Formation of Periderm in Plants - QS Study
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Formation of periderm:

Periderm is composed of the phellogen, phellem, and phelloderm. In woody plants, with the development of the key stem in thickness, new layers of cork cambium, and therefore sequential periderms, are formed in the secondary phloem, cutting off old non-functional phloem tissues.

Formation

(i) At the advent of extra-stelar secondary growth, the cells of the outer most layer of the hypodermis turn into phelogen or cork cambium after having incapable of cell division.

(ii) The cells of the phelogen or cork cambium undergo division and form cork cells or phellem towards the peripheral region and pleloderm or secondary cortex towards the inner side. When one cork cambium causes its function, another new one appears in the inner tissues.

The meristematic tissue which grows to reinstate the worn out epidermis of dicot stem is called cork cambium or phellogen. The phellogen cuts off cells on both sides. The outer cells distinguish into cork or phellem. The inner cells distinguish into secondary cortex or phelloderm. Cork is impervious to water due to suberin deposition in the cell wall. The cells of secondary cortex are parenchymatous. Phellogen, phellem and phelloderm are collectively called periderm.