fig. 109). These facts are useful in the identification of woods of dif- 

 ferent species. 



Grain of wood. When boards are cut from stems and polished, vari- 

 ous patterns of wood are often conspicuous. Such patterns are dependent 

 on the relative size of the cells and vessels of spring and summer wood, 

 on the width of the annual rings, and on the plane in which the saw 

 passes through the log with reference to the xylem rays (Figs. 110 and 

 111). "Quarter-sawed" boards are cut parallel to these rays. Tangential 

 or "slab" cuts at various angles to the rays result in a variety of patterns. 

 The summer wood is harder than the spring wood and takes a finer 

 polish; and this difference is the cause of the often attractive grain pat- 

 terns of furniture, panels, and cabinet work. In perfect radial sections 

 the grain appears as straight lines crossed by patches of xylem rays. 



