Simple permanent tissues are made of one type of cells that look similar to each other, while complex tissues are made of more than one type of cells that coordinate to perform a common function.

The primary function of cork cells is to provide a protective barrier for the underlying tissues, as they are dead and compactly arranged, impervious to gases and water due to the presence of suberin in their walls.

The outer layer of a branch, formed by secondary meristem in the cortex, undergoes changes to produce cork, while the outer layer of a young stem does not yet have this specialized tissue.