The Fading Potted Plant
Mei keeps two identical potted plants, A and B, on a shelf. Plant A stands beside a closed window that faces the morning sun; Plant B stands in the same room but inside a tall cardboard box open only at the top. Both pots hold the same soil and receive 200 ml of water every second day. After three weeks, Mei notices that A has broad, dark green leaves while B's new leaves are small and pale yellow. She also finds that B's soil stays wet longer than A's soil. Mei wonders whether the box, the water, or something else explains the difference.
1. Mei concludes that the box caused the pale leaves. Using the observations, explain why this conclusion is reasonable, and identify one observation that supports the idea that light is the key difference.
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The two plants are identical, share the same soil and the same watering schedule, and stand in the same room, so most conditions are controlled. The main difference is that the box blocks light from reaching B. Pale, small leaves are a typical sign of a plant grown with too little light, so blaming the box is reasonable.
The strongest supporting observation is that B's soil stays wet longer. With less light, B makes less food and grows more slowly, so it takes up less water, leaving more in the soil. This links the box to a change in the plant's activity, not just its colour.
2. A classmate says B's pale leaves are caused by too much water, since B's soil is wetter. Evaluate this alternative explanation.
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The alternative is weak because both plants receive exactly 200 ml every second day. The difference in soil wetness appears after watering, not because B is given more water. If extra water were the cause, we would expect the extra water to come from a different watering amount.
A fair test would give A and B the same water but different light, which is already the case. To test the water idea directly, Mei could give a third plant in a box less water and see whether its leaves stay pale. If they do, water is not the cause and light remains the better explanation.