7.1 - Adaptations of Plants

All living organisms need to adapt to the changes of the environment in order to survive and ensure the survival of their species. Adaptation refers to the adjustment of organisms to their environments. For plants, adaptations can be seen in the shape of their leaves, roots and stems.

Classifications of Plants based on Habitats

Adaptation of the plants differ based on varying habitats. Plants can be categorized into 4 types.

• Halophytes
• Hydrophytes
• Mesophytes
• Xerophytes

Hydrophytes

• Live in aquatic habitats, either on the surface of water or submerged under water.
• For instance, lotus and Elodea sp.

Halophytes

• Live in swampy habitats such as river mouth, the meeting point of fresh water and seawater and area rich in salts.
• For example, mangrove tree

Mesophytes

• Live in habitats that are not too dry or too wet.
• Live in habitats where there is enough supply of water.
• For example, mango tree, hibiscus plant and rubber tree.

Xerophytes

• Live in habitats where it is hot and dry such as the desert.
• For example, cactus plant and date palm.

Adaptive Features of Hydrophytes, Halophytes and Xerophytes

As explained above, halophytes are plants that live in swampy habitats where oxygen supply is limited and concentration of salt is high. These swampy areas are also exposed to high light intensity. There are some specific adaptive features yang allow the halophytes such as mangrove trees to adapt to these conditions.

Leaves

• Thick cuticle with sunken stomata to reduce the rate of transpiration.
• Succulent leaves to allow water storage.
• Special structures called hydatode which helps to eliminate excess salt.
• Old leaves can store salt and will fall off when the concentration of salt is too high.

Roots

• Branch widely with many different shapes and sizes.

  • Support the plant in soft and muddy soil.
  • Prevent the plants from being uprooted due to strong wind.

• Produce hundreds of breathing roots called pneumatophores which grow vertically above the surface of the soil.

• Contain numerous pores called lenticels which allow the exchange of respiratory and photosynthetic gases.

• The concentration of salt in the cell sap of the roots is higher than the surrounding water. Therefore, the cell sap of the roots will not lose water via osmosis. Instead, halophytes can obtain water and mineral salts from the seawater.

Hydrophytes are commonly found floating on the surface of water or submerged under the water. Most of these plants have fibrous roots which provide a large surface area and trap air bubbles to stabilize the plants and make them lighter. This enables them to float or stay upright in the water.

For instance, lotus is an example of hydrophyte that floats on the surface of water. Its roots are not anchored to the bottom of the lake. Its broad, thin and flat leaves allow maximum absorption of sunlight for photosynthesis. Most of the stomata are found on the upper epidermis of the leaves. Also, the presence of waxy and water-permeable cuticle on the upper epidermis ensures that the stomata are always open. Its stem is made up of aerenchyma tissues which are light with lots of air spaces.

Elodea sp. is the example of a submerged hydrophyte that grow completely inside the water. Its thin and small leaves increase the total surface area per volume and increases the rate of diffusion of water, mineral salts and dissolved gas through the epidermis. This plant does not have stomata and cuticle on its leaves. Its stem is small which helps the plant to float upright in the water and to reduce the resistance of water flow.

Xerophytes are able to adapt in habitat that is very dry due to the characteristics below.

• The roots grow widely and penetrate deep into the soil to absorb water and minerals.

• The stems can carry out photosynthesis to produce food.

• Its leaves are small with waxy cuticle to prevent the loss of excess water.

  • The leaves are modified into thorns to reduce the total surface area exposed to sunlight.
  • The thorns also help to gather dew which will be absorbed by the roots.
  • The thorns protect the cactus from being eaten by other animals.

• The stomata are sunken so that water evaporation can be minimized.