A protective waterproof plant tissue produced by the cork cambium. It develops in plants undergoing secondary growth and replaces the epidermis. Its cells, whose walls are impregnated with suberin, are arranged in radial rows and fit closely together except where the cork is interrupted by lenticels. Some cork cells become air-filled while others contain deposits of lignin, tannins, and fatty acids, which give the cork a particular colour. The cork oak (Quercus suber) produces cork that can be used commercially.