What is phellogen?

What is phellogen ? What does it produce ? Or What is periderm ? How does periderm formation take place in the dicot stem ?

Phellogen : (a) It is called cork cambium, (b) It is developed to produce the inner tissues in dicotyledonous stems, © It develops from hypodermal cells which are collenchymatous or even from epidermal cells near to the cortex, (d) Phellogen or cork cambium produce secondary tissues more on outer side than the inner side, (e) Periderm (cork) is formed on outside while phelloderm (secondary cortex) on inner side.