Bean Seeds on a Windowsill
A class plants 12 bean seeds in identical cups of the same soil and waters each with 30 mL every other day. They place the cups in four groups of three: Group A on a sunny windowsill, Group B in a dark closet, Group C on the sunny windowsill but watered with 90 mL every other day, and Group D in the closet but kept in a sealed clear bag so air cannot move in or out. After two weeks, Group A plants are green and about 10 cm tall. Group B plants are pale yellow, thin, and about 14 cm tall. Group C plants are green but wilted with yellow lower leaves and mold on the soil. Group D seeds have not sprouted. A student concludes, "Light makes plants grow tallest, so the closet plants must be healthiest."
1. Evaluate the student's conclusion using the data from all four groups. Which group is the best evidence against it, and why?
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The student confuses height with health. Group B plants were tallest (14 cm) but pale yellow and thin, which signals they were stretching toward light and could not make food normally.
Best evidence against the conclusion: Group A. Those plants had light, were green, and were sturdy, even though shorter. Green color indicates the plants were making their own food using light, which the pale closet plants were not doing well.
Group D is also useful: those seeds never sprouted at all, showing that something besides light (air) is required just to start growing. Height alone is a poor measure; a complete answer should name a better measure, such as color, leaf size, or number of healthy leaves.
2. Group C received three times the water of Group A and did worse. What does this tell you about the claim "more water is always better for plants"? Use Group C's symptoms as evidence.
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The claim is false. Group C's symptoms - wilted stems, yellowing lower leaves, and mold on the soil - point to too much water rather than too little.
Mold growing on wet soil shows the soil stayed soggy. Soggy soil can crowd out air around the roots, so roots cannot take in what they need even though water is plentiful. The wilting looks like a thirst symptom but is actually caused by drowned, damaged roots.
Reasoning pattern: More of a needed resource is helpful only up to a point. Group A (30 mL) was healthier than Group C (90 mL), so the useful amount is closer to the smaller value. A good answer recommends testing several water amounts to find the best range rather than assuming more is better.
3. Group D seeds never sprouted, even though they had water and soil like the others. Propose the most likely missing requirement and describe a follow-up test to confirm it.
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Most likely missing requirement: air. The sealed clear bag let light in but blocked air movement, and seeds need air (oxygen) to sprout.
Follow-up test: Use two identical sets of seeds in the closet - one in a sealed bag and one in an open cup - with the same soil, water amount, and temperature. If the open cup sprouts and the sealed one does not, air is confirmed as the cause.
Key control: The closet is a good choice because it removes light as a possible explanation, isolating air. A strong answer also notes that the sealed bag could trap excess moisture, so the open-cup comparison should use the same water amount to rule that out.