The Case of the Wilting Seedlings
A student grows four identical bean seedlings in identical pots of the same soil. Each pot gets 50 mL of water every second day. Pot 1 sits on a sunny windowsill. Pot 2 sits in a dark cupboard. Pot 3 sits on the sunny windowsill but its leaves are wrapped in clear plastic. Pot 4 sits on the sunny windowsill but is watered with 100 mL every second day. After ten days, Pot 1 is green and upright. Pot 2 is tall, pale yellow, and bent toward the cupboard door. Pot 3 is green but its leaves feel damp and one leaf has grey fuzz on it. Pot 4 is green but its lower leaves are yellow and the soil smells sour. The student concludes, "Too much water is the only thing that harms plants."
1. Evaluate the student's conclusion using all four pots.
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The conclusion is too narrow. Pot 2 was harmed without any change in watering: it grew tall and pale because it had no light for photosynthesis, and it bent toward the door, showing it grows toward light. Pot 3 had normal water but damp leaves and grey fuzz, likely a mould growing because the plastic trapped moisture and blocked airflow. Pot 4 does show harm from too much water - yellow lower leaves and sour-smelling soil suggest the roots are waterlogged and lack air. So light, airflow around leaves, and water amount can all affect plant health. The evidence supports 'several factors affect plant health', not 'only too much water harms plants'.
2. Which pots act as a fair comparison for the effect of light, and why?
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Pots 1 and 2 are the fair comparison for light. They have the same soil, pot, and watering (50 mL every second day), and the only planned difference is light: Pot 1 is on the sunny windowsill and Pot 2 is in the dark cupboard. Pot 3 differs in airflow and moisture around the leaves, and Pot 4 differs in water amount, so neither isolates light. To be a controlled comparison, only one variable should differ. Comparing Pot 1 with Pot 2 lets the class link the pale, tall, bent growth in Pot 2 to the lack of light.