What is the commercial source of cork? How is it formed in the plant?

The commercial source of cork is cork tissue of Quercus suber.

Due to the activity of vascular cambium the stem increases in girth. So the outer cortical and epidermis layers get broken and need to be replaced by new protective cell layers. Hence another meristematic tissue called cork cambium or phellogen develops in the cortex region.

Phellogen is a couple of layers thick. It is made of narrow, thin-walled and nearly rectangular cells. Phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.

The cork is impervious to water due to suberin deposition in the cell wall and provides protection to the underlying tissues.