What kind of adaptive feature develops in plants to avoid breaking of external tissue due to increase in thickness of stem and root as a result of secondary growth ? Give its salient features

Explanation

To avoid the breaking of external tissues due to secondary growth in thickness, the plants develop a new cambium ring, called cork cambium (phellogen) in the outer region. It develops from permanent tissue in the region of hypodermis, cortex or pericycle. It cuts dead cells of cork (phellem) towards outerside and living cells of secondary cortex (phelloderm) towards inner side. These layers of cork, cork cambium and secondary cortex constitute the protective covering, called periderm.