IB DP Biology: HL复习笔记9.1.4 Adaptations of Xerophytes

• Plants that live in conditions with a plentiful supply of fresh water have leaves with a short diffusion distance through to the stomata and a large surface area provided by the air spaces between the leaf cells
  • These factors make them vulnerable to water loss
• Plants that live in conditions where fresh water is limited often have very different leaf structure to that described above; they have evolved effective adaptations to conserve water e.g.
  • Very few stomata
  • Sunken stomata
  • Hairs surrounding stomata
  • Needle-shaped or small leaves
  • Thickened waxy cuticle
• Plants with adaptations to conserve water are described as xerophytic, or known as xerophytes.

Cacti

• Cacti are well-studied xerophytes usually found in the deserts of the USA
• They have several xerophytic adaptations
  • Their leaves are reduced to spines that can no longer photosynthesise
    • This reduces their leaf surface area and so reduces water loss
    • Photosynthesis occurs in the green stem which possesses chloroplasts
  • Stomata are located on the stem and they are more sparsely distributed than they would be on a regular leaf
  • The stem has a thick cuticle and is very large in diameter which allows it to store water
    • The stem can expand to take on water, enabling water storage when it is available
  • Cacti carry out a specialised form of photosynthesis known as CAM photosynthesis that enables them to keep their stomata closed during the day
    • This reduces water loss by evaporation in the heat of the day
  • There are both shallow and deep penetrating roots which allow access to all available water

Marram Grass

• Marram grass is commonly found on sand dunes, another example of a dry environment where plants have evolved to survive
• Marram grass leaves are well adapted to minimise water loss
  • Leaves are rolled up to reduce the exposure of surfaces to the wind and so reduce water loss by evaporation
  • The stomata are sunken in pits to reduce water loss by evaporation
  • The inner surface of the leaf possesses a large number of hairs which shield the stomata, again reducing water loss
  • The exposed surface has a thick waxy cuticle to reduce evaporation

Adaptations of Halophytes

• Halophytes are plants that are adapted to saline, or salty, conditions
  • Halo = salt
• Saline soils are soils that contain high concentration of salts, such as coastal salt marshes and land where the tide comes in and out
• Halophytes have physiological and structural adaptations to survive in these conditions, including
  • Halophytes have the ability to sequester, or store away, salts within their cell wall or vacuoles
  • Some halophytes can concentrate the salts they absorb in certain leaves, which then fall off the plant
  • Halophytes can shed their leaves to reduce water loss
    • In such conditions the stem is able to take over the role of photosynthesis
  • Water loss reducing adaptations such as reduced leaf surface area and sunken stomata can be found in some halophytes
  • Some halophytes have salt glands that actively excrete salt to stop it from building up
  • Halophytes have deep roots to reach fresh water underground