ECONOMIC WOODS OF THE UNITED STATES 



27 



rays in Sassafras are much more distinct than in Fraxinus; like- 

 wise in Celtis and Ulmus, Tilia and dZsculus, Acer and Betula. 

 In white oaks the height of the large rays averages considerably 

 greater than in the red or live oaks. 



In dicotyledonous species the rays are composed wholly of 

 parenchyma. In certain Gymnosperms (Pinus, Larix, Picea, 

 Pseudotsuga, Tsuga, and occasionally in others) ray tracheids are 

 present (Figs. 47). They are usually marginal, but often inter- 

 spersed and sometimes they compose entire rays, particularly 



r.tr. 



r.tr. 



\ 



110 



FIG. 6. Radial section of a ray of Pinus resinosa (red or Norway pine), 

 showing the dentations (d) or reticulations on the upper and lower walls of the ray 

 tracheids (r. tr.), and the presence in the lateral walls of the ray-parenchyma cells 

 (r. p.), of large simple pits (s. p.) communicating with the wood tracheids (w. tr.) 

 adjacent; 6. p., bordered pit. Magnified about 250 diameters. 



low ones. They can be distinguished from the ray-parenchyma 

 cells by the presence of bordered pits in the lateral and especially 

 the end walls. They are often irregular in outline and are devoid 

 of visible contents. They have their counterparts in the paren- 

 chymatous tracheids surrounding the epithelial cells of resin cysts 

 and ducts. In the young root, and sometimes in the young stem 

 as well, special upright or oblique forms occur which may be 

 considered as transitional from wood tracheids to ray tracheids. 



The character of the upper and lower walls of the ray tracheids, 

 whether smooth, as in soft pines, or dentate or reticulate, as in 



