Commercial source of cork is cork oak or Quercus suber.
Cork is formed on the outerside by phellogen or cork cambium. Phellogen is purely a secondary meristem which generally develops from an outer layer of cortex by dedifferentiation. The intercellular spaces disappear. Phellogen cells start dividing on their outer and inner sides. Cells formed on the outerside remain compartly arranged.
They undergo suberisation, get filled with tannins and die. The tissue of dead, tannin and air-filled suberised cells formed on outer side of phellogen is called phellen or cork. At some places, aerating structures called lenticels are also formed.