Investigation of the phototropic responses
Objective: To study the phototropic response of the shoot of the seedling(1 mark)
Biological principle and experiment design: (3 marks)
Plants can detect light (stimulus) and respond to them. However, the responses of plants are very slow, and usually involve growth of certain parts of the body towards or away from light.
To investigate the phototropic responses of the plant shoots, young seedlings receive the illumination of unilateral light (1).
To ensure that the responses of the plant are due to the unilateral stimulus, same numbers of same batch of
seedlings are placed on a clinostat (1), so that the effect of the unilateral light is removed. In this set-up, the seedlings receive light from different direction. This set-up is referred as control set-up (1)
The following labeled diagram shows the experimental set-up and the control set-up:
Correct experimental set-up (1)
Correct control set-up (1)
Lateral light (1)
Labels (1): seedlings, shoots, petri dish,
cotton wool etc
Materials and instruments:
Clinostat X 1 Culture medium (for growth of seedlings) appropriate amounts
Petri dish X 2 seedlings (germinating mung beans) appropriate amounts
Cotton wool appropriate amounts
Procedures: (1 mark)
1. Prepare two petri dishes with same amounts of cotton wool and culture medium. Place 10 mung beans on each of the petri dish respectively.
2. Fix one petri dish on a clinostat which rotates horizontally. Switch on the clinostat so that it rotates at
about 4 revolutions per hour. This is the control set-up.
Results: (4 marks)
The table below shows the growth of seedlings in different set-up
Ways of growth of shoot of the seedlings
Experimental set-up /
Set-up without clinostat
The shoots grow towards the light.
Control set-up /
Set-up with clinostat
The shoots grow vertically without bending.
Discussion: (4 marks)
1 Based on the experiment results, describe the growth response of shoots to light. What is the significance of such response to plants? (3 marks)
The growth of shoot is positively phototropic (1).
This allows the leaves to obtain more light (1)
to carry out photosynthesis more efficiently to produce food (1)
2 Both set-ups should be maintained at the same height. Why? (1 mark)
To ensure that the seedlings in these two set-ups receive same the amounts of light (1).
Conclusion: (1 mark)
Growth of the shoot of seedlings is positively phototropic (1)