MEDULLARY RAYS 



97 



O 





FIG. 31,^. PINUS ALBICUALIS. Tan- 

 gential section of a fusiform ray 

 showing a typical resin canal with (a) 

 thyloses, and (f>) rather thick-walled 

 parenchyma, x 280; fi. PINUS PA- 

 LUSTRIS. Tangential section of afusi- 

 form ray in part showing thin-walled 

 parenchyma broken out. x 280 



to progressive development of the genus. Such 

 broadening arises abruptly in Pseudotsuga, 

 Larix, and Picea, so that the terminals above 

 and below consist of a single series of cells 

 with the general structure of the one-seriate ray 

 (fig. 30). In Pinus the broadening is less ab- 

 rupt, diminishing in both direc- 

 tions somewhat gradually, thus 

 giving rise to a region of lenticu- 

 lar form which occupies upwards 

 of half the height of the ray, or 

 in some cases constitutes the 

 entire structure. From this it 

 follows that in such types as 

 P. palustris (fig. 3 i, B) the termi- 

 nals, which are often prolonged 

 to great length, may be linear 

 and one-seriate, while in P. pon- 

 derosa the whole ray is lenticular 

 in outline and the terminals con- 

 sist of only one or two limiting 

 tracheids (fig. 32). Within the 

 region of the central tract the 

 cells are all thick-walled in Pseu- 

 dotsuga, Larix, and Picea, but in 

 Pinus they are generally thin- 

 walled, and in the hard pines this 

 feature is emphasized by a degen- 

 eration of the tissue to such an 

 extent that it is readily broken 

 out in making sections, whence 

 it characteristically appears 

 either much broken up or entirely 

 wanting. The principal feature 

 of such rays, and the one which 

 determines their form, is the 



