What is periderm? How does periderm formation take place in the dicot stems?

Explanation

Periderm : Cork cambium (or phellogen) develops in the region outside the vascular tissue. This gives rise to cork (or phellem) and secondary cortex (or phelloderm). All the three layers, i.e., cork, cork cambium and secondary cortex together constitute periderm.
Formation : Cork cambium produces dead cells or cork (phellem) towards outer side and living cells of secondary cortex or phelloderm towards inside. These layers of cork, cork cambium and secondary cortex constitute the protective covering called periderm. The cork cells are highly lignified and check the loss of water from the general surface of stem. Sometime the layers of cork are interrupted by lenticels. Lenticals are chief aerating structure occur as a raised corky spot in the periderm of stem.