The Plant Experiment
A student wants to investigate how light affects the growth of bean plants. They plant 10 bean seeds in identical pots with the same type and amount of soil. They place 5 pots on a sunny windowsill and 5 pots in a dark closet. They water all pots with 50 mL of water every day. After two weeks, the plants on the windowsill are green and 15 cm tall, while the plants in the closet are yellow and 20 cm tall but with thin, weak stems. The student concludes that darkness makes plants grow taller. The student's teacher suggests that the experiment has a flaw: the plants in the closet may have grown taller because they were searching for light. The student has a ruler and a camera.
1. Identify the independent, dependent, and controlled variables in this experiment. Explain whether the student's conclusion is valid, and suggest one improvement to the experimental design.
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Independent variable: light condition (sunny windowsill vs. dark closet). Dependent variable: plant growth, measured as height (and possibly color/stem strength). Controlled variables: type and amount of soil, amount of water (50 mL daily), type of bean seeds, pot size, watering schedule. The conclusion that darkness makes plants grow taller is not valid because the taller growth in darkness is likely due to etiolation-a response where stems elongate to reach light-not because darkness is beneficial. The plants in the dark are less healthy (yellow, weak stems). The experiment lacks a measure of overall health and does not control for other factors like temperature (closet may be cooler). Also, only one light condition (full sun vs. full dark) was tested; a more valid conclusion would require multiple light levels. Improvement: use a growth chamber with controlled light intensity and temperature, or place plants at different distances from a window to vary light, and measure both height and biomass (dry mass) to assess true growth. Also, increase sample size and randomize pot positions.
2. The student wants to test whether the plants in the dark were indeed searching for light. Describe an experiment they could do using the available materials (ruler, camera) and additional simple materials, and explain what results would support the hypothesis.
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The student could set up a directional light source, such as a lamp on one side of a box, with the plant in the box. They could use the camera to take time-lapse photos to record the direction of stem growth. If the plants bend toward the light, that supports the hypothesis that they are searching for light (phototropism). They could also compare plants grown in the dark with a single light source from one side versus plants grown in uniform light. If the dark-grown plants bend toward the light, it shows they respond to light direction. Additionally, they could measure the stem curvature using the ruler and camera images. To further test etiolation, they could grow plants in the dark but with a support stick; if they still grow tall and weak, it's etiolation, not just searching. Results supporting the hypothesis: dark-grown plants show directed bending toward light and longer stems than light-grown plants when light is present. The camera can document the bending over time.