Transport in Plants
Structures and Functions in Living Organisms · 5 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 19% of your exam marks.
Transpiration and the roles of xylem and phloem are tested on almost every paper in recent years.
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 | Many desert plants |
| Stomata sunk into pits or grooves | The pit traps moist air just above the stoma, again raising local humidity and slowing diffusion out | Marram grass, pine needles |
| Rolled leaves | The leaf curls inwards so the stomata face an inner enclosed space, which becomes humid and reduces transpiration | Marram grass |
| Fewer stomata and stomata only on the lower epidermis | Fewer exits for water vapour | Many xerophytes |
| Stomata close during the day (open only at night when it is cooler) | Avoids the hottest, driest part of the day. CO₂ is taken in at night and stored for daytime photosynthesis | Cacti and other succulent plants |
| Swollen stems or leaves for water storage | Stores the rare rain water for use later | Cacti, succulents |
| Deep roots or wide spreading roots | Reaches water that other plants cannot | Mesquite, cactus |
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.