Describe contribution of cork cambium in secondary growth.

$\rightarrow$ As the stem continues to increase the girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cells layers. Hence sooner or later another meristematic tissue called cork cambium or phellogen develops in the cortex region.

$\rightarrow$ Phellogen is a couple of layers thick, it is made of narrow, thin walled and nearly rectangular cells. Phellogen cuts off cells on both sides.

$\rightarrow$ The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.

$\rightarrow$ The cork is impervious to water due to suberin deposition in the cells wall.

$\rightarrow$ The cells of secondary cortex are parenchymatous.

$\rightarrow$ Phellogen, phellem and phelloderm are collectively known as periderm.

$\rightarrow$ Due to the activity of the cork cambium. Pressure builds up on the remaining layer peripheral to phellogen and ultimately these layer die and slough off.

$\rightarrow$ Bark : Bark is a non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem.

$\rightarrow$ Early barks : Bark refers to a number of tissue types viz periderm and secondary phloem. Bark that is formed early in the season is called early or soft bark. Towards the end of the season, late or hard bark is formed.

$\rightarrow$ Lenticels: At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the. outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens shaped opening called lenticels.