As the plant grows older, the outer protective tissue, epidermis undergoes certain changes. A strip of secondary meristem, called phellogen or cork cambium replaces epidermis of stem. Cells on the outside are cut off from this layer. This forms the several-layer thick cork or the bark of the tree. Cells of cork are dead and compactly arranged without intercellular spaces. They also have a chemical called suberin in their walls that makes them impervious to gases and water.