198 



STEMS 



the lenticels already mentioned, the features most peculiar to 

 woody stems are the annual rings of the woody cyUnder and the 

 corky hark 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 



Fig. 177. — Cross section of an Oak branch from a region nine years old. 

 o, outer corky bark; i, 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- 



