6.14B - Plants in extreme environments
Explain how leaf size, shape, cuticle and stomata help plants conserve water in extreme environments. Use these mechanisms to judge adaptation evidence and the trade-off with photosynthesis.
6.14B - Keeping water in extreme environments
Leaves lose water when liquid water evaporates from moist cells and water vapour diffuses out, mainly through stomatal pores. Xylem brings replacement water from the roots.
In a hot, dry or windy environment, water may leave a leaf faster than roots can replace it. A cold, windy environment creates a similar problem because water in frozen soil is difficult to take up. Plants that survive these conditions often have several features that reduce water loss, but every feature works through the same ideas of exposed area, evaporation, diffusion and stomatal opening.
| Leaf feature | Direct effect | Survival outcome |
|---|---|---|
| Reduced leaf area: small, narrow or needle-shaped leaves | A smaller total leaf surface exposes less area to the air; at a similar stomatal density it also bears fewer pores overall. | Less water vapour is lost; this is useful in dry habitats and in cold habitats with frozen soil. |
| Rolled leaves | Shelter stomata inside the roll, where still, humid air accumulates. The water-vapour concentration gradient and air movement at the pores are reduced. | Diffusion of water vapour out of the leaf slows. |
| Thick, waxy cuticle | Forms a water-resistant barrier that slows water movement through the epidermis and evaporation from the leaf surface. | Non-stomatal water loss is reduced. |
| Fewer or closable stomata | Provide fewer open pores, or reduce pore size when water is scarce. | Less water vapour diffuses out. |
| Sunken stomata | Place each pore in a pit that traps still, humid air and protects it from moving air. | A less steep water-vapour gradient slows transpiration. |
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