Secondary growth is brought about by the activities of the

Analyze the question: Secondary growth is the increase in the girth (thickness) of woody plants, mainly dicots. We need to identify which meristems (lateral meristems) are responsible.

Explain the concept: Plants have two types of growth:

• Primary growth: increase in length, from apical meristems at the root and shoot tips (procambium is a primary meristem that forms primary xylem and phloem).
• Secondary growth: increase in thickness, from two lateral meristems: • Vascular cambium: produces secondary xylem (wood) inward and secondary phloem outward. • Cork cambium (phellogen): produces cork (phellem) outward and phelloderm inward, forming the protective bark that replaces the epidermis.

Go through the options:

• A. Phellogen and phelloderm: phelloderm is a tissue produced by the phellogen, not a meristem. Wrong.
• B. Phellogen and procambium: procambium is a primary meristem. Wrong.
• C. Vascular cambium and phelloderm: phelloderm is again just a tissue. Wrong.
• D. Vascular cambium and phellogen: both are lateral meristems that drive secondary growth. Correct.

Tip: Remember "Secondary = Sideways (lateral) meristems": vascular cambium + cork cambium (phellogen). Anything ending in "-derm" (like phelloderm) is a tissue, not a growth-producing meristem.