Bark is the protective covering of a tree, branches, trunks and roots. The bark has three layers. The outer layer is called periderm and is made of dead cells, such as seen in the cork tree. Periderm is usually thick, and protects the tree from weather, insects and diseases. The middle cortex layer is made of living, non-growing cells. The innermost cells, including the phloem transmit food made in the leaves down to the roots. The periderm of some trees has small openings called lenticels. Lenticels allow gases such as oxygen and carbon dioxide to enter and leave the plant. As a tree grows, wood is formed and a layer of cells near the outer side begins to divide and produce cork cells. The walls of these cells soon get heavily thickened with a waterproof substance. These cells die and form the bark. As time passes, bark is constantly added to form the outer protective shell of the tree. As the diameter of the twig, branch or trunk increases, the waterproof barrier is maintained. The outer bark often splits and cracks or becomes flaky. The bark contains substance for malaria that repel insects and resist fire. Bark often has commercial uses. The actual cork comes from the bark of the Mediterranean cork oak. The drug quinine, a medicine for malaria is obtained from the bark of cinchona trees and cinnamon comes from the bark of the cinnamon tree. Medicines for cough and cold and many other useful substances are also obtained from the bark.