26 



ECONOMIC WOODS OF THE UNITED STATES 



75 cells wide and several hundred high. These large rays give rise 

 to the handsome figure of quarter-sawed (i.e., radially cut) oak 

 lumber. Besides the large rays in Quercus there are numerous 

 intermediate ones, mostly uniseriate and 1-20 cells high (Plate 

 III, Fig. 1). In Platanus the rays are uniformly broad (10-15 

 cells), while in Fagus only a portion of the rays are broad (15-25 

 cells), the intermediate ones being uniseriate. In some of the 

 evergreen oaks, Carpinus and species of Alnus (Plate V, Figs. 3, 4), 

 the large rays appear to be composed of numerous small ones 



r.tr. 



rjtr. 



Fig. 5. — Radial section of a ray of Pinus edulis (pinon pine), showing the 

 smooth upper and lower walls of the ray tracheids (r. ir.), and the presence in the 

 lateral walls of the ray-parenchyma cells (r. p.) of small semi-bordered pits (s. b. p.), 

 communicating with the wood tracheids (w. tr.) adjacent; s. p., simple pit; 6. p., 

 bordered pit. Magnified about 250 diameters. 



separated by wood fibres. Such rays are termed aggregate or 

 compound rays; sometimes also false rays. Every ray, regardless 

 of its width at the middle, tapers to an edge so that the upper 

 and lower margins are a single cell wide.* 



The comparative distinctness which rays on cross section 

 present to the unaided eye is important in separating certain 

 woods which bear superficial resemblance. For instance, the 



* For this reason cross sections often do not afford a correct idea of ray 

 width. 



