Periderm is a protective tissue found in the stems and roots of woody plants. It replaces the epidermis as the plant grows and develops. The periderm consists of three main layers: the outermost layer called the phellem or cork, the middle layer known as the phellogen or cork cambium, and the innermost layer called the phelloderm. The primary function of periderm is to protect the plant from environmental damage, pathogens, and water loss. It also helps in gas exchange through small openings called lenticels, which allow for the exchange of gases between the plant and its surroundings.