Identify the tissue involved in secondary growth that is couple of layers thick and is made of narrow, thin-walled and nearly rectangular cells.

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. Phellogen, phellem, and phelloderm are collectively known as periderm.