A level OCR Biology on plant adaptations to water availability

1. Adaptations of Xerophytes
and Hydrophytes

2. Marram grass and Cacti (xerophytes)
Most plants have adaptations to conserve water. Especially plants found in very hot
and dry conditions as water will evaporate very quickly so they have evolved many
adaptations that enable them to live where water availability is very low and they
are called xerophytes. Plants found in very cold and icy conditions are also
xerophytes as the water in the ground is frozen and not available to them.

3. Adaptations
• A thick waxy cuticle- In most plants some water can be lost by transpiration through the cuticle, however a thick and waxy
cuticle helps minimise water loss, therefore reducing the transpiration rate in the leaf. The cuticle helps seal in the water,
making the leaves virtually waterproof as it is impermeable to water.
• Sunken Stomata- Most xerophytes have their stomata located in pits, which reduce air movement, producing a microclimate
(the climate of a very small or restricted area, especially when this differs from the climate of the surrounding area) of still
humid air that reduces the water vapour potential gradient and so reduces transpiration.
• Reduced stomata- Reducing the number of stomata in the leaves reduce the amount of water loss by transpiration but it does
reduce the amount of gas exchange can take place as there are less of them
• Reduced leaves- An example of reduced leaves is Cacti have very few leaves or they are thin needles. This means that they have
a significantly reduced SA :V ratio, minimising the amount of water lost in transpiration.

4. • Hairy leaves- This creates a microclimate of still, humid air, reducing the water vapour potential gradient and
minimising the loss of water by transpiration from the surface of the leaf. Marram grass and cacti both have this
adaptation as marram grass has micro hairs in the stomata pits and cacti normally have spines which are hairy.
• Curled leaves- This confines all of the stomata within a microenvironment of still, humid air to reduce diffusion of
water vapour from the stomata. The leaves they minimize the surface area of the leaf directly exposed to sunlight
and wind, thus minimizing evaporation and water loss. Marram grass is an example of having curled leaves.
• Succulents- They can store water in specialised tissues in their stems and roots called parenchyma. Water is stored
when it is in good supply and then it is used in times of a drought. The storage of water often gives succulent
plants a more swollen or fleshy appearance than other plants. Cacti’s are examples of succulents.
• Leaf loss- Plants can prevent water loss by simply losing their leaves when water is not available.

5. • Root adaptations- Long roots that grow deep into the ground that can penetrate 7 several meters down so
they can access water that is long way underground. Also, a lot of roots that are widespread and are shallow
are able to absorb water that is available near the surface. Many Cacti show both if these adaptations.
Marram grass’s root system consists of long vertical roots that go many meters into the sand. They also have
horizontal modified stems from which many more roots develop to form extensive networks that help to
change their environment and enable the sand to hold more water.
• Avoiding the problems- Some plants will die or become dormant, leaving their seeds behind to grow when
rain falls again. A few plants can withstand dehydration and recover as they appear dead but when rain falls
the cells recover and the plant comes green again. The ability to survive in this way is related to the
disaccharide trehalose, which appears to enable the cells to survive unharmed.

6. Water Lilies (Hydrophytes)
• These plants actually live in water and require adaptations to cope with growing
in water or in permanently saturated soil. It is important that the leaves float in
surface water plants and water-logging is a major problem for hydrophytes. The
air spaces need to be full of air not water for the plant to live.

7. Adaptations
• Very thin or no waxy cuticle- Thy do not need to conserve water so water loss by transpiration is not an issue
• Many always-open stomata on the upper surface- Unlike other plants there is no risk to the plant of loss of turgor as
there is always water available, so they are always open for gaseous exchange and the guard cells are inactive. With water
lilies the stomata need to be on the upper surface of the leaf so they are in contact with the air.
• Reduced structure to the plant- the water supports the leaves and the flowers so there is no need for strong supporting
structures.
• Wide, flat leaves- Water lilies have wide, flat leaves that spread across the surface of the water to capture as much sun
light as possible.
• Small roots- Water can diffuse directly into stem and leaf tissue so there is less need for uptake by roots.

8. • Large surface area of stems and roots under water- This maximises the area for photosynthesis
and for oxygen to diffuse into submerged plants.
• Air sacs- Some have air sacs to enable the leaves and flowers to float to the surface of the water.
• Aerenchyma- Specialised parenchyma tissue forms in the leaves, stems and roots of the plants. It
has many large air spaces, which seem to be formed at least in part by apoptosis ( programmed
death) in normal parenchyma. It has several different functions within the plants, making the
leaves and stems more buoyant and forming a low-resistance internal pathway for the movement
of substances such as oxygen to tissues below the water. This helps the plant to cope with
extreme low oxygen conditions in mud, by transporting oxygen to the tissues these are normally
found in crop species.