The periderm is a protective tissue that replaces the epidermis in the stems and roots of woody plants. It consists of three layers: the outermost layer called the cork, the phellogen (cork cambium), and the phelloderm. This tissue helps prevent water loss and protects the plant from pests and diseases. In many trees, the periderm becomes thicker as the plant ages, contributing to the formation of bark. The cork cells in the periderm are dead at maturity and are filled with suberin, a waxy substance that enhances its protective qualities. This adaptation is crucial for the survival of woody plants.