200 STEMS 



the dead outer bark. In a few trees like the Beech and Fir the 

 original cork cambium may renew its activity year after year, but 

 usually the cork cambium is replaced each year by a new one 

 formed just beneath. The inner bark consists of the inner cortex 

 and the elements of the phloem made up of sieve tubes, com- 

 panion cells, parenchyma cells, and bast fibers. After years of 

 growth the outer layers of phloem die and thus on trunks of trees 

 of much age, the inner living bark contains only the inner layers 



Fig. 179.— Cross section throug;h the stem of a Black Oak, showing 

 heartwood and sapwood. From Pinchot, U. S. Dept. of Agr. 



of phloem, the older layers of phloem having become a part of the 

 outer bark. Due to the addition of cork and the increase of the 

 phloem and woody cylinder in thickness, the bark, which is un- 

 able to increase in circumference except in a few cases, as in 

 Beeches, is usually broken and slowly exfoliated. It is usually 

 broken into furrows, which are thought to serve the same purpose 

 as lenticels in letting air into the stem tissues beneath. 



The woody cylinrler, consisting of the xylems of numerous vas- 

 cular bundles closely joined, functions chiefly in the conduction 



