Two Variables in a Bean Investigation
Third graders plant bean seeds in cups. Group A places cups on the windowsill and waters 50 mL every other day; Group B places cups in a dark cupboard and waters 10 mL every other day. After two weeks, Group A plants are taller and greener. Students conclude 'sunlight makes plants grow.' Mr. Ellis wants them to write a claim-evidence-reasoning paragraph. One student asks why the cupboard plants 'didn't die if plants need sun to make food.' Mr. Ellis has 12 cups, one sunny window, one cupboard, and two weeks before the unit ends.
1. Identify the design flaw and describe a revised investigation that fits Mr. Ellis's constraints.
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The investigation changes two variables-light and water-so the taller, greener plants in the window cannot be attributed to sunlight. The conclusion is confounded.
With 12 cups, use 6 per condition. Keep water amount, watering schedule, soil, seed type, and cup size identical; vary only light. Measure height, leaf number, and color every two days. If possible, rotate cup positions at the window to reduce draft or heat differences. The revised claim can only be about light under these conditions, and the class should acknowledge that one trial is limited.
2. How should Mr. Ellis respond to the student's question about the cupboard plants not dying?
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The dark plants did not die immediately because the seed contains stored food in the cotyledon. The seedling grows initially by using that reserve and by cell elongation; in darkness it becomes pale and spindly (etiolated). Without light it cannot photosynthesize enough to sustain growth, so it would yellow and die after the reserves run out.
Also, in the flawed design the dark cups received less water, so their fate is not a clean test of light. Use the question to distinguish 'grows for a while' from 'grows sustainably.' That distinction is exactly the reasoning the CER paragraph should include.
3. What should a strong claim-evidence-reasoning paragraph look like for this class, and how should it be scored?
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A strong paragraph states a claim that matches the design: 'In our investigation, bean plants in the window grew taller than plants in the cupboard.' Evidence: mean height and leaf color on day 14. Reasoning: 'Because the only planned difference was light, the difference in growth is consistent with light being needed for sustained growth; however, seeds can grow briefly without light using stored food.'
If the original design is kept, the claim must say the results are inconclusive because water also differed. Have students add a next-investigation sentence. The rubric should score whether the claim matches the design and whether evidence is used accurately, not whether the paragraph matches a memorized answer.