198 STEMS 
the lenticels already mentioned, the features most peculiar to 
woody stems are the annual rings of the woody cylinder and the 
corky bark which replaces the epidermis and some or all of the 
cortex. Also the medullary rays are commonly better developed 
in woody stems than in herbaceous stems. These features are 
directly associated with the perennial habit and the capacity to 
add new layers of xylem and phloem each year and thus increase 
m 
{\ 
Fic. 177. — Cross section of an Oak branch from a region nine years old. 
o, outer corky bark; 7, inner bark; c, cambium; a, annual rings; m, medul- 
lary rays; p, pith. 
in diameter. In well developed woody stems, as shown in Figure 
177, there are three regions, bark, woody cylinder, and pith, al- 
though the latter is often so small in amount as to appear absent. 
The bark, consisting of outer and inner bark, the latter of which 
contains the active phloem, extends centerward to the cambium, 
which, although distinctly separating the bark and wood, is so 
inconspicuous, except under the microscope, that bark and wood 
appear directly joined. The annual rings are the circles in the 
wood, and the medullary rays show as radiating lines travers- 
