8± 



PLANT RELATIONS. 



Fig. 78. Section across a young twig of box 

 elder, showing the four stem regions : e, 

 epidermis, represented by the heavy bound- 

 ing line ; c, cortex ; w, vascular cylinder ; 

 j), pith. 



are arranged in a hollow 

 cylinder, just inside of 

 the cortex, leaving Avhat 

 is called pith in the 

 center (see Fig. 78). 

 Sometimes the pith dis- 

 appears in older stems or 

 parts of stems and leaves 

 the stem hollow. When 

 the vessels are arranged 

 in this way and the stem 

 lives more than a year, it 

 can increase in diameter 

 by adding new vessels 

 outside of the old. In 



the case of trees these additions appear in cross-section like 



a series of concentric rings, and as there is usually but one 



growth period during the year, they are often called annual 



rings (see Fig. 79), and the age of a tree is often estimated 



by counting them. 



This method of ascer- 

 taining the age of a 



tree is not absolutely 



certain, as there may 



be more than one 



growth period in some 



years. In the case of 



trees and shrubs the 



epidermis is replaced 



on the older parts by 



layers of cork, which 



sometimes becomes 



very thick and makes 



up the outer part of 



what is commonly 



called bark. 



Fig. 79. Section across a twig of box elder three 

 years old, showing three annual rings, or growth 

 rings, in the vascular cylinder. The radiating 

 lines (m) which cross the vascular region (w) rep- 

 resent the pith rays, the principal ones extending 

 from the pith to the cortex (c). 



