240 ELEMENTARY STUDIES IN BOTANY 



xylem is conducting water to the leaves, the new layers 

 enable it to conduct more water, and more leaves can be 

 ^ supplied. 



When a stem increases in diameter, it is very seldom that 

 the epidermis grows in proportion. Hence it is usually 

 sloughed off and a new protective covering is developed by 

 the cortex. Either the outermost layer of the cortex or 

 some deeper one becomes a cambium, which means that it is 

 able to form new cells. This cambium is called the cork 

 cambium, since it forms at its outer surface layer after layer 

 of cork cells, which are peculiarly resistant to water. If the 

 cork cambium is formed deep in the cortex, all the cells 

 outside of it die, since they are cut off from the water supply 

 in the plant. The cork cambium is often renewed year 

 after year, and two prominent kinds of bark are formed. 

 In some cases the successive cork cambiums form zones 

 completely about the stem, and the cork is then deposited 

 in concentric layers, forming the ringed bark. Such bark 

 often becomes very thick, and the surface becomes seamed 

 or furrowed. In the cork oak, for example, there is a very 

 great accumulation of cork, which is stripped off in sheets, 

 from which corks of commerce are made. In other cases 

 the successive cork cambiums, instead of passing completely 

 around the stem, run into the next outer one, thus cutting 

 out segments which presently loosen and flake off, forming 

 scaly bark, as in hickory, apple, etc. 



The layers of cork and other cells that may lie outside 

 of the cork cambium form the outer bark, which is dead and 

 dry. The tissues between the cork cambium and the cam- 

 bium of the vascular cylinder, that is, more or less of the 

 cortex and all of the phloem, form the inner bark, which 

 contains some living cells. To remove the outer bark does 

 not injure a tree; but removing the inner bark kills it, 

 because it interrupts the work of conduction carried on by 

 the sieve vessels. In the process known as girdling, not only 



