Bark is the outer protective coating of the trunk and branches of trees and shrubs and includes all the tissues outside of the vascular cambium. A typical bark consists of several layers of different types of tissue. The inner bark, or bast, is living and contains the conductive tissue, called phloem, by which sugars are transported from the leaves in the crown of a tree to the roots, and from storage tissues to other parts of the plant. The outer bark is layered, with the inner layer consisting of the cork cambium, a meristem that produces cork cells to the outside. The cork cells are usually tightly packed and have fatty substances deposited in their thick walls. In contrast to the cork cambium, cork cells are dead and filled with air, making cork lightweight and insulating.

The appearance of a bark depends on the type of cork cells...