1. Plant Responses

2. Plant Responses
Do plants have feelings? Do plants have senses? Can plants detect stimuli (and if so, which)? Can plants respond to stimuli?

3. Plant Responses
Plants don’t have a nervous system, but they do have senses, and they can respond to stimuli in their environment.
Plant responses are called tropisms. Tropisms are directional growth responses by plants. Tropisms can be positive – growing towards the stimulus, or negative – growing away from the stimulus.
Tropisms help plants survive by helping them find light or water. The stimuli that plants can detect include light, water, touch and gravity.
The response of plants to light is called phototropism.
The response of plants to gravity is called geotropism.

4. Investigating Phototropism in Cress
Method
Sow cress seeds in damp cotton wool in three Petri dishes.
Place the Petri dishes in a cardboard box as shown. Dish A has light from the side. Dish B has light from above. Dish C has no light.
Place the box in a window for 5 days.

5. Investigating Phototropism in Cress
Results and Conclusions
Draw the appearance of the cress seedlings in each dish.
Describe the appearance of the cress seedlings in each dish.
For each dish A-C explain why the seedlings have grown this way. What stimulus are they responding to? What kind of tropism is shown in each case?

6. Geotropism
Whichever way up a seed germinates, the shoot always grows up and the root always grows down. The shoots and roots are responding to gravity – geotropism.
Photo and Geotropism from ScienceBank (5min)

7. More Geotropism

8. Auxin
What causes shoots and roots to bend in tropisms? Bending is caused by a plant growth hormone called auxin. Normally auxin is distributed equally and the shoot grows straight. But if the auxin collects on one side that side grows faster, so the shoot bends.
Auxin from KeyScience (4min)

9. Summary of Plant Responses
Copy and complete this table
Tropism
Part of plant
Direction of growth
Advantage
Positive phototropism
Negative phototropism
Positive geotropism
Negative geotropism

10. Summary of Plant Responses
Tropism
Part of plant
Direction of growth
Advantage
Positive phototropism
Stem
Towards light
To find light for photosynthesis
Negative phototropism
Positive geotropism
Negative geotropism

11. Summary of Plant Responses
Tropism
Part of plant
Direction of growth
Advantage
Positive phototropism
Stem
Towards light
To find light for photosynthesis
Negative phototropism
Root
Away from light
To find water
Positive geotropism
Negative geotropism

12. Summary of Plant Responses
Tropism
Part of plant
Direction of growth
Advantage
Positive phototropism
Stem
Towards light
To find light for photosynthesis
Negative phototropism
Root
Away from light
To find water
Positive geotropism
Towards gravity
Negative geotropism

13. Summary of Plant Responses
Tropism
Part of plant
Direction of growth
Advantage
Positive phototropism
Stem
Towards light
To find light for photosynthesis
Negative phototropism
Root
Away from light
To find water
Positive geotropism
Towards gravity
Negative geotropism
Away from gravity

15. Investigating Geotropism with Dandelions
Take a plastic pipette, and snip off the end at a point where it is just wide enough to insert the base of your dandelion stem.
Fill the pipette with water by holding it under a tap.
Push the base of the stem into the pipette.
Gently push blu-tac around the stem and the pipette to prevent water leaking out.
Cut the bud off the top of the stem.
Stick the pipette down on the bench with blu-tac. Make sure that the stem is horizontal. Record the time.
Position a piece of graph paper behind the stem, to make it easier to see the movement.
Observe the stem at intervals, and record its height against the graph paper.

16. Place graph paper
behind the stem.
Record the height
of the stem at regular
intervals