STEMS 55 



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 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 

 cambium of the vascular cylinder, that is more or less 

 cortex and the bast, 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 is the 

 bark cut through, but the young wood is cut into. This 

 interferes with the movement of water up the stem as well 

 as with conduction by the sieve vessels. If a small portion 

 of the bark is removed, the incision extending only to the 

 wood, as in the making of inscriptions on trees, the wound 

 is healed, unless too large, by the growth of tissue from all 

 sides until it is closed over. In this new tissue a cork 

 cambium is developed, and presently there may be no 

 surface indication of the wound. But if the wound 

 has gone deeper and entered the wood, the record of 

 it may always be found m the wood by removing the 



