Biology ATAR Unit 2
Heinemann Biology 1
Chapter 13; pages 254-256
ADAPTATIONS TO ARID
ENVIRONMENTS
Keywords
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Transpiration
Evaporation
Potometer
Stomata
Guard cells
Flaccid
Turgid
Phyllode
C4 & CAM
photosynthesis plants
Succulents
Photosynthetic stems
Acquifer
Learning outcomes
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Understand, describe and explain
the adaptations plants have to be
able to survive in hot, dry
conditions
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
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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
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Plant adaptations
Plants in different habitats
different adaptations:
possess
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Mesophytes
plants adapted
adequate water
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Xerophytes
plants adapted to a dry habitat
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Halophytes
plants adapted to a salty habitat
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Hydrophytes
plants adapted to a freshwater habitat
to
a
habitat
with
Effect of temperature
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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
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Leaf adaptations
Leaf adaptations
aim to:
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Reduce rate of
evaporation
Reduce leaf
temperature –
reflect heat
Reduce exposure
to the sun’s heat
– surface area
Create humid
microenvironments –
slows evaporation
Leaf adaptations
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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)
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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.
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.
Root hairs
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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
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)
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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
Structural adaptations
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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.
Structural adaptations
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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.
Annual desert plants
Annual desert plants also
have special
adaptations:
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Short life cycle
Rapid growth and
flowering period
Produce seeds quickly
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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
Xerophyte adaptations summary
Adaptation
How it works
Example
thick cuticle
stops uncontrolled
evaporation through leaf cells
small leaf surface
area
less surface area for
evaporation
low stomata
density
smaller surface area for
diffusion
sunken stomata
maintains humid air
around stomata
marram grass, cacti
stomatal hairs
(trichores)
maintains humid air
around stomata
marram grass, couch
grass
rolled leaves
maintains humid air
around stomata
marram grass
extensive roots
maximise water uptake
cacti
conifer needles,
cactus spines