Dry-environment plants reduce water loss
Definition
Xerophyte
A plant adapted to live where water is scarce, with features that reduce the rate of transpiration.
- Xerophyte means a plant adapted to live where water is scarce, with features that reduce transpiration.
- Water loss threatens cell turgor, transport and photosynthesis, so xerophyte leaves reduce evaporation or trap moist air near stomata.
- Each useful adaptation changes leaf size or shape, the cuticle, or the number and position of stomata.
Example
Examples: cacti reduce their leaves to spines, while marram grass rolls its leaves and has stomata inside the roll.
Smaller leaves reduce surface area
- Small leaves or spines provide less surface area from which water can evaporate.
- A low surface-area-to-volume ratio reduces the exposed surface compared with the amount of tissue containing stored water.
- Some plants shed their leaves during dry periods, removing a major surface for transpiration.
Common Mistake
Link the feature to reduced water loss; writing only that a plant has spines or small leaves does not explain the adaptation.
Leaf shape traps humid air
- Rolled leaves enclose stomata inside a sheltered space where water vapour accumulates.
- Leaf hairs trap still, moist air and reduce the movement of air across stomata.
- The smaller water vapour concentration gradient between the leaf and the trapped air slows diffusion out of the leaf.
- Marram grass combines rolled leaves, hairs and stomata on the inner surface, so all three features act on the same gradient.
Key Idea
Mechanism: trapped moist air, smaller concentration gradient, slower diffusion of water vapour, lower transpiration.
A thick cuticle reduces evaporation
- A thick, waxy cuticle forms a waterproof barrier over the epidermis.
- This reduces evaporation through the leaf surface and limits water loss when stomata are closed.
- The cuticle remains transparent, allowing light to reach photosynthesising tissues.
Stomata limit vapour loss
- Having fewer stomata reduces the total pore area through which water vapour can diffuse.
- Sunken stomata sit in pits that shelter them from air movement and trap humid air.
- Closing stomata during the hottest or driest conditions conserves water, although it also reduces carbon dioxide uptake and photosynthesis.
Example
Cactus: spines reduce leaf surface area, a thick cuticle limits evaporation, and water-storing stem tissue helps the plant survive long dry periods.
Explaining an unfamiliar adaptation
- Identify how the feature changes exposed surface area, evaporation, air movement, stomatal opening or the water vapour gradient.
- Then link that change to slower diffusion of water vapour and a lower rate of transpiration.
Exam technique
- Two-mark explanation: a sunken stoma traps moist air, which reduces the water vapour concentration gradient and slows diffusion out of the leaf.
- Comparison questions: compare the same feature in both plants, such as leaf area or stomatal position, before explaining the consequence.
Self review
- What is a xerophyte?
- How do small leaves or spines reduce water loss?
- Why do rolled leaves and hairs reduce transpiration?
- How does a thick waxy cuticle conserve water?
- Why do sunken stomata reduce the diffusion of water vapour?
