ECONOMIC WOODS OF THE UNITED STATES 23 



some cases the formation of imperfect resin ducts or resin cysts 

 (Fig. 10). (See Resin Ducts.) 



In Pinus (Fig. 8) wood parenchyma is found only in association 

 with resin ducts, isolated resin cells being absent; while in Larix 

 and Pseudotsuga resin cells are occasionally found on the extreme 

 outer face of the late wood. In Abies resin cells are remote and 

 inconspicuous; in Thuya plicata they are present, though often 

 zonate in widely separated growth rings, thus often apparently 

 absent. In Sequoia (particularly S. sempervirens) the resin cells 

 are partially filled with dark resin masses which appear on longi- 

 tudinal surface as fine dotted lines, or under lens as rows of black 

 or amber beads. 



References 



DeBary, a.: Comparative Anatomy, pp. 485-486. 

 Penhallow, D. p.: North American Gymnosperms, pp. 109-122. 

 BouLGER, G. S.: Wood, pp. 28-29. 



Saxio, Carl: Bot. Zeitung, Vol. XXI, No. 12, pp. 93-98. 

 Kny, L.: Ueber Krystallbildung beim Kalkoxolat, Berichte der deutschen 

 Bot. Gesellschaft, Vol. V, 1887, pp. 387-395. 



RAYS 



Medullary or pith raj^s, for brevity termed simply rays, 

 appear on the cross section of a stem as radial lines crossing the 

 growth rings at right angles and extending into the bark (Fig. 1). 

 A few of them originate at the pith and are commonly known as 

 primary rays. Successively, as the stem increases in size, addi- 

 tional or secondary rays arise between those already formed. 

 A ray may arise in the cambium layer at any point, and once formed 

 its growth is continuous.* 



Under the microscope the line formed by the ray becomes a 

 radial series of mostly elongated cells usually with transverse 

 end walls (Plates II-IV). Viewed radially a ray appears as a 

 muriform structure composed of from one to many tiers of brick- 

 shaped cells (Plate IV, Figs. 5, 6). In tangential section the ends 

 of the rays are visible, showing to advantage their height, shape, 



* When on cross or radial sections a ray appears to be discontinuous, it is 

 probable that it has merely been missed by the plane of section. This empha- 

 sizes the importance of making cross sections exactly at right angles to the 

 axis of growth, and radial sections as nearly as possible parallel with the rays. 



