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Evolution - adaptations: Part 6. More
on plant adaptations including those living in extreme conditions - plant
extremophiles
Doc Brown's GCSE level Biology exam study revision notes
There are various sections to work through.
Sub-index for evolution - adaptations biology notes
(6)
Adaptations in plants
- emphasis plants living in extreme environments
You should be able to describe physical
and chemical defence responses by plants to
resist e.g. invasion of microorganisms, though most plants would not
be considered as extremophiles. Typical examples of adaptations of
'non-extremophiles' are summarised below
Chemical plant defence responses:
Production of antibacterial
chemical e.g. mint and witch hazel.
Production of poisons to
deter herbivores eg tobacco plants, foxgloves and deadly
nightshade.
Plants physical defences adaptations
to respond to and resist microorganisms
Tough cellulose cell walls
Tough
waxy cuticle on leaves to reduce water loss.
Layers of dead cells around stems (bark
on trees) which fall off taking pathogens with them.
Mechanical physical adaptations
Thorns and hairs on plants to deter
animals from eating or touching them.
Leaves which droop or curl
when touched.
Mimicry to trick animals into
not eating them or not laying eggs on the leaves i.e the plant
looks like something the animal would not want to eat.
Some types of plant adaptation, often classed as
extremophiles
Adaptations of
hydrophytes
Hydrophytes are plants that are especially adapted to
living in aquatic environments.
They are also referred to as macrophytes to
differentiate them from algae and other microscopic aquatic plants.
Hydrophytic plants, in terms of the aquatic environment,
emergent, submerged or floating on the surface.
Adaptations that are commonly seen in hydrophytes
include floating leaves which are thin, flat and have large air spaces
inside to give them buoyancy. This keeps them close to the surface
of the water where there is more light for photosynthesis.
Adaptations of
xerophytes
These are plants that have well developed roots.
They store water in succulent water storing
parenchymatous tissues.
They have small sized leaves with waxy coating.
Some of the xerophytes complete their life cycle within
a very short period when sufficient moisture is available.
Cacti galore!
(pictures
of cacti from the Leicester University Botanic Garden)
Know and understand that plants may be adapted to survive in dry environments
by means of structural adaptations ...
Examples of plant adaptations are
described below, many are to do with controlling water uptake and retention.
Plants are adapted to live in a variety of environments including extreme environments that are very hot
and/or very dry like deserts.
These adaptations affect, in
particular, the size and shape of a plant's leaves, cuticle and the
number and position of the stomata.
Plants grow well in warmer climates,
particularly summer, as conditions favour photosynthesis to build up
food stores for the colder winter.
See
factors affecting rate of photosynthesis.
Changes to surface area, particularly
roots, and the leaves
- through which water is naturally lost by transpiration
In dry climates, minimising surface area can also
minimise water loss by evaporation - the final stage of transpiration.
So many plants show adaptations to cope with these low
rainfall areas.
In dry conditions of 'non-dry' climate areas, the leaves
of many plants will curl up to minimise surface area - a sort of
temporary adaptation, since the leaves return to their normal shape and
surface area after rainfall.
Even narrow roots are further covered
in tiny root hairs that greatly increase the surface area even more and
hence increase the efficiency (rate) of water absorption.
This adaptation means the water has
only got to move a short distance to the xylem and transported up
through the whole of the plant.
In contrast, in hot climates, to reduce the surface area,
to
reduce water loss by evaporation, plants like cacti have a rounded shape with thin spines instead of broader
leaves.
Plants like cacti have evolved
systems of storing water in their tissues to conserve water.
Spines also deter animals from
feeding on the plants like cacti.
See
Plant diseases and defences against pathogens
and pests
Pine trees grow up in a cone shape
to expose the most thin pine needles to the sun - increase in
surface area, increases the rate of photosynthesis.
Marram grass grows on sand dunes and
has adaptations to reduce water loss by transpiration in dry windy
conditions.
The leaves are compacted and rolled
with the stomata sunk into pits in the thick waxy cuticle.
Also, interlocking leaf hairs retain
water vapour, so all these features reduce water loss.
Water storage and water retention tissues
Plants are well adapted to live in
extreme environments e.g. very hot and very dry environments.
Adaptations are often to do with the
size and shape of leaves, cuticle structure and the number and position
of stomata.
Many adaptations are about reducing
the rate of evaporation of water and retaining or storing water.
Illustrates many of the adaptations described here.
Plants in hot and very dry environments tend to
have thick waxy cuticle to reduce water loss by evaporation.
Plants like cacti, living in deserts,
tend to have adaptations which help them to conserve water - retain as
much water as they have access too.
Plants like cacti have
relatively thick fleshy stems which contain groups of specialised cells that
store water.
Some giant cacti like the saguaro cactus in the deserts of
Arizona (USA) can be 20m high and hold in storage several tonnes of water -
more than enough to see it through the driest of dry seasons and survive
long periods of drought.
Cacti also have a water repellent
thick waxy layer (the cuticle) which further reduces water loss by
evaporation.
Leaves can be curled or have hairs on
their surface.
This reduces air flow over the leaf keeping more water
vapour near the surface and so reducing the diffusion of water vapour
from the leaf surface to the surrounding air.
Needle-like spines or very small
leaves, reducing surface area, also have a similar
effect in reducing loss of water by evaporation - spines also deter
animals from eating them and reduce air flow.
A lot of water is lost by
evaporation from the stomata.
Most stomata are on the underside of
leaves and so evaporation of water is less affected by environmental
conditions such as sunlight or wind. See also ...
Plant cells
- transport, transpiration, absorption of nutrients, leaf & root structure
Plants with fewer stomata on their
leaves or have stomata that only open at night are adaptations that
reduce water loss by evaporation.
Stomata can be sunk in pits below the
leaf surface, this reduces flow of air across
the stomata and so less water vapour carried away - helping water
retention.
Extensive root systems
Cacti generally have one of
two kinds of root system
(i) Some have relatively few roots, but roots
that can burrow deep into the ground to seek out underground water.
(ii)
Most other cacti have many shallow spread out roots that can rapidly absorb
water quickly over a large area eg if it rains, which may be very infrequent in desert regions.
Other 'defences':
 Know and understand that plants may be adapted to cope with
specific features of their environment, these specialised features to deter
predators include thorns and
poisons to deter 'predators' e.g.
Roses have thorns, cacti have sharp spines to deter
animals (herbivores) eating them, turtles, armadillos and tortoises have
hard protective shells. These are examples of organisms having a sort of
'armour' for protection!
Plants like ivy contain poisons that
deter animals from eating them.
Some desert shrubs secrete toxic
compounds into the soil to prevent other plants growing nearby.
Some species of epiphytes grow in
rainforests and exhibit several adaptations to survive by growing above
ground level e.g.
(i) They can grow on other plants,
preferentially capturing sunlight through the trees for photosynthesis.
They are not parasites and do not extract
nutrients from the host plant.
(ii) These epiphyte type of plants have
roots that rely on nutrients from the air, falling rain, and the compost
(leaf litter) that lies on tree branches.
(iii) They can have upturned leaves that
capture and store rainwater or dew.
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