STEMS 



239 



m 



w 



stems to shrubs and trees. Taking the trees as an illustra- 

 tion, the increase in diameter occurs as follows. Between 

 the xylem and the phloem of each bundle is a layer of very 

 active cells called the cambium (Fig. 206), which soon extends 

 across the intervening pith rays (Fig. 205), and so forms 

 a complete cylinder of cambium. This cambium has the 

 power of adding new xylem cells to the outer surface of the 

 xylem, and new phloem 

 cells to the inner surface of 

 the phloem, as well as of 

 adding to the pith rays 

 where it traverses them. 

 In this way a new layer of 

 wood is laid down on the 

 outside of the old wood; 

 and usually these layers, 

 added year after year, are 

 so distinct that a section 

 of wood shows a series of 

 concentric rings (Fig. 208). 

 Ordinarily one such layer 

 is added each year, and 

 hence the layers are called 

 annual rings. The age of a 

 tree is usually estimated by 



counting these rings, but occasionally more than one ring may 

 be added during a single year. The new layers added to the 

 phloem are not persistent ; but the xylem accumulates year 

 after year, until in an ordinary tree the stem is a great mass 

 of xylem covered with thin layers of phloem and cortex. 

 It is this mass of wood that supplies our lumber. 



This annual increase in diameter enables the tree to put 

 out an increased number of branches, and hence leaves, each 

 succeeding year, so that its capacity for leaf-work becomes 

 greater year after year. A reason for this is that since 



FIG. 208. Cross-section of a branch of box- 

 elder three years old, showing three 

 annual rings in the vascular cylinder ; the 

 radiating lines (m) that cross the vascular 

 cylinder (w) represent the pith rays, the 

 principal ones extending from pith to 

 cortex (c). 



