Biology notes, chapter 7
Adaptations of Plants in Different Habitats
Textbook Biology Form 5 (DLP), pages 132–140
What this chapter covers
- Classify plants into mesophytes, hydrophytes, halophytes and xerophytes according to habitat (7.1.1)
- Describe the adaptive features of halophytes, hydrophytes and xerophytes for uptake of water and mineral salts, gaseous exchange, support and photosynthesis (7.1.2)
- Explain how the adaptive features help a plant survive in its habitat (7.1.2)
7.1Adaptations of plants
Adaptation is the adjustment of an organism to its environment so that the species survives. In plants it shows in the shape of the leaves, roots and stem.
Classification of plants by habitat (7.1.1)
| Type | Habitat | Examples |
|---|---|---|
| Mesophyte | Not too dry and not too wet; adequate water. Most plants are mesophytes | Mango tree, hibiscus, rubber tree |
| Hydrophyte | Aquatic: floating on the surface or submerged | Lotus, Elodea sp., Hydrilla sp., Eichhornia sp. |
| Halophyte | Swampy river mouth where fresh water meets sea water; salty soil with low oxygen | Mangrove tree |
| Xerophyte | Hot, dry habitat with very little water, such as the desert | Cactus, date palm |
A tree grows in a habitat that always has enough water. Which type of plant is it?
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A mesophyte (habitat neither too dry nor too wet).
Adaptive features of halophytes (7.1.2)
The mangrove swamp is salty, has soft muddy soil with little oxygen, and gets strong light and wind.
| Part | Feature | Function |
|---|---|---|
| Leaves | Thick cuticle and sunken stomata | Reduce the rate of transpiration |
| Leaves | Succulent leaves | Store water |
| Leaves | Hydathodes; old leaves that store salt and fall off | Remove excess salt |
| Roots | Widely branching roots of many shapes and sizes | Support in soft mud; prevent uprooting by strong wind |
| Roots | Pneumatophores (breathing roots) with lenticels | Grow upwards out of the mud; gaseous exchange with the air |
| Roots | Cell sap more concentrated in salt than sea water | Roots do not lose water by osmosis; they take in water and mineral salts |
Lenticels are the pores on the pneumatophores. Do not confuse them with stomata, which are on leaves.
Adaptive features of hydrophytes (7.1.2)
| Feature | Function |
|---|---|
| Fibrous roots with a large surface area that trap air bubbles | Make the plant stable and lighter so it floats or stays upright (with buoyant force) |
| Floating plants (lotus): broad, thin, flat leaves | Absorb maximum sunlight for photosynthesis |
| Stomata mostly on the upper epidermis, covered by waxy waterproof cuticle | Stomata stay open for gaseous exchange |
| Stem with aerenchyma tissue (many air spaces) | Light tissue gives buoyancy and stores air |
| Submerged plants (Elodea): thin, small leaves; no stomata or waxy cuticle | Large surface area per volume; water, mineral salts and gases diffuse directly through the epidermis |
| Small, hollow stem | Floats upright and reduces resistance to water flow |
Submerged plants have no stomata and no waxy cuticle. Only floating plants have stomata, on the upper surface.
Adaptive features of xerophytes (7.1.2)
| Feature | Function |
|---|---|
| Roots spread widely and go deep | Absorb water and mineral salts; water is stored in roots, stem and leaves |
| Small leaves, or leaves modified into thorns | Reduce surface area exposed to the sun, so less water loss; thorns collect dew and deter animals |
| Thick waxy cuticle | Reduces evaporation |
| Embedded (sunken) stomata | Reduce water evaporation from leaves |
| Green fleshy stem | Stores water and carries out photosynthesis (cactus) |
Arctic plants are also xerophytes, because they cannot absorb water when the ground is frozen.
Worked example
A plant has thorns, a thick waxy stem and deep roots. Type? Feature that reduces water loss?
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Xerophyte. Thorns (reduced leaves) and the thick waxy cuticle reduce the surface area and evaporation.
Why do mangrove roots not lose water to the sea by osmosis?
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The cell sap of the roots has a higher salt concentration than sea water, so water enters the roots instead.