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MAKING SENSE OF TROPISMS IGCSE Biology 3.3 Chemical Coordination in Plants Brought to you by MrExham.com Copyright © 2014 Henry Exham Terms and Conditions Copyright © 2014 Henry Exham 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. Brought to you by MrExham.com Copyright © 2014 Henry Exham • 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? Copyright © 2014 Henry Exham • Its not any animals that can respond to their environments. Click on the images to watch the video about each plant Copyright © 2014 Henry Exham • A growth response of a plant to directional stimuli is called a tropism. • Grows towards stimuli – POSITIVE • Grows away from stimuli – NEGATIVE Copyright © 2014 Henry Exham A tropism where the stimuli is light is called phototropism. WHY? • The shoot of a plant grows towards the light to maximise photosynthesis. • Called positive phototropism. Positive Phototropism Copyright © 2014 Henry Exham Copyright © 2014 Henry Exham • If there is no light under the soil, how does a germinating seed know which way to grow? Copyright © 2014 Henry Exham A tropism where the stimuli is gravity is called geotropism. WHY? • The root of a plant grows towards gravity to uptake water & mineral ions • Called positive geotropism. Positive Geotropism POSITIVE PHOTOTROPISM Stems grow upwards NEGATIVE GEOTROPISM Stems grow upwards NEGATIVE PHOTOTROPISM Roots grow downwards POSITIVE GEOTROPISM Roots grow downwards Copyright © 2014 Henry Exham • AUXIN = growth hormone produced by plants PRODUCED AT: Tip of the shoot – diffuse downwards ROLE OF AUXIN: It causes cell elongation & division. Copyright © 2014 Henry Exham 1. Auxin is produced at the tip of the shoot and causes it to grow. 2. 3. Auxin diffuses AWAY from the light into the shadowed side The shaded side elongates faster than the illuminated side and the shoot bends towards the light