In woody plant stems, secondary growth occurs through the activity of the vascular cambium. This cambium produces new phloem cells toward the outside and new xylem cells toward the inside. As new phloem is continuously added, the stem diameter increases, putting pressure on the outer tissues including the epidermis.
As the vascular cambium continues to produce new phloem, the stem diameter increases significantly. The original epidermis, being a thin protective layer, cannot accommodate this expansion and begins to crack and peel off. To maintain protection, cells in the outer cortex dedifferentiate and form a new meristematic tissue called cork cambium or phellogen.
The cork cambium is a lateral meristem that divides in two directions. Toward the outside, it produces cork cells, also called phellem. Toward the inside, it produces phelloderm cells. The cork cells undergo suberization, a process where suberin is deposited in their cell walls, making them waterproof and impermeable. These suberized cork cells die and form the protective outer layer called periderm.
The complete protective system that replaces the epidermis is called the periderm. The periderm consists of three distinct layers: the outermost cork or phellem layer made of dead suberized cells, the cork cambium or phellogen in the middle that produces new cells, and the innermost phelloderm layer of living parenchyma cells. Together, these three layers form an effective barrier that protects the inner tissues of the woody stem.
To summarize the process: First, the vascular cambium produces new phloem, causing the stem diameter to increase. Second, the original epidermis cannot accommodate this expansion and cracks and peels off. Third, cells in the outer cortex dedifferentiate to form cork cambium. Fourth, the cork cambium divides to produce cork cells outward and phelloderm inward. Finally, the cork cells undergo suberization and die to form the protective periderm. Therefore, the new protective cells outside the phloem are not epidermis cells, but cork cells produced by the cork cambium through secondary growth.