Which of the following is involved in secondary thickening in plants?

Analyze the question: It asks which structures are involved in secondary thickening (secondary growth), the increase in girth of stems and roots in dicots and gymnosperms.

Explain the concept: Secondary growth comes from two lateral meristems:

1. Vascular cambium produces secondary xylem (wood) towards the inside and secondary phloem towards the outside. This is the main cause of increase in stem thickness.
2. Cork cambium (phellogen) forms in the cortex or epidermis region. It produces cork (phellem) outwards and phelloderm inwards. Together these make the periderm, which replaces the epidermis as the stem expands.

Both are needed. The vascular cambium thickens the stem, and the cork cambium forms new protective tissue as the stem gets wider and the epidermis splits.

Check the options:

• A. Collenchyma and xylem cells: Collenchyma is a primary supporting tissue and does not produce new tissue, so this is wrong.
• B. Vascular cambium: This is correct as far as it goes, but it is incomplete, because it leaves out the cork cambium.
• C. Cork cambium and sclerenchyma: Sclerenchyma is a permanent supporting tissue, not a meristem, so this is wrong.
• D. Vascular cambium and cork cambium: This includes both lateral meristems responsible for secondary growth. Correct.

Answer: D. Vascular cambium and cork cambium.

Exam tip: Remember that secondary growth means lateral meristems: Vascular cambium and Cork cambium. If an option lists both, it is usually the best answer. Primary growth, in contrast, is from apical meristems, which increase length.