Which tissue is responsible for secondary growth in diameter by producing new vascular tissue?

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

Answer: (D)

Secondary growth in diameter comes from a lateral meristem called the cambium. The vascular cambium sits between the inner xylem and outer phloem and divides to add new vascular tissue: inner cells become secondary xylem (wood) and outer cells become secondary phloem. This activity thickens the stem and creates tree rings as cambial growth varies with seasons. The cortex is outside this area and doesn’t produce vascular tissue, while xylem and phloem are the products of cambial activity, not the growth layer themselves. The cork cambium handles outer bark, not the vascular tissue responsible for widening the stem.