Variation and Selection: Chapter 18, Bio WRP 29.7.22 Part 2.3: Adaptions to Arid conditions and living in water
Plants that are modified to cope with lack of water are called xerophytes.
Pine tree:
Small, compact needle- like leaves
The small surface area offers less resistance to high winds, which means it also helps to resist wind damage and reduce water loss by transpiration.
Sunken stomata creates high humidity and reduces transpiration.
Thick, waxy cuticle prevents evaporation from the surface of the leaf.
Cacti:
Adapted to hot, dry conditions.
They have no leaves, or the leaves are reduced to spines. This reduces the surface area for transpiration and also acts as a defence against herbivores.
Photosynthesis is carried out by a thick green stem, which offers only a small surface area for evaporation.
Are succulent; Store water in their fleshy tissues and draw on this store for photosynthesis
The special stomata of cacti:
Marram grass:
Ammophila lives on sand dunes where water drains away very quickly.
Has very long roots to search for water deep down in the sand.
Its leaves roll up into straw-like tubes in dry weather due to the presence of hinge cells, which become flaccid as they lose water. Leaf rolling, along with the fact that the stomata are sunken, helps to increase humidity around the stomata, reducing transpiration.
The presence of fine hairs around the stomata reduces air movement so humidity builds up and transpiration is reduced.
Hydrophytes:
Plants adapted to living in water are called hydrophytes. E.g. Water lily
The leaves contain large air spaces to make them buoyant, so they float. This enables them to gain light for photosynthesis.
The lower epidermis lacks stomata to prevent water entering the air spaces, while stomata are present on the upper epidermis for gas exchange.
The roots of hydrophytes, which can be poorly developed, also contain air spaces.
Stems lack much support as the water they are surrounded by provides buoyancy for the plant.