22 



OF WOOD IN GENERAL. 



relatively large spaces, the resin-passages, each surrounded by a 

 layer of thin-walled cells, the resin-epithelium (Fig. 13). These 



FIG. 12. 1. Piece of wood of Spruce (Picea excttsa) with the bark removed, 

 natural size. 2. A portion from the nearest upper outside angle of 1, 

 showing wood near the outside of an annual ring, magnified 100 times. 

 From Hartig's Timbers and how to know them, by permission of Dr. Somer- 

 ville and Mr. David Douglas. 



resin-passages are not cells or vessels, but intercellular spaces, into 

 which the resin oozes from the surrounding epithelium (Fig. 14). 

 They generally occur singly, though sometimes in groups, and 

 are most readily detected on a very smooth surface, or are often 

 more easily seen on radial or tangential sections. On these they 

 appear as fine lines or scratches running longitudinally. The 

 whole mass of xylem is traversed radially by pith-rays, most of 

 which appear in the transverse section of the stem as only one 

 cell in width and made up of cells elongated radially. In a 



