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 ; 

 p, pith. 



are arranged in a hollow 

 cylinder, just inside of 

 the cortex, leaving what 

 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 XfeS^fSfflMlfeSO/tf 



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 FlG - 79 - %** on across a twi ^ of box elder three 



J years old, showing three annual rings, or growth 



Up the Ollter part Of rings, in the vascular cylinder. The radiating 



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



resent the pith rays, the principal ones extending 

 Called OarlC. from the pith to the cortex (c). 



US 



