List the characteristics of cork. How are they formed? Mention their role.

(N/A) Characteristics:
- The cells of cork are dead at maturity.
- These cells are compactly arranged without any intercellular spaces.
- The cell walls possess a chemical substance called $suberin$, which makes them impervious to gases and water.
- They are several layers thick.
Formation:
- As plants grow older, the epidermis of the stem is replaced by a strip of secondary meristem known as the $phellogen$ or cork cambium.
- The cells cut off on the outer side by this meristem differentiate into cork cells.
Role:
- Cork acts as a protective tissue for older stems, twigs, and branches.
- It prevents water loss and protects the plant from mechanical injury and microbial infection.