Transpiration and xerophytes The loss of water vapour by evaporation from plant leaves

Transpiration-unavoidable consequence of gas exchange During the hours of daylight plants need to absorb carbon dioxide and remove waste oxygen. This process of gas exchange takes place mainly through open stomata. Inevitably water is lost by diffusion of water vapour out through the open stomata

The need to conserve water Plants need to be able to survive dry periods or seasons when there is little rainfall. Need to be able to survive when water in the soil is frozen.

Plants conserve water Layer of wax called the cuticle reduces evaporation from the leaf epidermis Stomata mainly found on the lower epidermis which is more shaded from the sun. Stomata open during the day and close at night in many plants Deciduous plants lose their leaves in the winter

Surviving in a very dry environment Some plants have adaptations which help them to survive in very dry conditions These plants are called xerophytes They can live in environments where there is less competition from other plants.

Xeromorphic adaptations Small leaves reduce the surface area for evaporation of water The leaves are often cylindrical rather than flattened, eg pine needles

Cacti? The leaves are so small that they are just spines These are not used for photosynthesis at all but protect the plants from animal grazing The stomata and chloroplasts are in the swollen stem. This gives a small surface area for water loss Q Why do cacti grow so slowly?

Xeromorphic adaptations Thicker waxy cuticle Low water potential in leaves Water stored in leaves

Xeromorphic adaptations Hairs on the underside of the leaf trap a layer of humid air so reduce the rate of water loss by diffusion as the water vapour potential gradient is not so steep Stomata may be sunken in the leaf

Marram grass Found on sand dunes Leaves are rolled around the under surface Stomata are inside the rolled leaf Water vapour is trapped in the rolled leaf (raising the water potential) so transpiration rate is reduced Leaf hairs help to prevent moist air being blown away There is a thick waxy cuticle on the outer surface