Interpreting a Plant Growth Experiment
A student grows four groups of bean plants to test whether coloured light affects growth. Each group has 10 plants in identical pots with the same soil and water. Group 1 is under white light, Group 2 under red light, Group 3 under blue light, and Group 4 under green light. All groups are on the same shelf near a window, and the coloured lamps are switched on for 8 hours a day. After three weeks, average heights (cm) are: White 14.2, Red 15.1, Blue 13.8, Green 9.4. The student concludes: 'Red light makes plants grow tallest, and green light stops growth.' A classmate notices that the window also lets in daylight, and that the green-light group was placed at the end of the shelf nearest the door, which is opened often.
1. Evaluate the student's conclusion. Identify at least two problems with the experimental design and explain how each could affect the results.
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The conclusion outruns the design:
- Confounding light source. The window lets in white daylight for all groups, so the coloured lamps are not the only light. The 'red' group also received daylight; we cannot isolate the effect of red light.
- Position effect. The green group was nearest a frequently opened door - likely cooler, draughtier, and possibly shaded differently. Lower height (9.4 cm) could reflect temperature or disturbance, not green light.
- No replication of position. Each colour was tested in only one location, so colour and position are confounded. A fair test would rotate or randomise positions.
- 'Stops growth' is too strong. 9.4 cm is lower, not zero; the plants grew.
Judgement: The data are consistent with 'red group grew tallest' but do not establish causation by light colour. The green result is especially unreliable.
2. Propose an improved design that would let the student make a valid claim about light colour, and state what result would support the original hypothesis.
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Improved design:
- Use light-proof boxes with the coloured lamps as the only light source (remove window daylight), or use a dark room.
- Randomise and rotate pot positions regularly (e.g., weekly) so no group sits permanently near the door.
- Keep temperature and humidity equal across groups; measure them.
- Increase replication (more pots per colour) and record individual plant heights, not just averages.
- Include a control (white light) and keep all other variables identical.
What would support the hypothesis: If, with daylight excluded and positions randomised, the red group is consistently and significantly taller than white, blue, and green groups across repeated trials, then 'red light promotes greater height growth' is supported. The green group would need to be re-tested under controlled conditions before claiming it 'stops growth'.