ECONOMIC WOODS OF THE UNITED STATES 



25 



Rays consisting wholly of procumbent cells may be said to be 

 homogeneous; those which contain both upright and procumbent 

 cells, heterogeneous (Fig. 3). Heterogeneous rays are characteristic 

 of many dicotyledonous woods, and are features of importance in 

 classification. For example, Celtis has heterogeneous rays, while 

 those of Ulmus are homogeneous. The same distinction obtains |^ 

 between Salix and Populus, Sassafras and Fraxinus. The rays of 

 Sassafras are peculiar in having a few of the marginal cells abnor- 

 mally large and rounded or ovate (Fig. 3, A) ; these are oil cells. 



The rays in the wood of Gymnosperms are for the most part 

 one cell wide, i.e., uniseriate, and from 1 to 20 cells high. It is not 



r.tr. 



r.p. 



r.tr. 



Fig. 4. — Radial section of ray of Pinus strobus (white pine) ; showing the 

 smooth upper and lower walls of the ray traeheids {r. tr.), and the presence in the 

 lateral walls of the ray-parenchyma cells (r. p.) of large simple pits (s. p.), com- 

 municating with, the wood traeheids (w. tr.) adjacent; b. p., bordered pits. Magni- 

 fied about 250 diameters. 



uncommon to find biseriate rays, and those which contain resin 

 ducts {Pinus, Picea, Larix, Pseudotsuga) are multiseriate. The 

 latter, because of their shape as seen on tangential section, are 

 called fusiform rays (Fig. 9) . 



In woody Dicotyledons there is more variation in the rays. 

 In some instances {e.g., Msculus [Plate VI, Fig. 6], Salix, Populus) 

 low uniseriate rays only are present. At the other extreme is 

 Quercus (Plate III, Fig. 1), where the largest rays are from 25 to 



