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Difficulty: MediumFormulating and Modifying Hypotheses

A student hypothesized that under constant light and temperature, the rate of transpiration in bean plants increases as the relative humidity of the surrounding air increases. To test this hypothesis, the student measured the transpiration rates of several identical bean plants at different relative humidity levels. The results are shown in the table below:

Relative Humidity (%)Transpiration Rate (mg/dm2/hrmg/dm^2/hr)
3015.2
5010.4
705.8
901.3

Based on these results, how should the student modify their hypothesis to accurately reflect the relationship between relative humidity and transpiration rate?

  1. A
    The student should hypothesize that the transpiration rate increases as relative humidity increases, because a lower relative humidity slows the rate of evaporation from the leaves.
  2. B
    The student should hypothesize that the transpiration rate remains constant at all relative humidity levels, because relative humidity has no effect on transpiration.
  3. The student should hypothesize that the transpiration rate decreases as relative humidity increases, because a higher humidity decreases the water vapor concentration gradient between the leaf interior and the air.Answer
  4. D
    The student should hypothesize that the transpiration rate decreases as relative humidity increases, because a higher relative humidity increases the rate of water absorption by the leaves from the air.

Answer

The student should hypothesize that the transpiration rate decreases as relative humidity increases, because a higher humidity decreases the water vapor concentration gradient between the leaf interior and the air.
The correct option states that the transpiration rate decreases as relative humidity increases, which matches the trend in the data. It also provides the correct scientific explanation: higher humidity reduces the water vapor concentration gradient between the leaf's wet interior and the air, reducing the rate of diffusion.

Step-by-Step Solution

1
Analyze the experimental data in the table to determine the relationship between the independent variable (relative humidity) and the dependent variable (transpiration rate).
As relative humidity increases from 30% to 90%, the transpiration rate decreases from 15.2 mg/dm^2/hr to 1.3 mg/dm^2/hr.
This establishes the empirical trend that refutes the student's initial hypothesis of a positive correlation.
2
Identify which proposed hypothesis modifications align with the observed inverse relationship.
The modifications stating that transpiration rate decreases as relative humidity increases align with the trend.
This eliminates hypotheses predicting increasing or constant trends.
3
Evaluate the scientific reasoning behind the remaining options to select the correct physical/biological mechanism.
A higher humidity reduces the concentration gradient of water vapor between the humid interior of the leaf and the outside air, which reduces the evaporation/transpiration rate.
The correct hypothesis must couple the correct trend with scientifically sound reasoning.

Key Concept

Formulating and Modifying Hypotheses
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Topics

Formulating and Modifying Hypotheses

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