Module 3: Plant Adaptations to Water
These free OCR A Level Biology Plant Adaptations to Water revision notes have been written for specification point 3.1.3(e).
Plant Adaptations To Water
Plant species are adapted to the environments in which they evolved, and will have adaptations that maximise their chances of survival.
Regarding water, plants can be categorised as:
- Xerophytes: Adapted to dry environments by minimising water loss and maximising water retention
- Hydrophytes: Adapted to aquatic or very wet environments by increasing access to oxygen as well as mineral ions by maximising the transpiration stream
Adaptations To Dry Environments
The table below outlines some adaptations found (in some) xerophytes:
| Adaptation | Function |
|---|---|
| Thick waxy cuticle | Reduces evaporation (water loss) through the cellulose cell wall |
| Sunken stomata | Traps moist air to decrease the water vapour potential gradient by reducing the wind |
| Rolled leaves | Traps moist air to decrease the water vapour potential gradient by reducing the wind |
| Hairy surfaces | Traps moist air to decrease the water vapour potential gradient |
| Reduced leaf surface |
– Spines or needle-like leaves reduce the surface area available for transpiration – Photosynthesis occurs on the fleshy plant stems instead (like a cactus) |
| Fleshy stems | Provides storage space for water, and can swell to accommodate more |
| Deep or widespread roots | Maximise water uptake by accessing deep or widespread soil moisture |
Adaptations To Wet Environments
The table below outlines some adaptations found (in some) hydrophytes:
| Adaptation | Function |
|---|---|
| Air spaces in the tissue | Enable floating on/in water to access the air (for oxygen) and light, and to promote gas exchange |
| Stomata on the upper surface | So that gas exchange can occur with the atmosphere (in floating leaves) |
| Reduced vascular tissue |
– Xylem is not required for water transport, saving resources and energy – Less mechanical support is needed |
| Thin, flexible stems with airspaces |
– Allows flexibility with water movement to reduce breakage, so less lignin is made (saving energy and resources) – Helps the plant float, and provides a shorter diffusion pathway for oxygen to the roots |
Required Examples: Marram Grass, Cactus and Water Lilies
These are the three plants whose adaptations you are expected to know for the OCR A level Biology course.
The adaptations they each have are listed in the table below:
| Marram Grass (Xerophyte) | Cactus (Xerophyte) | Water Lily (Hydrophyte) |
|---|---|---|
| Rolled leaves | Spines instead of leaves | Stomata on the upper surface |
| Hairy leaf surfaces | Photosynthetic stem | Large air spaces in tissues |
| Sunken stomata | Fleshy stem for water storage | No waxy cuticle |
| Thick waxy cuticle | Thick waxy cuticle | Reduced vascular tissue |
| Extensive root system (deep) | Shallow, widespread root system | Thin, flexible stems |
| Sunken stomata | Minimal root system |