Answer: Stem spine (thorn).
- A Stem tendril
- B Stem spine (thorn)
- C Phylloclade
- D Cladode
Correct answer: B. Stem spine (thorn)
Explanation: Bougainvillea has stem modifications into hard, sharp thorns (spines) that are modified axillary buds or stems. They provide defense. Contrast with leaf-spine (cactus) and prickle (rose - epidermal outgrowth).
In a dicot stem, vascular bundles are arranged in a single ring (with cambium between xylem and phloem, enabling secondary growth); in a monocot stem, bundles are scattered throughout with no cambium, so monocots can't undergo secondary growth (no wood formation).
Concept context
External structure of roots, stems, and leaves plus internal tissue organization. High marks in Class 11 and NEET.