Interpreting a Plant Growth Experiment
A student grows four groups of bean plants under different light conditions for three weeks. Group A receives 12 hours of white light daily; Group B receives 12 hours of red light; Group C receives 6 hours of white light; Group D receives 12 hours of white light but is watered half as often as the others. All groups start with 10 seeds in identical soil and pots at the same temperature. At the end, average heights are: A, 14 cm; B, 9 cm; C, 8 cm; D, 6 cm. The student concludes that red light is harmful to bean plants and that white light is best. A classmate points out that Group D's result may not reflect light at all.
1. Evaluate the student's conclusion about red light using the design of the experiment.
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The conclusion about red light is not well supported. Group B received 12 hours of red light and grew less than Group A, which received 12 hours of white light, so red light produced less growth under these conditions. But the experiment did not test red light against darkness or against a range of red intensities, so calling red light 'harmful' goes beyond the data. The comparison also confounds color with total light quality, since white light contains many wavelengths. A defensible statement is that, over three weeks, 12 hours of white light produced more average height than 12 hours of red light in this setup, without claiming harm.
2. Identify the flaw involving Group D and explain how it affects the interpretation of Groups A, C, and D together.
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Group D differs from Group A in two ways at once: it receives the same light but half the water. Because water is a second variable, Group D cannot be used to isolate the effect of light. Its low height of 6 cm may result from underwatering rather than from light conditions. This matters because the student might otherwise compare A, C, and D as if light duration were the only difference. In fact, only Groups A, B, and C vary light while holding watering constant, so conclusions about light duration should rest on A versus C. Group D is better treated as a separate test of watering, or the design should be revised so that water is held constant across all light comparisons.