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SITEMAP   School Biology revision notes: Evolution: Part 6 More on plant adaptations

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

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(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.

The upper side of a leaf is smoother and greener - richer in chloroplasts to capture the sunlight The under side of a leaf is rougher - more 'porous' for efficient gas exchange and the veins more prominent

See photosynthesis page for details of leaf adaptations to favour efficient rates of photosynthesis

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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