A xerophyte is a plant that lives in a place where water is scarce: a desert, a sand dune, a sunny rocky cliff, or a salt marsh. Xerophytes have evolved a set of adaptations to minimise water loss by transpiration, while still leaving enough surface area for photosynthesis. The famous examples are cacti (deserts) and marram grass (dry sand dunes), but plenty of other species use the same tricks.
The adaptations to know
| Adaptation | What it does | Example |
|---|
| Thick waxy cuticle | Forms a thicker waterproof barrier on the leaf surface, cutting transpiration through the cuticle to almost zero | Cactus stems, holly leaves |
| Reduced leaf surface area (e.g. leaves reduced to spines or needles) | Less surface area means less area for water to evaporate from | Cactus spines, pine needles |
| Hairs on the leaf surface | Hairs trap a layer of moist air close to the leaf, raising the local humidity and reducing the water-vapour gradient between leaf and air. They also reflect sunlight |
Why xerophyte adaptations don't make every plant a xerophyte
Most of these tricks come at a cost. A thick cuticle blocks some light. Hairs and sunken stomata also slow CO₂ uptake, so photosynthesis runs more slowly. Small leaf surface area means less photosynthetic capacity overall. In a place with plenty of water, a plant with thin, broad, smooth leaves can photosynthesise much faster and out-compete a xerophyte. Xerophytes only win where water shortage is the bigger limit on survival.