periderm

The periderm is a protective outer layer of tissue found in many plant roots and stems. It is formed during secondary growth, which increases the girth of woody plants.12

The periderm consists of three parts:
* Phellem (cork): The outermost layer, made of dead cells whose inner walls are lined with suberin, a substance impermeable to gases and water.2
* Phellogen (cork cambium): A lateral meristem that produces cork cells to the outside and parenchyma cells to the inside.2
* Phelloderm: Parenchyma cells produced on the inner side of the cork cambium.2

As a plant grows wider, the periderm replaces the epidermis, which is eventually sloughed off. The periderm's primary function is to protect the plant from water loss and environmental damage.2