6. Adaptions of plants to the availability of water

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definitions

hydrophyte = a plant adapted to living in water or where the ground is very wet.

Xerophyte = a plant adapted to living in dry conditions.

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Terrestrial plants

During the day plants take up a lot of carbon dioxide, for use in photosynthesis. The stomata must be open during the day for this and alos the removal of oxygen. Whilst the stomata is open, it is easy for water to be lost. This water must then be replaced.

Plants living on land must be adapted to:

  • reduce the water loss
  • replace the water lost

Adaptations:

  • Thick waxy cuticle on the leaf to reduce water loss due to evaporation through the epidermis.
  • Stomata are often found on the under-surface of the leaves, reducing the evcaporation due to direct heat from the sun.
  • most stomata are closed at night, when there is no light for photosynthesis to reduce water loss.
  • Deciduous plants lose their leaves in winter, whent he ground may be frozen and temperatures are too low for photosynthesis.
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Marram grass

Specialises in living on sand dunes. The conditions are particularly harsh, because any water in the sand draines away quickly. The sand may be salty qand the leaves are often in windy conditions. Marram grass is a xerophyte.

Adaptations:

  • The leaf is rolled longitudinally so that air is trapped inside - the air becomes humid reducing the water loss.
  • Thick waxy cuticle on the outer side of the rolled leaf to reduce evaporation.
  • The stomata are on the inner-side of the rolled leaf, so they are protected by the enclosed air space.
  • The stomata are in pits in the lower epidermis, which is also folded and covered by hairs. This reduces air movement and loss of water vapour.
  • The spongy mesophyll is very dense, with few air spaces - less SA for for evasporation.
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Cacti

Cacti show other features to overcome arid conditions/;

  • Cacti are succulents - they stopre water in their stems which become fleshy and swollen. The stem is often ribbed or fluted so that it can expand when water is available.
  • The leaves are reduced to spines (made smaller). This reduces the SA, so less water is lost by transpiration.
  • The stem is green for photosynthesis.
  • The roots are very widespread, in order to take advantage of any rain that does fall.
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Other xerophytic features:

  • closing the stomata when water availability is low will reduce water loss and so reduce the need for water uptake.
  • some plants have a low water potential inside their leaf cells. This is achieved by maintaining a high salt concentration in the cells. The low water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced.
  • A very long tap root that can reach water deep underground.
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Hydrophytes

eg) water lilies.

These plants ahve easy access to water but face other problems, like; getting oxygen to their submerged tissues and keeping afloat whilst keeping their leaves in the sunlight for photosynthesis.

Adaptions:

  • Many large air spaces in the leaf. This keeps the leaves aflooat so thast they are in the air and can absorb sunlight.
  • The stomata are on the upper epidermis, so they are exposed to the air to allow gaseous exchange.
  • The leaf stem has many air spaces (aerenchyma), which helps with buoyancy and allows oxygen to diffuse quickly to the roots for aerobic respiration.
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How do they transpire?

Transpiration is the loss of the water vapour from the surface of the leaves by evaporation.

Water can't evaporate into water or into high humidity. If the water can't leave the plant, then the transpiration stream stops and the plant won't be able to transporate mineral ions to the leaves.

Many plants contain specialised structures at the tips or margins of their leaves called hydathodes.

Hydathodes can release water droplets which can then evaporate from the leaf surface.

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