Part 75. Cork is the light, waterproof, compressible yet elastic 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 substances which* is known as suberin. Very many plants produce cork in their outer parts, but only the cork oaks form masses sufficiently large to be of economic use.

Fig. 267. Cork Oak (Quercus Suber, Beech Family, Fagaceoe). A, fruiting branch. B, twig with staminate flower clusters. C, staminate flower. D, pistillate flower. (Redrawn after Schneider.) Tree growing 15 m. tall; bark thick and spongy; leaves whitish, hairy beneath; flowers yellowish; fruit brownish. Native home, Southern Europe, and Northern Africa.

Fig. 267.-Cork Oak (Quercus Suber, Beech Family, Fagaceoe). A, fruiting branch. B, twig with staminate flower-clusters. C, staminate flower. D, pistillate flower. (Redrawn after Schneider.)-Tree growing 15 m. tall; bark thick and spongy; leaves whitish, hairy beneath; flowers yellowish; fruit brownish. Native home, Southern Europe, and Northern Africa.

Fig. 268. Cork Oak. Wedge of trunk cut across to show wood, with strong pith rays and annual rings, and the thick bark consisting of the outer virgin cork (light colored) and the inner cork mother (dark colored).

Fig. 268.-Cork Oak. Wedge of trunk cut across to show wood, with strong pith-rays and annual rings, and the thick bark consisting of the outer "virgin cork" (light colored) and the inner "cork mother" (dark colored). (Figuier.)

Fig. 269. Harvesting Cork.

Fig. 269.-Harvesting Cork. (Figuier.)

The imperviousness to water, the elasticity, and the firmness 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 comparatively 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 conservatories, and the like, is all the tree ever produces if left undisturbed. 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 clean the outer surface, are then flattened under pressure with the aid of heat, and finally tied in bundles for shipment.

By far the most important use of cork is for stoppers. It is estimated that the daily consumption amounts to twenty million. Cork stoppers are cut either by hand or by machinery. Large flat corks have to be cut so that the channels pass from top to bottom. Such corks require, therefore, the use of some sealing material such as wax, to make them impervious. Smaller corks are cut so that the channels go from side to side and hence are air-tight without sealing. In the cutting, about half the material, or more, becomes waste chips. So valuable are the properties of cork, however, that even in this form it may be utilized in important ways. Thus, pulverized and mixed with rubber or with boiled linseed-oil it forms when spread on canvas a floor covering at once durable and sound-deadening. Coarsely ground cork serves well on account of its softness and elasticity as packing for fruit, especially grapes; and, when glued to paper forms a safe wrapping for bottles in transportation. The same remarkable properties make masses of cork most effective buffers for vessels. In the form of thin sheets it has long been used as a material for insoles and hat linings. The lightness of cork has especially recommended it for artificial limbs, handles, net floats, and life-preservers; while the uniform texture and the ease with which it may be shaped have made it valuable to model makers and even to turners and carvers.

Although cork was known to the ancient Greeks and Romans, and there is record of its use by them for the soles of shoes and as stoppers for wine vessels, it has been generally used only within the last few hundred years.