THE ANNUAL RINGS 291 



494. Secondary Thickening of Stems. — Dicotyledonous (or 

 exogenous) stems with open collateral bundles may increase 

 in diameter each year. If they are perennial, they may add 

 a ring of growth each spring. (Fig. 461.) These rings may be 

 counted on the smooth cross-cut surface of a tree, and the 

 age of the tree usually can be very closely determined. All 



461. White pine stem five years old. The outermost layer is hark. 



growth in thickness due to the formation of new cells out- 

 side of the primary wood is called secondary thickening. 



495. As we have seen (490), there is a cambium or grow- 

 ing layer in every open collateral bundle just between the 

 xylem and phloem. Each spring the cells of this layer divide 

 many times and form new cells both inside and outside the 

 cambium ring. (Figs. 462, 463.) Those formed inside 

 become thick-walled and are xylem. Those formed to the 

 outside of the ring are gradually changed into phloem. The 

 crowding of the cells within the cambium ring causes the 



