1. Exchange and Transport
13.10 Limiting water loss in plants

2. Learning outcomes Students should be able to understand the following:
How terrestrial plants are adapted to efficiently exchange gases and conserve water
How Xerophytic plants are adapted to live where water loss may exceed their water uptake
How Xerophytes are adapted to limit water loss through transpiration

3. Learning outcomes Candidates should be able to:
Explain structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants

4. Balancing gas exchange with water loss Insects and plants compared
Both have waxy / waterproof coverings over their gas exchange surfaces to limit water loss by evaporation
Both can close the openings to their gas exchange systems (spiracles / stomata) to regulate water loss
In plants stomata are predominantly on the underside of leaves to limit water loss by evaporation
Insects have evolved small SA : Volume ratios to limit water loss from the body surface
But the leaves of plants require a large surface area in order to absorb light and exchange gases for photosynthesis

5. Adaptations of plants
Plants in different habitats are adapted to cope with different problems of water availability:
Mesophytes - plants adapted to a habitat with adequate water
Xerophytes - plants adapted to a dry habitat
Halophytes - plants adapted to a salty habitat
Hydrophytes - plants adapted to a freshwater habitat

6. small leaf surface area stomata on lower surface of leaf only
Some adaptations of xerophytes are:
Adaptation
How it works
Example
small leaf surface area
less area for evaporation
conifer needles, cactus spines
low stomata density
fewer gaps in leaves
stomata on lower surface of leaf only
more humid air on lower surface, so less evaporation
most dicots
sunken stomata
maintains humid air around stomata
marram grass, pine
stomatal hairs
marram grass, couch grass, heather

7. shedding leaves in dry/cold season succulent leaves and stem
Some adaptations of xerophytes are:
Adaptation
How it works
Example
thick cuticle
stops uncontrolled evaporation through leaf cells
Most dicots
Evergreens
e.g. Holly
shedding leaves in dry/cold season
reduce water loss at certain times of year
deciduous plants
folded leaves
maintains humid air around stomata
marram grass,
succulent leaves and stem
stores water
cacti
extensive roots
maximise water uptake

8. All Cacti are xerophytes
Very dry habitat – not much rainfall

9. Left and right Epidermis of the cactus Rhipsalis dissimilis.
Left: View of the epidermis surface. The crater-shaped depressions with a guard cell each at their base can be seen.
Right: X-section through the epidermis & underlying tissues. The guard cells are countersunk, the cuticle is thickened. These are classic xerophyte adaptations.
Cacti have sunken stomata, thick waxy cuticle, spines (reduced SA of leaves) and store water in stems

10. Transverse Section Through Leaf of Xerophytic Plant

11. Marram grass
Habitat is different to cacti – plentiful rainfall but sand dunes with windy conditions so high water loss a problem

12. Marram grass
Marram grass has sunken stomata, stomatal hairs and curl up leaves (decreases water potential from IN to OUT)

13. Marram grass
Marram grass also has extended root system to help get to water which drains away rapidly through sand dunes

14. Written tasks AQA AS Biology textbook pg 201 Summary questions 1-4
2. Read ‘Not only desert plants have problems obtaining water’ on page 201 then answer application questions 1-4
3. Complete the exam style question about Xerophytic plants

15. Mark scheme - exam style question
Answers – Xerophytes question
1.(a) E. superba; Largest proportion of shallow roots; 2
(b) Roots go to greatest depths; Able to get water when surface soil dried out; or Greatest root mass; Able to store more water; 2
(c) Curled leaves; Thick cuticle; Sunken stomata; Hairs; max 2
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16. Homework
Referring to the AQA AS Biology textbook and other (internet) sources:
Use pictures and brief explanations to show how a variety of Xerophytic plants are adapted conserve water in dry habitats

17. Learning outcomes Students should be able to understand the following:
How terrestrial plants are adapted to efficiently exchange gases and conserve water
How Xerophytic plants are adapted to live where water loss may exceed their water uptake
How Xerophytes are adapted to limit water loss through transpiration