INCREASE IN THE CORTEX 55 



rupts the cork layer and allows the air to enter through its inter- 

 cellular spaces (Fig. 22). This aerating tissue is known as a 

 lenticel. It commonly arises beneath a stoma by the division 

 of phellogen cells lying at the same depth as those that form the 

 cork, and as it increases in size the stoma above it is crowded out- 

 ward and a rent is made in the epidermis. 



In woody plants, as a rule, the cork soon comes to take the 

 place of the epidermis, as can be seen by the roughening of the 

 surface where the epidermis has disappeared. The phellogen 

 first formed near the surface does not remain active indefinitely 

 and a new one is formed deeper in, which, after a time of cork 

 building, is replaced by a still deeper one, and so on. Sometimes 

 phellogen layers are formed as deep in as the tissues of the 

 pericycle, and even within the secondary phloem, and large 

 masses of tissues called borke, thus cut off from the water supply 

 by cork which the phellogen builds, die, dry up, and fall off. 

 This is well seen in the shell-bark hickory, sycamore, grape 

 vine, and birch. Frequently the borke clings with great tenacity 

 and is furrowed by many clefts as it is stretched beyond its 

 strength by the increase in diameter of the stem, as seen in the 

 oak, hackberry, and elm. After a time it may come about, as 

 the borke falls away, that all of the original primary cortex is 

 gone and its place is taken by cork, phellogen, and phelloderm; 

 or, to use the collective term, by the periderm; and, as has been 

 said, even the pericycle and old phloem tissues may be thus 

 replaced. 



The periderm and additions to the phloem by the cambium 

 are collectively called the secondary cortex. It will be seen from 

 the foregoing that the word cortex has three different applica- 

 tions: First there is the primary cortex, extending from the epi- 

 dermis to the pericycle; then there is the secondary cortex as 

 just defined; and finally cortex without qualifying adjective 

 is applied to all of the tissues outside the cambium ring and is 

 synonymous with bark. Borke is a Germna word meaning 

 bark or rind. As frequently used by the German botanists, it 

 has not the same application as our bark, which includes every- 



