Secondary (diameter) growth in trees is produced by which tissue?

• A. Cambium
• B. Epidermis
• C. Cortex
• D. Phellogen

Answer: (A)

Secondary (diameter) growth in trees is produced by the vascular cambium, a lateral meristem that lies between the primary xylem and phloem. The cambium divides to add new tissue outward and inward: toward the inside it forms secondary xylem (wood) and toward the outside it forms secondary phloem. This continuous production adds layers and steadily increases the stem’s girth as the tree ages. The epidermis and cortex are primary tissues that don’t drive this thickening, and they’re eventually replaced as protection shifts to the periderm formed by the cork cambium (phellogen). Thus, the tissue responsible for diameter growth is the cambium.