Question

How does the expose tissue of cortex become protected when primary epidermis fall off in stem

Answer 1

So that it can protect exposed tissue, a layer of meristematic cells, cork cambium, or phellogen rises from the cortex.

The cells of phellogen divide and create new cells.On both sides towards the cortex are parenchymatous, they form a secondary cortex or phelloderm while the cells on the outside of the phelloderm are rectangular and arranged in radial rows. They secrete suberin inside their wall then die off. These cork tissues functions as a barrier of sorts to protect the plant from outside threats.

Suberin - Waxy substance 

Parenchymatous - The basic tissue of plants 

Hope this helps six days late