9.5- Plant Adaptations To Water Availability

Question 1

What are xerophytes?

Answer 1

Plants that are adapted to survive in dry habitats where water availability is very low
- cacti, marram grass, conifers etc.

Question 2

What adaptations of the stomata do xerophytes have?

Answer 2

1. Sunken stomata- located in pits which reduces air movement. Creates microclimate of still, humid air so reduces vapour potential gradient and therefore transpiration.
2. Reduced numbers of stomata- reduces water loss but also their gas exchange ability.

Question 3

Adaptations of the leaves in xerophytes?

Answer 3

1. Reduced leaves- often reduced to thin needle like structures which have a greatly reduced SA:V. Minimising transpiration.
2. Hairy leaves- create microclimate of still humid air, reducing water vapour potential gradient, reducing transpiration.
3. Curled leaves- confines all stomata in still humid air, reducing diffusion of water vapour from stomata.
4. Leaf loss- some plants lose their leaves when water isn’t available. Trunk and branches turn green and photosynthesise with minimal water loss.

Question 4

Other adaptations in xerophytes?

Answer 4

1. Thick waxy cuticle- most plants lose 10% of water loss by transpiration through cuticle. This minimises it.
2. Succulents- succulent plants store water in specialised parenchyma tissue in stem/roots. Water is used in times of drought.
3. Root adaptations:
   - can have long tap roots to reach deep into ground, to access water below.
   - can have shallow widespread roots with large SA to absorb max before rainfall evaporates.
4. Some plants become dormant or die and leave seeds behind to germinate.

Question 5

What are hydrophytes?

Answer 5

Plants that live in or on top of water.

• need adaptations to cope with growing in water or permanently saturated soil.
• water lilies, water cress, duckweeds etc.
• water logging is a major problem. Air spaces need to be full of air not water for survival.

Question 6

Adaptations of hydrophytes?

Answer 6

1. Thin/ no waxy cuticle- don’t need to conserve water.
2. Many always open stomata in upper surface- maximises gas exchange.
3. Reduced structure- water supports leaves/flowers.
4. Wide, flat leaves- spread across water surface to capture max light for photosynthesis.
5. Small roots- water can diffuse directly into stem/leaf tissue.
6. Large SA of stems/roots under water- max area for photosynthesis and oxygen to diffuse into submerged plants.
7. Air sacs- enable leaves/flowers to float on water surface.

Question 7

What is aerenchyma?

Answer 7

• specialised spongy tissue that forms in leaves/stem/roots of hydrophytes.
• has many large air spaces (filled with O2), formed by apoptosis in normal parenchyma.
  Functions in plants:
• making leaves and stems more buoyant.
• forming a low-resistance internal pathway for movement of substances like oxygen to tissues below the water. (Helps cope with anoxic, low oxygen, conditions in mud)

Question 8

What are pneumatophores?

Answer 8

Special aerial roots that grow upwards into the air.
They have many lenticels (raised pores), which allow the entry of air into the woody tissue.
- grow in situations where there is plentiful water supply and the roots can become waterlogged.