The Shaded Tray: What Really Limited the Bean Plants?
Ms. Okafor's Grade 4 class is growing bean plants in the school greenhouse to test the claim that plants need light to grow. Two trays were planted on the same day with the same seeds and the same soil mix. Tray A sits on a sunny shelf; Tray B sits on a lower shelf in shade. Both trays get 100 mL of water every Monday, Wednesday, and Friday. After three weeks, Tray A plants average 14 cm with dark green leaves, while Tray B plants average 9 cm with pale yellow-green leaves. Two other differences went unnoticed at first: the lower shelf also blocks airflow, and Tray B's soil stays damp for about two days longer after each watering. The class wants to write that light alone caused the difference.
1. Evaluate the claim "light alone caused the difference." Name the strongest alternative explanation available in the setup and explain the mechanism by which it could produce the same result.
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The data are consistent with light mattering, but they do not show light alone caused it.
- The 5 cm height gap and the darker leaves fit the idea that shaded plants make less food and grow less.
- But Tray B's soil stays wet about two days longer. Roots in constantly wet soil get little air, and root damage slows growth and yellows leaves - the same symptoms, with no role for light.
- Still air on the lower shelf is a second candidate: higher humidity and warmer leaves change how much water the plant loses.
So the honest conclusion names the pattern and the limits: sunny-shelf plants grew taller and greener, but the shaded tray was also wetter and stiller, so light is not yet isolated as the cause.
2. Describe one concrete change to the setup that would separate the effect of light from the effect of soil moisture, and explain why it works.
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Replace the solid shelf with a frame draped in black breathable cloth, so Tray B gets shade but the same airflow as Tray A.
Then water by need rather than by calendar: push a finger or a soil-moisture meter 2 cm into each tray and water only when the reading matches, or set both trays in saucers and give the shaded tray less water so both soils dry at the same rate.
Why it works: the two trays now differ in one variable - light reaching the leaves. Airflow and soil wetness are held the same, so if Tray A still outgrows Tray B, light becomes the best remaining explanation. Adding three plants per tray and swapping tray positions weekly also spreads out any shelf-to-shelf differences in temperature.
3. The class places a thermometer in each tray and records temperature every day at noon. What pattern of readings would weaken the light explanation, and what pattern would leave it standing? Explain your reasoning.
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Pattern that weakens it: the shaded tray reads clearly cooler (say 19 °C vs 24 °C) every day. Cold slows enzymes and growth, so temperature alone could explain shorter, paler plants, and light would no longer be the only cause.
Pattern that leaves it standing: the two trays read within about 1 °C of each other all three weeks. Then temperature is unlikely to explain a 5 cm gap, and light (plus the still-uncontrolled soil wetness) remains the best candidate.
Either way, one week of readings is weak evidence - the class should look at the whole record, not a single noon. A thermometer does not fix the design; it only tests whether temperature is a live rival explanation.