24 



ECONOMIC WOODS OF THE UNITED STATES 



width, and distribution, and also the outline in cross section of 

 the component cells (Plate III, Fig. 1; Plate IV, Figs, 3, 4; Plate 

 VI, Figs. 3, 4, 6). 



Ray cells are usually elongated in the radial direction. This is 

 normally the case in Gymnosperms and usually so in the woody 

 Dicotyledons, Not infrequently in the latter, however, part or 

 all of the cells are upright, i.e., with their greatest diameter vertical, 

 or are square. The marginal cells are sometimes upright and the 

 interior cells radially elongated or procumbent (Fig.3). The upright 

 cells are often very irregular, especially the outermost marginal 

 rows; sometimes they are nearly square; again they are in pali- 

 sade arrangement. When these upright or square cells are in 



y.c. 



pr.c. 



%' J>*-P- 



Fig. 3. — Radial sections of heterogeneous rays. A, Sassafras sassafras (sassa- 

 fras) ; B, Nyssa sylvatica (black gum) ; C, Msculus octandra (buckeye) , showing 

 large pits (I. p.) in upright cells (up. c), where they adjoin vessels; and small pits 

 (s. p.) , in procumbent cells (pr. c.) . No pits are shown in A and B. Magnified 

 about 150 diameters. 



contact with large vessels the separating walls are characteristically 

 marked with very large pits whose polygonal or oval outlines 

 present the appearance of lattice work (Fig. 3, C). The lateral 

 walls of similarly located procumbent cells usually contain few 

 small pits. Moreover, in proximity to large vessels the walls 

 between all ray cells are usually thicker and much more abun- 

 dantly pitted than elsewhere. Upright cells are not always easy 

 to distinguish from the cells of wood-parenchyma fibres, especially 

 where they cross the latter, on account of the similar direction of 

 their longitudinal diameters. 



