Corkbark

Cork (bark), the periderm, secondary cortex or outer-bark of dicotyledonous perennials. It consists of layers of muriform parenchyma, or brick-shaped cells, filled with air, the walls of which have undergone the molecular change known as cuticularisation or suberisation. This consists in the conversion of the permeable cellulose into the highly elastic impermeable cutin, which turns yellow with iodine or acid, and resists the action of the latter even when concentrated. The formation of cork, which is practically confined to the axis, begins at certain points beneath the epidermis of young green shoots, known as lenticels, where it ruptures the epidermis. It then extends throughout a layer known as the phellogen or cork cambium, the cells of which on dividing give rise to an inner layer of chlorophyll-containing cells, the phelloderm, and externally to successive layers of cork cells, the periderm. The formation of this layer of air-filled cells, which are, of course, physiologically dead, cuts off all external structures from the vital fluids of the plant, and it is thus that the fall of the leaf is brought about, the layer of cork extending across its base. The periderm may be of considerable thickness, the lenticels (q.v.) growing with it so as to form perforations, or tubes filled with loose dried powdery cells, extending through it to the surface, as is familiar in thin slices of cork. With its growth, and that of the wood of the stem, the outer portion of the cork may be split by longitudinal furrows as in oak, elm, chestnut, etc., or it may flake off, as in the birch and plane, or by cell-division radial as well as periclinal it may accommodate itself to the increased girth, as in beech, sycamore, etc. The term cork is specially applied to the periderm of Quercus Suber, the Cork Oak, a native of the Mediterranean region. The first crop, taken off trees ten to fifteen years old, which is furrowed, is known as virgin cork, and is used to ornament ferneries. Subsequent crops are taken every seven years or thereabouts, care being taken not to injure the phellogen. The stripping is said to accelerate growth, though it probably renders the wood less dense. The curved sheets are flattened by weights when under water and charred to close their pores (lenticels). Great Britain imports about 22,000 tons of cork in the rough and over 2,000 tons manufactured. The best comes from Spain, France and Portugal also contributing. Besides its use for bungs and bottle-corks, cork is used for boot-soles and to line hats, and refuse cork treated with rubber is the material of the floorcloth known as kamptulicon, now generally superseded by linoleum.