Explain how the structural and physiological adaptations of a xerophyte, such as marram grass (Ammophila arenaria), reduce the rate of transpiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Leaves are rolled or inrolled, creating a humid micro‑environment inside the leaf and reducing the vapour pressure gradient between leaf interior and air.
2. Stomata are sunken into pits or situated on the inner (adaxial) surface of the rolled leaf, limiting air flow over them and keeping a localised high‑humidity zone.
3. A thick waxy cuticle on the outer (abaxial) epidermis is largely impermeable to water, thereby reducing cuticular transpiration.
4. Epidermal hairs (trichomes) on the inner leaf surface reflect radiation and/or trap humid air, lowering leaf temperature and the vapour pressure deficit.
2. Stomata are sunken into pits or situated on the inner (adaxial) surface of the rolled leaf, limiting air flow over them and keeping a localised high‑humidity zone.
3. A thick waxy cuticle on the outer (abaxial) epidermis is largely impermeable to water, thereby reducing cuticular transpiration.
4. Epidermal hairs (trichomes) on the inner leaf surface reflect radiation and/or trap humid air, lowering leaf temperature and the vapour pressure deficit.
Examiner tips
- Use the exact phrases from the mark scheme; each point counts as one mark.
- Show the causal link: adaptation → reduced gradient/air flow → lower transpiration.
- Keep sentences concise and avoid unnecessary words.
Common mistakes
- Confusing adaxial/abaxial surfaces; mislabeling leaf surfaces.
- Lumping all adaptations into one sentence, losing individual marks.
- Failing to mention the role of the waxy cuticle or trichomes.
Mark scheme (4 marks)
- Leaves are rolled/inrolled, trapping humid air and reducing the water vapour concentration gradient between the leaf interior and the atmosphere
- Stomata are sunken into pits or located on the inner (adaxial) surface of the rolled leaf, reducing air movement over them and maintaining a localised high-humidity zone
- A thick waxy cuticle on the outer (abaxial) epidermis reduces cuticular transpiration by being largely impermeable to water
- Epidermal hairs (trichomes) on the inner leaf surface reflect radiation and/or further trap humid air, reducing leaf temperature and the vapour pressure deficit
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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