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.