peridermis

Periderm is a protective tissue produced during secondary growth in many vascular plants, forming the outer protective layer of stems and roots after the epidermis has been replaced. It is part of the bark and serves to protect underlying tissues, reduce water loss, and provide a barrier against pathogens and mechanical damage.

The periderm consists of three tissues. The cork cambium, or phellogen, is a meristem that divides periclinally to produce cork (phellem) toward the outside and phelloderm toward the inside. The cork consists of numerous dead, suberized cells that form a waterproof barrier. The phelloderm is made of living parenchyma and can contribute to storage and protection. Together these layers replace the epidermis and may be renewed by successive cork cambia during continued secondary growth.

Lenticels are specialized structures within the periderm that allow gas exchange between the internal tissues and the atmosphere, facilitating respiration and diffusion of gases in woody organs where the outer surface is impervious.

In woody dicots and many gymnosperms, the periderm develops as part of bark, replacing the epidermis in regions that undergo thickening. The persistence and renewal of the periderm enable plants to expand in girth while maintaining protective and water-regulating functions.