CJORK 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 laj'ers formed 

 by a cambium. It differs from wood in having the inner 

 layers the j^ounger, in being non-fibrous, and in containing 

 about 70-80% of a mixture of waxy and tallow-like .sub- 

 stances which is known as subcrin. Very many plants pro- 

 duce cork in their outer parts, but only the cork oaks form 

 ma.sses .sufficiently large to be of economic use. 



The imperviousn('s.s to water, the elasticity, and the firm- 

 ness of cork, upon which its economic value mainly depends, 

 render it in the fir.st 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 bj' the increasing bulk of the 

 \\'ood within, antl by the new laj^ers 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 u.ses. 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 unchs- 

 turbed. But in cultivation when the trees are from fifteen 

 to twenty j^ears 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 j^ears, slabs of fine cork 5-20 cm. 

 thick are ijeeled 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 



