Explain how the structural adaptations of a xerophyte reduce water loss.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Xerophytes are plants adapted to survive in environments where water availability is extremely limited, such as deserts or exposed rocky habitats.
Model answer (4 marks)
1. A thick, waxy cuticle coats the leaf surface, forming a barrier that slows the diffusion of water vapour out of the plant.
2. Stomata are positioned in sunken pits or grooves; the surrounding tissue retains humid air, lowering the water‑vapour concentration gradient across the stomatal pore and thus reducing transpiration.
3. Leaves are small or transformed into spines/needles, which cuts down the total surface area through which water can be lost.
4. Some xerophytes roll or curl their leaves so that the stomata are enclosed in a humid micro‑environment, further decreasing the vapour pressure difference and the rate of water loss.
2. Stomata are positioned in sunken pits or grooves; the surrounding tissue retains humid air, lowering the water‑vapour concentration gradient across the stomatal pore and thus reducing transpiration.
3. Leaves are small or transformed into spines/needles, which cuts down the total surface area through which water can be lost.
4. Some xerophytes roll or curl their leaves so that the stomata are enclosed in a humid micro‑environment, further decreasing the vapour pressure difference and the rate of water loss.
Examiner tips
- Use the exact terms – cuticle, stomata, pits, surface area, micro‑environment. Show the link between each adaptation and the reduction of the water‑vapour gradient. Keep each point concise and separate with numbers or bullets.
Common mistakes
- Confusing cuticle with stomatal density. Failing to explain how a reduced surface area limits transpiration. Using vague phrases like "less water loss" without linking to the mechanism.
Mark scheme (4 marks)
- Thick, waxy cuticle reduces evaporation of water from the leaf surface
- Stomata are sunken in pits or grooves, which trap humid air and reduce the water vapour concentration gradient, decreasing the rate of transpiration
- Leaves are reduced in size or modified into spines/needles, decreasing the surface area available for transpiration
- Leaves may roll or curl, enclosing the stomata in a humid microenvironment, which reduces the water vapour concentration gradient and therefore the rate of transpiration
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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