xerophytes and hydrophytes

Question 1

What are xerophytes?

Answer 1

These are plants that have adapted to live in very dry habitats, where this is little access to water. This could be:
- very cold icy conditions, where water is frozen.
- Hot dry desert conditions where there is very little rainfall.
- Sand-dune environments along the coat.

Question 2

How does a thick waxy cuticle help xerophytes?

Answer 2

Helps minimise water loss. Common in evergreen plants and helps them survive both dry summers and cold winters with frozen water.

Question 3

How does sunken stomata help xerophytes adapt?

Answer 3

• stomata located in pits, which reduce air movement, producing a microclimate of still humid air that reduces the water vapour potential gradient and so reduces transpiration. Seen in marram grass, cacti and conifers.

Question 4

How does reduced number of stomata help xerophytes adapt?

Answer 4

Reduces their water loss by transpiration but also reduce their gas exchange capabilities.

Question 5

How does reduced leaves help xerophytes adapt?

Answer 5

• By reducing leaf area, water loss can be greatly reduced.
• The leaves on conifers are reduced to thin needles. These narrow leaves, which are almost circular in cross-section, have a greatly reduced SA:V ratio, minimising the amount of water loss in transpiration.

Question 6

How do hairy leaves help xerophytes adapt?

Answer 6

Creates a microclimate of still, humid air, reducing the water vapour potential gradient and minimising the loss of water by transpiration from surface of the leaf.

Question 7

How do curled leaves help xerophytes adapt?

Answer 7

• Reduces water loss by transpiration. Confines all of the stomata within a microenvironment of still, humid air to reduce diffusion of water vapour from the stomata.

Question 8

How do succulents help xerophytes to adapt?

Answer 8

• Succulent plants store water in specialised parenchyma tissue in their stems and roots. Often have a swollen or fleshy appearance. Water is stored when it is in plentiful supply and then used in times of drought.

Question 9

How does leaf loss help xerophytes adapt?

Answer 9

• Some plants prevent water loss by losing their leaves when they water is not available.

Question 10

What is a good example of a tree that loses leaves to help xerophytes adapt?

Answer 10

Palo Verde is a desert tree that loses all of its leaves in dry seasons.
The trunk and beaches turn green and photosynthesise with minimal water loss to keep it alive.

Question 11

What are the root adaptations that help xerophytes adapt?

Answer 11

Long tap roots : Can penetrate several metres so they can access water that is a long way below the surface.
Mass of widespread shallow roots: Large surface area, able to absorb any available water before a rain shower evaporates.

Question 12

How may a plant avoid the problems to help xerophytes adapt?

Answer 12

• Plants may lose their leaves and become dominant, or die completely, leaving seeds behind to germinate and grow rapidly when rain falls again.
• Others survive as storage organs such as bulbs, corms or tubers.

Question 13

How to cacti adapt to its conditions?

Answer 13

• Cacti have a thick, waxy layer on the epidermis - this reduces water loss by evaporation because the layer is waterproof.
• They have spines instead of leaves - this reduces the surface area for water loss.
• Cacti also close their stomata at the hottest times of the day when transpiration rates are the highest.

Question 14

Why are sand-dunes such harsh conditions for plant growth?

Answer 14

• Sand does not hold water, it quickly drains away- not much water available to the roots.
• Usually windy conditions- increased transpiration rate, so more water vapour is lost from the leaves.
• Salty sand lowers the water potential around the roots - more difficult for plants to take up water by osmosis.

Question 15

How does marram grass adapt to its condition?

Answer 15

• Has stomata that are sunk in pits, so they’re sheltered from the wind. This traps moist air in the pits to slow transpiration down by lowering the water potential.
• It also has a layer of hairs on the epidermis - this is also traps moist air around the stomata, which reduces the water potential gradient between leaf and air slowing transpiration down.
• In hot or windy conditions roll their leaves - traps moist air slowing down transpiration. Also reduces exposed surface area for losing water and protects the stomata from wind.
• Thick waxy layer on epidermis reducing water loss by transpiration.

Question 16

What are hydrophytes?

Answer 16

Plants that live in water, partially submerged.
Examples include water lilies, watercress and duckweed.

Question 17

How does very thin or no waxy cuticle help hydrophytes adapt?

Answer 17

Hydrophytes do not need to conserve water as there is always plenty available so water loss by transpiration is not an issue.

Question 18

How does always-open stomata on the upper surfaces help hydrophytes adapt?

Answer 18

Maximising the number of stomata maximises gaseous exchange. Guard cells are inactive. In plants with floating leaves such as water lilies the stomata need to be on the upper surface of the leaf so they are in contact with the air.

Question 19

How does reduced structure help hydrophytes adapt?

Answer 19

The water supports the leaves and flowers so there is no need for strong supporting structures.

Question 20

How does wide, flat leaves help hydrophytes adapt?

Answer 20

Some hydrophytes, including water lilies, have wide, flat leaves that spread across the surface of the water to capture as much as possible.

Question 21

How does small roots help hydrophytes adapt?

Answer 21

Water can diffuse directly into stem and leaf tissue so there is less need for uptake by roots.

Question 22

How does large surface areas of stems and roots under water help hydrophytes adapt?

Answer 22

This maximise the area for photosynthesis and for oxygen
to diffuse and into submerged plants.

Question 23

How does air sacs help hydrophytes adapt?

Answer 23

Enables the leaves and flowers to float to the surface of the water.

Question 24

How does aerenchyma help hydrophytes adapt?

Answer 24

• Specialised parenchyma tissue forms in the leaves, stems and roots of hydrophytes.
• Many large air space.
  Functions include:
  -Making the leaves and stems more buoyant.
• 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 anoxic (extreme low oxygen conditions) by transporting oxygen to the tissues.

Question 25

How does hydrophytes prevent the roots becoming water-logged?

Answer 25

Have special roots which grow upwards into the air, allowing them to take in oxygen , which is then passed down into the submerged parts.

Question 26

How do hydrophytes transpire?

Answer 26

Many plants contain specialised structures at the tips or margins of their leaves called hydathodes. Those structures can release water droplets which may then evaporate from the leaf structure.