[Chap. XXVIII TISSUES AND PROCESSES OF STEMS 



285 



Fig. 105. Radial longitudinal sections of wood of (A) white oak and (B) long 

 leaf pine. In the former wood fibers, xylem rays, and vessels filled with tyloses 

 may be seen. In the latter section tracheids with bordered pits, xylem rays, and a 

 resin duct are visible. Photos from Forest Products Lab. 



these stems there is a region composed of several layers of thin-walled 

 cells. The cells near the middle of this region are very small and thin- 

 walled. These are the cambial cells from which additional xylem and 

 phloem develop by cell division, enlargement, and differentiation and 

 bring about an increase in the diameter of stems. Adjacent to and on 

 both the phloem and the xylem sides of this cambium there are a few 

 layers of cells which are thin-walled but inceasingly larger at greater 

 distances from the cambium. One may therefore see from one to several 

 layers of cells in the processes of enlargement and differentiation be- 

 tween the cambium and the mature phloem on the one side and the 

 cambium and the mature xylem on the other ( Fig. 106 ) . 



In the larger stems the cambium is a closed c)'lindrical sheath between 

 the xylem and the phloem, that is, between the xylem and the bark; but 

 in the younger part of the stem it is present only in the individual vas- 

 cular bundles, which are separated from one another by broad rays of 



