Mature cork cells are plant cells that form the protective water-resistant tissue in the outer covering of stems or trunks. Cork cells are genetically programmed not to divide, but instead to remain as they are, and are considered dead cells. Each cell wall is comprised of a waxy substance known as suberin, which is highly impermeable to gases and water. Depending upon the species of woody plant, the cork cell may be filled with air or may contain traces of lignin, tannins, or fatty acids. Thickness of cork tissue varies from one plant to the next. Packed closely together, the cells are generally arranged in radial rows and separation is achieved by structures called lenticels. These pore-like structures allow gases to be exchanged between the plant stem and the outside environment. The layer of dead cells formed by the cork cambium provides internal plant tissue, including the vascular system, with extra insulation and protection.