Xerophytes

• what are xerophytes?

  plants with xeromorphic characteristics
• what do xeromorphic adaptations do?

  xeromorphic adaptations such as sunken stomata reduce water loss from the leaf by reducing the water potential gradient between the inside of the leaf and the atmosphere
• how have xerophytes adapted?

  they have adapted to living with low water availability and have modified structures which prevent excess water loss
• where do xerophytes live?

  they may live in hot, dry desert regions, cold regions where the soil water is frozen for much of the year or exposed, windy locations
• what is an example of a xerophyte?

  • ammophila arenaria (marram grass)
  • it colonises sand dunes
• where does marram grass live?

  there is no soil, rainwater drains away rapidly, there are high wind speeds, salt spray and a lack of shade from the sun
• how does marram grass hold its leaves?

  vertically
• what are the modifications that the leaves of marram grass have?

  • rolled leaves
  • sunken stomata
  • hairs
  • thick cuticle
  • fibres of sclerenchyma
• how does marram grass having rolled leaves help it?

  • large thin-walled epidermal cells, called hinge cells, at the bases of the grooves become plasmolysed when they lose water from the excessive transpiration, and the leaf rolls with its adaxial surface inwards
  • this reduces the leaf area exposed to air, and so reduces transpiration
• how does the leaves of marram grass having sunken stomata help it?

  • stomata occur in grooves on the adaxial surface, but not the outer (abaxial) surface, of the leaf
  • the stomata are in pits or depressions and humid air is trapped in the pit, outside the stomata
  • this reduces water potential gradient between the inside of the leaf and the outside and so reduces the rate of diffusion of water out through stomata
• how does the leaves of marram grass having hairs help it?

  stiff, interlocking hairs help trap water vapour and reduce the water potential gradient between the inside of the leaf and the outside
• how does the leaves of marram grass having a thick cuticle help it?

  • the cuticle is a waxy covering over the outer (abaxial) leaf surface
  • wax is waterproof and so reduces water loss
  • the thicker this cuticle, the lower the rate of transpiration through the cuticle
• how does the leaves of marram grass having fibres of sclerenchyma help it?

  fibres of sclerenchyma are stiff so the leaves shape is maintained even when the cells become flaccid
• where does the adaxial leaf surface face?

  it faces towards the central axis of a plant - its the upper surface of a leaf held horizontally
• where does the abaxial surface of a leaf face?

  it faces away from the central axis of a plant - its the lower surface of a leaf held horizontally
• what do many plants show a rhythm of?

  they show a diurnal (24h) rhythm of opening and closing stomata, independently of other factors, many open their stomata in the morning
• what is this an image of?

  diagram of a section of lea of marram grass