1. Adaptations to arid environments
Biology ATAR Unit 2
Heinemann Biology 1
Chapter 13; pages
Adaptations to arid environments

2. Keywords Transpiration Evaporation Potometer Stomata Guard cells
Flaccid
Turgid
Phyllode
C4 & CAM photosynthesis plants
Succulents
Photosynthetic stems
Acquifer

3. Learning outcomes
Understand, describe and explain the adaptations plants have to be able to survive in hot, dry conditions

4. Photosynthesis & respiration
During the day, the rate of photosynthesis in plants is greater than the rate of respiration
More CO2 is used up in photosynthesis than can be produced by respiration
The net result is that during the day plants take up CO2 and release O2
At night plants take in O2 and release CO2

5. Transpiration
Transpiration is the movement of water up through the xylem from the roots to the leaves
Transpiration is driven by evaporation at the leaves
Evaporation creates a pulling force that continually draws more water up from the roots

6. Plant adaptations
Plants in different habitats possess different adaptations:
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

7. Effect of temperature If water loss ˃ water gain:
High temperatures increase the rate of evaporation from the leaves
This increases the rate of transpiration
Roots need to absorb increasing amounts of water to keep pace with the rate of transpiration
If water loss ˃ water gain:
Cells become flaccid (lose water)
Stomata close
No CO2 diffuses into the plant
Photosynthesis slows down & stops
Plant wilts
Plant dies unless situation is reversed

8. Leaf adaptations Leaf adaptations aim to: Reduce rate of evaporation
Reduce leaf temperature – reflect heat
Reduce exposure to the sun’s heat – surface area
Create humid micro- environments – slows evaporation

9. Leaf adaptations Long narrow leaves Fewer stomata
Stomata underneath the leaf
Sunken stomata
Hairs on leaf
Leaves that hang vertically
Waxy cuticle
Rolled up leaf
Modified leaf structure (phyllodes in Acacia)
Open stomata at night, close during day (C4 & CAM photosynthesis plants)

10. Sagebrush grows in drier climates and areas with high exposure to sunlight.  The leaves are small and tightly clustered.  When it gets dry enough, sagebrush will often look dead. Sometimes the bush is blown off its roots by the wind.  This is one way its seeds are distributed.

11. The Utah juniper is a medium-sized tree
The Utah juniper is a medium-sized tree.  It has extremely small leaves that are often referred to as needles.  The leaves grow in tight clusters and have a dry texture. 

12. Root hairs All plants have root hairs
Root hairs are extensions of root epidermal cells
These extensions greatly increase the surface area for water absorption and maximise water uptake
Root hairs are found near the root tip

13. Root adaptations
Extensive, large root systems that cover a large volume of soil
Roots that form a network along the surface to absorb dew
Tap roots that penetrate to the acquifer (underground water)

14. Succulents Succulents have a few specialised adaptations:
Their stem and/or leaf cells can store water
Some have photosynthetic stems (eg. cacti)
This reduces evaporation while increasing energy production

15. Structural adaptations
Plants lose a lot of water through their leaves.
The mulga tree leaves are reduced to spines, which reduces water loss through the smaller leaf surface.
The boab tree has its own water storage area in a swollen trunk. .

16. Structural adaptations
The mulga tree's root system lies close to the base of the tree.
The tree's spiny leaves grow upward.
When it rains the leaves capture the water and funnel it down along the branches to the center of the tree.
The water then falls to the ground near the trunk of the tree where tree roots are concentrated.

17. Annual desert plants
Annual desert plants also have special adaptations:
Short life cycle
Rapid growth and flowering period
Produce seeds quickly
When significant rain falls over a short period their seeds germinate and grow quickly while there is enough water available.
The seeds produced must be heat and drought tolerant

18. Xerophyte adaptations summary
How it works
Example
thick cuticle
stops uncontrolled evaporation through leaf cells
small leaf surface area
less surface area for evaporation
conifer needles, cactus spines
low stomata density
smaller surface area for diffusion
sunken stomata
maintains humid air around stomata
marram grass, cacti
stomatal hairs (trichores)
marram grass, couch grass
rolled leaves
marram grass
extensive roots
maximise water uptake
cacti 18