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, ete. 
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. Ifa 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 in the wood by removing the 
5 
