What is a periderm? How does periderm formation take place in dicots?

A periderm is a protective tissue that replaces the epidermis in woody plants during secondary growth. It consists of three main types of cells: cork cells, phelloderm cells, and cork cambium (also known as phellogen).

In dicots, periderm formation occurs as follows:

Initial Formation: Periderm formation begins when the cork cambium, a lateral meristem, becomes active in the deeper layers of the cortex or in the secondary phloem. This typically happens as a response to the initiation of secondary growth, which occurs due to the activity of the vascular cambium.

Production of Cork Cells: The cork cambium divides periclinally, meaning that the new cells are produced perpendicular to the surface of the plant. The cells produced on the outer side of the cork cambium become cork cells, which are dead at maturity. These cork cells are compactly arranged and have suberin in their walls, making them impervious to water and gases, thus providing protection to the plant.

Production of Phelloderm: The cells produced on the inner side of the cork cambium become phelloderm cells. These cells are living and may function in storage or in other supportive roles. They are typically parenchymatous in nature.

Sloughing off of Epidermis: As the periderm develops, the older layers of the epidermis and cortex are gradually sloughed off or otherwise replaced. This is essential for accommodating the increasing girth of the stem or root due to secondary growth.

Continued Growth and Renewal: The cork cambium continues to produce cork cells outward and phelloderm cells inward, contributing to the growth of the periderm. This process goes on throughout the life of the plant, ensuring ongoing protection and support.

Overall, periderm formation in dicots is a vital process that allows woody plants to protect themselves from environmental stresses, pathogens, and mechanical damage as they undergo secondary growth.