7.1 - Adaptations of Plants

Classification of Plants Based on Habitats

Adaptation: Adjustment of an organism to the environment.
Plant adaptations are observed in the shape of leaves, roots and stems; they enable plants to live in their habitats and ensure species survival.
Plants are classified as halophytes, hydrophytes, mesophytes and xerophytes according to habitat.

Live in swampy habitats at river mouths, where fresh water meets seawater; the swamp is rich in salt.
Example: Mangrove tree.

Live in habitats that are neither too dry nor too wet and have an adequate water supply.
Most plants are mesophytes.
Examples: Mango tree, hibiscus plant and rubber tree.

Live in hot, dry habitats with a minimum presence of water and very high temperatures, such as deserts.
Examples: Cactus and date palm.

The adaptive features involve the uptake of water and mineral salts, gaseous exchange, support and photosynthesis.

Live in habitats with a high salt concentration and low oxygen content; mangrove swamps are also exposed to high light intensity.
Leaves:
- Thick cuticle and sunken stomata reduce the rate of transpiration.
- Succulent leaves store water.
- Hydatodes eliminate excess salt.
- Old leaves store salt and fall off when the stored salt concentration is too high.
Roots:
- A widely branching root system of various shapes and sizes supports the plant in soft, muddy soil and prevents uprooting by strong winds.
- Hundreds of breathing roots called pneumatophores grow vertically above the soil surface.
- Numerous pores called lenticels enable gaseous exchange with the atmosphere.
- Root cell sap has a higher salt content than seawater; it does not lose water by osmosis, and water and mineral salts from seawater enter the roots.

Adapt to wet habitats by floating on the water surface or remaining submerged.
Most have fine, fibrous roots that provide a large surface area and trap air bubbles, making the plants more stable and lighter.
The roots and buoyant force of the surrounding water enable the plants to float or remain upright.
Floating plants such as lotus:
- Roots do not anchor to the bottom of the lake.
- Broad, thin and flat leaves absorb maximum sunlight for photosynthesis.
- Most stomata are distributed on the upper epidermis.
- A waxy, waterproof cuticle covers the upper epidermis and ensures that the stomata remain open.
- The stem contains light aerenchyma tissue with many air spaces between cells.
Submerged plants such as Elodea sp.:
- Thin, small leaves increase the total surface area per volume and the diffusion rate of water, mineral salts and dissolved gases directly through the epidermis.
- Leaves have no stomata or waxy cuticle.
- Small, hollow stems keep the plants floating upright and reduce water-flow resistance.

Live in deserts with very little rainfall and withstand extreme dryness.
Roots grow widely and penetrate deep into the soil to absorb water and mineral salts.
Absorbed water is stored in the roots, stems and leaves.
Cactus stems carry out photosynthesis.
Cacti have small leaves and thick, waxy cuticles on stems and leaves.
Leaves modified into thorns reduce the total surface area exposed to the sun and reduce water loss.
Thorns collect dew; the dew drops onto the ground and is absorbed by the roots.
Thorns prevent the plant from being eaten by animals.
Embedded stomata reduce water evaporation from the leaves.