200 STEMS 



by the cork formed beneath and gradually thickened as the stem 

 grows older. In some cases the cork cambium produces cortex 

 cells on its inner side as well as cork on its outer side, in which 

 case the cortex is increased in thickness. Since cork is imper- 

 vious to water, the tissues on its outside, having their water 

 supply cut off, soon die and with the epidermis and cork form 

 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- 



FiG. 179. — Cross section through the stem of a Red Oak, showing 

 heartwood and sapwood. 



panion cells, parenchyma cells, and bast fibres. 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 

 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 fiurows, 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 



