20 



OF WOOD IN GENERAL 



running longitudinally. The whole mass of xylem is traversed 

 radially by pith-rays, most of which appear in the transverse sec- 



Pig. 14.— Resin-duct in coniferous wood, in transverse section, highly magnified, 

 showing the epithelial cells surrounding the duct. 



tionjof the stem as only one cell in width and made up of cells 

 elongated radially. In a longitudinal and radial section (Pig. 15) 



Pig, IS.—Badial section of Silver JFir {Jhies pectindta), showing a medullary ray, 

 with simply pitted, parenchymatous cells, crossing wid,e tracheids of spring wood, 

 and narrower ones of autumn wood, with bordered pits. Magnified 100 times. (From 

 Hartig's Timbers and how to know themy by permission of Br. Somerville and Mr. Pavid 

 Doug&s.) 



it appears that the tracheids are from ^ to ^ inch long, 50-100 

 times as long, that is, as they are wide; that they have their 



