24 



OF WOOD IN GENERAL. 



carpenter's chisel. The pith -rays in longitudinal sections are 

 seen to extend only a short way longitudinally, each appearing 

 on radial sections as a band of 8 to 10 rows of cells elongated at 

 right angles to the elongation of the tracheids like bricks in a 

 wall 8-10 bricks high, with bordered pits on the cells of the 



FIG. 15. Radial section of Silver Fir (Abies pectindta), showing a medullary 

 ray, with simply pitted, parenchymatous cells, crossing wide tracheids of 

 spring wood, and narrower ones of autumn wood, with bordered pits. Mag- 

 nified 100 times. From Hartig's Timbers and how to know them, by permission 

 of Dr. Somerville and Mr. David Douglas. 



upper and lower rows, in Pines and Spruces, and simple pits on 

 the others. On tangential sections the rays appear as vertical 

 series of 8-10 pores tapering above and below. In Pines there 

 are some larger pith-rays containing horizontal resin-passages. 



The development of this comparatively simple type of wood 

 from the cambium can be readily traced. The cambium is a 

 cylindrical sheet of very thin-walled cells, each of which is 

 rectangularly prismatic, broader in a tangential direction and 

 tapering above and below to a radially-directed chisel-edge. 



