What is a bark and what is their function?

Bark is the dead tissue, lying outside the cork cambium. It includes ruptured epidermis, hypodermis and cork. The outer layers of bark are constantly sloughed off on account of the formation of new secondary tissue inside.
Function : (i) Bark acts as heat screen and also protect the inner tissue against micro-organisms and insects. E.g., Quinine - a drug for malaria is obtained from the bark of Cinchona.