13. What is periderm? How does periderm formation take place in the dicot stems?
The periderm refers to the collective term used to signify phellogen, phellem, and phelloderm in a plant stem. Periderm formation in plants, mainly occur to replace the existing epidermis.
Formation of periderm
The formation of periderm occurs during secondary growth. During this process, to replace the broken outer epidermal layer and the cortical layer, the cells of the cortex turn meristematic. As a result of this, cork cambium or phellogen is formed. The phellogen is composed of thin-walled, narrow and rectangular cells. Later on, the phellogen cuts off cells on its either side. The cells of the outer side give rise to the phellem or cork which due to the deposition of suberin in its cell wall is impervious to water. Similarly, the inner side forms secondary cortex or phelloderm which is mainly parenchymatous in nature.