The Fading Tomato Seedlings
A student grows tomato seedlings in two identical trays on a windowsill. Tray A receives full sunlight for 8 hours a day. Tray B is covered with a cardboard box for 6 of those hours, receiving only 2 hours of light. Both trays get the same amount of water and are kept at 22 °C. After 14 days, Tray A seedlings are 9 cm tall with broad dark-green leaves. Tray B seedlings are 13 cm tall but pale yellow with small, narrow leaves. The student concludes that light harms plant growth because the shaded plants grew taller. A classmate argues the result shows light is needed for healthy leaves but not for stem elongation. The teacher asks the class to evaluate both claims using the data and their knowledge of photosynthesis and plant responses.
1. Evaluate the student's conclusion that light harms plant growth. Use the measurements and leaf appearance to justify your judgement.
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The conclusion is not supported. Taller stems in Tray B (13 cm vs 9 cm) do not mean better growth. The pale yellow colour shows less chlorophyll, so photosynthesis is reduced. The narrow leaves give less surface area for light absorption. The extra height is etiolation: stems elongate to reach light, which is a survival response, not healthy growth. A valid conclusion needs more than one measure. Here height alone misleads; leaf colour, leaf width and likely dry mass all favour Tray A. So light is not harmful - it is needed for chlorophyll formation and normal leaf development, while low light triggers elongation. The student confused one measurement with overall health.
2. The classmate says light is needed for healthy leaves but not for stem elongation. Is this a fair reading of the evidence? Explain what further measurement would strengthen the judgement.
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It is partly fair but incomplete. The evidence does support a role for light in leaf health: Tray A leaves are dark green and broad; Tray B leaves are pale and narrow. The stem result also fits the claim that elongation is not simply driven by light, since shaded stems were taller. However, the claim implies elongation is independent of light, which is wrong: the elongation is a response to low light (etiolation), so light does influence stem growth, just in the opposite direction. A fairer statement is that light promotes leaf development and suppresses excessive stem elongation. To strengthen the judgement, measure stem dry mass and leaf area per plant, and repeat with several plants per tray to reduce the chance that one unusual seedling drives the result.