The cross section below presents cork cells from a tree. Cork (phellem) is a non-living, water-resistant, protective tissue that displaces the stem epidermis as the plant undergoes an increase in diameter during the secondary growth phase.

During growth cycles, within the (phloem) layer of tissue there develops a meristem tissue comprised of cells in a state of undifferentiation until, at an appropriate time, they demonstrate their embryonic-like capability to actively divide and develop into cells of specialized types. The lateral meristem tissue responsible for cork cell development is the cork cambium, also known as phellogen. The increasing number of cork cells produced by the meristematic tissue of the cork cambium push the older, non-dividing phloem cells toward the outer margins of the stem where they are crushed causing them to eventually die and slough off. This process can repeat itself many times over giving rise to a new cork cambium, which will displace its predecessor as the tree grows. Generally, the phloem together with the cork cells is considered to comprise the outer bark.

A mature cork cell is non-living and has cell walls that are composed of a waxy substance that is highly impermeable to gases and water called suberin. 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 and may vary in thickness from one to the next. Packed closely together, the cells are generally arranged in radial rows. Separation among the cells is achieved by structures arising from the cork cambium 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 the internal cells of the plants with extra insulation and protection.

Trees such as the Quercus suber, known as the cork oak, have been cultivated for cork that exhibits the commercially valuable properties of shock absorbency, gas and water impermeability, and insulation. Probably the most commonly known use for cork is as a bottle stopper used for wine and champagne. Cork from this oak, which is native to Southern Europe and Northern Africa, is generally harvested every several years after the thick layers have accumulated.