bark

Protective outer layer on the stems and roots of woody plants, composed mainly of dead cells. To allow for expansion of the stem, the bark is continually added to from within, and the outer surface often becomes cracked or is shed as scales. Trees deposit a variety of chemicals in their bark, including poisons. Many of these chemical substances have economic value because they can be used in the manufacture of drugs. Quinine, derived from the bark of the Cinchona tree, is used to fight malarial infections; curare, an anaesthetic used in medicine, comes from the Strychnus toxifera tree in the Amazonian rainforest.

Bark technically includes all the tissues external to the vascular cambium (the phloem, cortex, and periderm), and its thickness may vary from 2.5 mm/0.1 in to 30 cm/12 in or more, as in the giant redwood Sequoia where it forms a thick, spongy layer.

The bark from the cork oak Quercus suber is economically important and harvested commercially. The spice cinnamon and the drug cascara (used as a laxative and stimulant) come from bark.

The fissured bark of species such as oak provides shade and protection for invertebrates; acccording to a German study 1995 it contains 20 times as many bark-dwelling animals as smooth bark.

The white bark of silver birch reflects the Sun's rays, thereby preventing overheating and enabling the colonization of open spaces.