What is periderm? How does periderm formation takes place?

Activity of vascular cambium increases the girth of dicot stems. In this activity the outer cortical and epidermal layers get broken.

The need arises to replace these broken layers by forming new protective cell layers. For this a new meristem arises in the cortical region called cork cambium or phellogen.

Phellogen is few layers thick made up of narrow, thin walled rectangular cells. It cuts of cells on both sides. The outer one is called cork or phellem while the inner one is called phelloderm (secondary cortex). Cork is impervious to water due to suberin deposition in its cell walls.

The cells of phelloderm are parenchymatous. Due to the activity of phellogen pressure builds upon the cells outside the phellogen. These cells die and slough off.

Phellogen,Phellem and Phlloderm are collectively known as Periderm.Bark includes periderm also.