A layer of new wood on a tree just under the bark is called __________.

• A. Cambium
• B. Phloem
• C. Cortex
• D. Xylem

Answer: (A)

Growth in diameter of a tree happens in the cambium, a thin meristematic layer between the bark and the wood. The cambium divides to form new xylem on the inside (which becomes wood) and new phloem on the outside (which stays in the bark). Because it sits just under the bark and is responsible for generating new wood, this layer is described as the source of new wood. Phloem is the outer bark tissue that transports nutrients, cortex is an outer stem layer not involved in wood formation, and xylem is the wood produced by the cambium rather than the growth layer itself.