Explain how the structural adaptations of a xerophyte leaf reduce water loss by transpiration.

IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A xerophyte leaf has a thick, waxy cuticle that limits evaporation from the epidermis. Its stomata are sunken in pits or located on the lower surface, creating a humid micro‑environment that reduces the water‑potential gradient between leaf and air. Many xerophytes also roll or fold their leaves, enclosing the stomata and further decreasing the gradient. Finally, a dense covering of trichomes slows air movement over the stomata, thickening the boundary layer and lowering transpiration.

Examiner tips

Mention cuticle, sunken stomata, leaf rolling/folding, and trichomes in separate points
Use terms ‘water‑potential gradient’, ‘boundary layer’, ‘abaxial surface’
Show how each feature reduces transpiration

Common mistakes

Confusing cuticle with stomatal density
Failing to explain the role of the boundary layer
Using vague terms like ‘dry’ instead of ‘water‑potential gradient’

Mark scheme (4 marks)

Thick/waxy cuticle reduces evaporation of water from the epidermal surface
Stomata sunken in pits/crypts or located on the lower (abaxial) surface, which traps humid air and reduces the water potential gradient between leaf and atmosphere
Rolled/folded leaves (e.g. Marram grass) enclose stomata, creating a humid microenvironment that reduces the water potential gradient driving transpiration
Dense covering of leaf hairs (trichomes) on the leaf surface reduces air movement/wind speed over stomata, maintaining a thicker boundary layer and reducing transpiration

Key terms in this question

xerophyte · transpiration