Periderm is a secondary tissue produced by the Cork Cambium or Phellogen. Periderm is a protective tissue. Its cells are called Cork or Phellem. They are dead at maturity but their walls are impregnated with Suberin. Suberin is a waxy material like Cutin & it stains positively with Phloroglucinol & Sudan. Suberin is waterproof and resists microbial degradation. Consequently, cork cells protect the stem or root from excess water loss and the entrance of pathogens. It also acts as an insulator against extreme temperatures. Periderm is commonly called Bark. We will say more about this term later.

Eucalyptus Bark 
Note the colors of the shedding bark and its underlying cells. The green color indicates the presence of living chlorenchyma. The red/brown cells of the bark are dead.


Periderm development in the stem often starts in subepidermal Parenchyma cells.
The image above is what the Cortex of a typical dicot stem would look like prior to the initiation of Periderm.