The outer layer of a young plant changes as it matures. Tissues called secondary meristems replace the epidermis, forming several layers of thick cork called BARK. this is how the bark of a tree is formed.
1 5 1
As plants grow older , a secondary meristem  replaces the epidermis of the plant. the cells of this layer are cut off from the inside. this layer forms a thick layer of bark or is found in old woody plants. it is divided into inner bark and outer bark.inner bark consists of living tissues and outer bark consists of dead tissues. they contain a chemical called suberin in their walls that make them impervios to gases and water.
1 5 1