3.3.6 (Adaptations of plants to the availability of water)

What must terrestrial plants be adapted to do?

• They must be adapted to reduce the loss of water through stomata during the day

• They must be adapted to replace the water that is lost through the stomata

What adaptations do most terrestrial plants have to reduce water loss?

• A waxy cuticle on the leaf to reduce water loss due to evaporation through the epidermis

• The stomata are often found on the under-surface of leaves not on the top surface - this reduces the evaporation due to direct heating from the sun

• Most stomata are closed at night, when there is no light for photosynthesis

• -add plants in winter losing leaves

What are the 2 types of terrestrial plants?

• Xerophytes

• Hydrophytes

What are 2 examples of xerophytes?

• Marram grass

• Cacti

What is another name for marram grass?

• Ammophila

Where does marram grass live and what conditions do they live in?

• They specialise in living in sand dunes

• They live in arid conditions:

  • A lot of sunlight

  • A lot of wind

  • Not a lot of water

  • Sand may be salty

What adaptations do marram grass have to reduce water loss?

• The leaf is rolled longitudinally so that air is trapped inside - this air becomes humid. They can roll more tightly in very dry conditions

• There is a thick waxy cuticle on the upper epidermis of the rolled leaf to reduce evaporation

• The stomata are on the lower epidermis of the rolled leaf, so they are protected by the enclosed air space

• The stomata are in sunken pits in the lower epidermis, which are also folded and covered by hairs. These adaptations help to reduce air movement

• The spongy mesophyll is very dense, with few air gaps - so there is less surface area for evaporation of water

What adaptations do cacti have to reduce water loss?

• They are succulents - they store water in their stems which become flesh and swollen. The stem is often ribbed or fluted so that it can expand when water is available

• The leaves are reduced to spines. This reduces the SA of the leaves. When the lead total SA is reduced, less water can be lost

What adaptations do cacti have to replace the water that is lost?

• They have wide spread roots in order to absorb as much water as possible in any rainy day

What other features do xerophytes have?

• Closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water

• Some plants have a low water potential inside their leaf cell. A low water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between cells and the leaf air spaces is reduced

• A very long tap root that can reach water deep underground

What are are hydrophytes?

• They are plants which live in water

what is an example of a hydrophyte?

• Water lillies

What adaptations do water lilies have for gas exchange of oxygen?

• Many large air spaces in the leaf which keeps the leaves a float so that they are in the air and can undergo gas exchange

• The stomata are on the upper epidermis, so that they are exposed to the air to allow for gas exchange

how do hydrophytes transpire if they are underwater?

• They cannot transpire normally due to there being to water vapour potential gradient due to the high humidity

• They contain specialised structures at the tips or margins of their leaves called hydathodes which release water droplets which evaporate from the leaves surface