Unit 2 Biology

Plant Behaviour

WEEK 3: Plant BehaviourLEARNING OUTCOMESBy the end of this week, you should: Have an understanding of the behavioural

adaptations in plants. Recognise the importance of behaviours to the

survival of individuals and groups.

Plant BehaviourPlant growth and

development is influenced by internal and external factorsInternal factors –

hormones e.g. auxins

External factors – (name 4 examples)

Watch the Mythbusters

clip “Talking to Plants”

How do plants respond?TROPISMS:A directional growth response of a plant to

an environmental stimuluse.g. light, water, gravity, touch, day length,

water.Positive tropism: plant growing towards the

stimulus.Negative tropism: plant growing away from

stimulus.

What does it mean?Look at the following tropisms and discuss with

the person next to you what they might be responses to:

Gravitropism

Hydrotropism

Galvanotropism

Thigmotropism

Chemotropism

Phototropism

Photoperiodism Heliotropism

What is happening here?Which stimulus made

these plants grow this way?

How did the plants do it?

Why would they grow this way?These plants are not

necessarily moving in a certain direction, but growing in that direction.

TropismsPhototropism: growing towards the light.Geotropism: responding to gravity

Negative geotropism: a shoot grows upwards.

Positive geotropism: a root grows downwards.

Photoperiodism: responding to changes in day-length.

Plant Hormones - AuxinPlants have hormones that control their

growth and development.Auxin is produced in the coleoptile (the

growing tip of the shoot).Where present, auxin causes elongation of

cells.When affecting growth in a shoot:

Receptor: tip of shootEffector: growing region below tip

Auxins and phototropismAuxin is deposited on the ‘dark side’ of the

growing shoot tip (away from the light source).

Therefore cells on the darker side grow faster.

Uneven growth = bending of tip.

1 2 3

Draw and label this:

Now explain what is

happening to the

person next to you.

Phototropism experiment

Can you explain this?1. The shoot did not bend because the tip was

removed and this is where the auxin is produced.2. The shoot did not bend because the tip was covered

and so was not affected by the light.3. The shoot did bend because it was covered by a

transparent cap and so the light could affect the tip.4. The shoot did bend because auxin is not produced

in the base of the shoot. 5. The shoot did bend because the auxin can diffuse

through the gelatin block. 6. The shoot did not bend because auxin cannot pass

through the mica.

12 3 4 5 6

What is happening here?

How do plants always grow the right way up?

GeotropismGeotropism (also called

gravitropism) is a plant response to the Earth's gravitational field.

Roots are positively geotropic because they grow ‘towards gravity’ (down).

Shoots are negatively geotropic because they grow ‘away from gravity’ (up).

What part of the plant might respond to a positive geotropism?

What part of the plant might respond to a negative geotropism?

GeotropismNegative geotropism is

advantageous because:Leaves and flowers are

lifted off the ground.The plant can better

compete for light and CO2.

Flowers brought into good position for wind and insect pollination.

Seed dispersal is more effective.

What are the advantages of

negative geotropism in tall shoots?

GeotropismPositive geotropism is

advantageous because:

Roots penetrate the soil.

This is the source of water and mineral salts for the plant.

It also anchors the plant.

What are the advantages of

positive geotropism in roots?

Auxins and GeotropismWhen present in a root, auxin inhibits

elongation – they grow slower.So it is the upper part of the root that

grows faster so the root will bend downwards towards gravity.

What about the shoots?In a horizontal seedling, auxin accumulates

along the lower horizontal part of the shoot causing the cells in that part to grow faster.

Hence, the shoot turns upwards, away from gravity.

What is going to happen?

EXPERIMENT: Geotropism

Can you explain this?What happens to a plant when it falls

on its side?Explain it to the person next to you.

Draw a diagram – include where the auxin works.

Extension:

Which tropism?

Other TropismsThigmotropism – response to

touch/contact.Circadian rhythm – like a biological clock,

an activity that follows a 24-hour cycle. This movement is called nastic movement (not influenced by direction of stimulus).

Heliotropism – solar tracking (following the sun).

Photoperiodism – response to periods of light and dark. Flowering is affected by this.

Short or long day plant?

What is going on here?