The Phellem and Phelloderm formed from the cork cambium or Phellogen are collectively called periderm. Formation of different layers of peridermAs the stem continues to increase in grith due to the activity of vascular cambium, the outer cortical and epidermal layers get broken which has to be replaced by new protective layers. The protective layers formed are merismatic tissues called cork cambium. Phellogen or cork cambium is a couple of layers thick. These layers are made of narrow, thin-walled and nearly rectangular cells. Cork cambium cuts off cells on both sides of these the outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.