A TEXT-BOOK OP BOTANY 



m 



of concentric rings (Fig. 54). 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 occasion- 

 ally more than one ring 

 may be added during a 

 single year. The new 

 layers added to the bast 

 are not persistent; but 

 the wood accumulates 

 year after year, until in 

 an ordinary tree the 

 stem is a great mass of 

 wood covered with thin 

 layers of bast and cor- 

 tex. It is this mass of 

 wood that supplies our 

 lumber. 



This annual increase 

 in diameter enables the 

 tree to put out an in- 

 creased number of branches, and hence leaves, each suc- 

 ceeding year, so that its capacity for leaf work becomes 

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

 wood is conducting water to the leaves, for food manufac- 

 ture, the new layers enable it to conduct more water, and 

 more leaves can be supplied. 



When a stem increases in diameter it is very seldom 

 that the epidermis grows in proportion. Hence it is usu- 

 ally sloughed off and a new protective covering is de- 

 veloped by the cortex. Either the outermost layer of the 

 cortex or some deeper one becomes a cambium, which 

 means that 'it is able to form new cells. This cambium is 

 called the cork cambium, since it forms at its outer surface 

 layer after layer of cork cells, which are peculiarly resistant 



FIG. 54. Cross-section of a branch of box 

 elder three years old, showing three an- 

 nual rings in the vascular cylinder; the 

 radiating lines (m) which cross the vascu- 

 lar ring (w) represent the pith rays, the 

 principal ones extending from pith to cor- 

 tex (c). 



