PLANT STUDIES 
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 
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 - f* ci acro * 8 a twig \ box elder thre h e 
* years old, showing three annual rings, or growth 
Up the Outer part of rings, in the vascular cylinder. The radiating 
What is Commonly lines (m) which cross the vascular region (w)rep- 
resent the pith rays, the principal ones extending 
Called bark. from the pith to the cortex (c). 
