This PowerPoint answers the following questions: Do you understand that plants respond to stimuli? Can you give an example of positive phototropism? How do plant roots and stems respond to gravity? It covers section 3.3 of the IGCSE Edexcel Biology Course.

1. IGCSE Biology – 4BI0
MAKING SENSE OF
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3. Copyright © 2015 Henry Exham
3.3 Chemical Coordination in Plants - Tropisms
Objectives
3
• Do you understand that plants respond to
stimuli?
• Can you give an example of positive
phototropism?
• How do plant roots and stems respond to
gravity?

4. Copyright © 2015 Henry Exham
3.3 Chemical Coordination in Plants - Tropisms
Plants can respond to their environment
4
• Its not any animals that can respond to their
environments.
Click on the images to watch the video about each plant
Venus Fly TrapMimosa Pudica

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3.3 Chemical Coordination in Plants - Tropisms
Tropisms
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• These examples are very quick responses
which are not typical in plants.
• A typical response in a plant is a change in
growth.
• For example a plant may detect a certain
stimuli like light, and start growing towards it.

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3.3 Chemical Coordination in Plants - Tropisms
Tropisms
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• A growth response of a plant to directional
stimuli is called a tropism. This is much slower
as it involves cell division to see the results.
• If it grows towards the stimuli it is a positive
tropism. If it grows away it is a negative
tropism.

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3.3 Chemical Coordination in Plants - Tropisms
Phototropism
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• Tropisms are given
different names dependent
on the stimuli.
• A tropism where the
stimuli is light is called
phototropism.
• The shoot of a plant grows
towards the light to
maximise photosynthesis.
• This is called positive
phototropism.
Cress seeds grown on a window sill

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3.3 Chemical Coordination in Plants - Tropisms
Phototropism
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3.3 Chemical Coordination in Plants - Tropisms
Geotropism
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• If there is no light under the soil, how does a
germinating seed know which way to grow?

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3.3 Chemical Coordination in Plants - Tropisms
Geotropism
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• If there is no light under the soil, how does a
germinating seed know which way to grow?
• Because the shoot is also negatively geotropic.
• So will detect gravity and grow in the opposite
direction.
• The roots are positively geotropic. So will grow
down to find mineral ions and water.

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3.3 Chemical Coordination in Plants - Tropisms
Hydrotropism
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• The roots of some species show positive
hydrotropism.

12. Copyright © 2015 Henry Exham
3.3 Chemical Coordination in Plants - Tropisms
What causes these responses?
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• The plant produces many different plant
hormones (plant growth substances).
• The most important one of these is called
auxin.
• It’s produced at the tip of the shoot and
diffuses downwards.
• It causes cell elongation
and division.

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3.3 Chemical Coordination in Plants - Tropisms
What causes phototropism?
13
• Auxin is produced at the tip of the shoot and
causes it to grow.

14. Copyright © 2015 Henry Exham
3.3 Chemical Coordination in Plants - Tropisms
What causes phototropism?
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• When light comes from one direction, the
auxins get transported to the shaded side.

15. Copyright © 2015 Henry Exham
3.3 Chemical Coordination in Plants - Tropisms
What causes phototropism?
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• The shaded side elongates faster than the
illuminated side and the shoot bends towards
the light

16. This PowerPoint is protected under copyright.
It is designed for educational use. Either personal study or to be
presented to a class. It may be edited or duplicated for these
purposes only.
It must not be shared or distributed online in any format.
Some images used are under a separate creative commons license,
these are clearly marked.
Copyright © 2015 Henry Exham
Brought to you by MrExham.com
TERMS AND CONDITIONS
16