The cork cambium (Phellogen) is the lateral meristem that produces cork or Phellem towards the outside and secondary cortex or Phelloderm towards the inner side. The cork cells or Phellem are waxy, impregnated with suberin, a lipid material that makes them waterproof but also causes them to die. Areas of loosely packed cells penetrate cork layers and enable gas exchange to occur. Phelloderm consists of living parenchyma cells, which may be photosynthetic and store nutrients. Phellogen produces much more Phellem on the outerside than Phelloderm on the inner. The cells constituting Phellem or called cork cells.
Phellogen, Phelloderm and Phellem together constitute the periderm. Due to internal increase in thickness, periderm replaces the epidermis, becomes protective in function. All the dead cells lying outside the active Phellogen constitute the bark.
In Betula bhojpatra bark peels off like sheets of paper. Ancient manuscripts are still preserved on them. Cork tissue becomes very thick in Cork tree (Quercus suber) and is used commercially as, bottle-stoppers, insulators, shoe soles etc.