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 cambiuin 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 imade up of sieve tubes, com- 
panion cells, parenchyma cells, and bast fibers. After years of 
growth the outer layers of phlocm die and thus on trunks of trees 
of much age, the inner living bark contains only the inner layers 
Fie. 179. — Cross section through the stem of a Black Oak, showing 
heartwood and sapwood. From Pinchot, U. 8. Dept. of Agr. 
of phloem, the older layers of phloem having become a part of the 
outer bark. Duce 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 cylinder, consisting of the xylems of numerous vas- 
cular bundles closely joined, functions chiefly in the conduction 
