CORK 279 
75. Cork is the light, waterproof, compressible yet elas- 
- tic material forming the outer bark of the cork oak (Figs. 267- 
269). Like true wood it is built up of annual layers formed 
by a cambium. It differs from wood in having the inner 
layers the younger, in being non-fibrous, and in containing 
about 70-80% of a mixture of waxy and tallow-like sub- 
stances which is known as suberin. Very many plants pro- 
duce cork in their outer parts, but only the cork oaks form 
masses sufficiently large to be of economic use. 
The imperviousness to water, the elasticity, and the firm- 
ness of cork, upon which its economic value mainly depends, 
render it in the first place useful to the tree as a protection 
for the tender inner bark where processes of vital importance 
are carried on. Since these processes cannot proceed without 
free access of air the thick cork layer is found to be pierced 
by numerous breathing channels extending radially to the 
surface. Besides these channels rifts naturally occur in the 
outer bark as it is stretched by the increasing bulk of the 
wood within, and by the new layers of bark. 
In the young tree the first few layers of cork are compara- 
tively thick while those formed later are only about 1-2 mm. 
in thickness and soon become so brittle and so badly cracked 
as to be unfit for finer uses. Such inferior cork, suitable 
only for fuel, packing, fish-net floats, rustic work in conserva- 
tories, and the like, is all the tree ever produces if left undis- 
turbed. But in cultivation when the trees are from fifteen 
to twenty years old all of this ‘virgin cork,” as it is called, is 
cut away, great care being taken not to injure the tender 
part within known as the “cork mother” because it includes 
the cambium. The effect of this operation upon the tree is 
in every way beneficial. Henceforth the cork produced is 
more abundant, softer, and more homogeneous; the breathing 
channels are farther apart; and the cracks become far less 
troublesome. For a century and a half or even longer, at 
intervals of eight to fifteen years, slabs of fine cork 5-20 cm. 
thick are peeled from the trunk in the manner illustrated 
(Fig. 269). The harvesting takes place in summer when the 
inner bark adheres most firmly to the wood. After being 
stripped from the tree the slabs of cork are scraped so as to 
