Define cambium and its role in tree growth.

• A. A thin meristematic layer between bark and wood that produces new xylem inward and phloem outward, enabling secondary (diameter) growth.
• B. A layer of cork producing bark.
• C. The tissue that stores carbohydrates as starch.
• D. The primary xylem tissue that conducts water.

Answer: (A)

Secondary growth in trees is driven by cambium, a thin meristematic layer between the wood (xylem) and the bark (phloem). Because cambium keeps dividing, it continually adds tissue: new secondary xylem on the inside and new secondary phloem on the outside. This outward–inward production builds the tree’s girth over time and creates the annual growth rings you see in temperate species. It’s the vascular cambium that does this work, enabling diameter growth and the continued transport of water and nutrients as the trunk thickens. Cork cambium, which produces outer bark, is a different layer, and primary xylem or storage tissues are not cambium and don’t drive this diameter growth.