To investigate the effect of wind speed on the rate of transpiration, students placed four… — ACT

To investigate the effect of wind speed on the rate of transpiration, students placed four identical oak saplings in potometers. Each sapling was exposed to a fan at a different distance to vary the wind speed, with Trial 4 serving as the control group (no fan). The trials were conducted sequentially inside a greenhouse, and the ambient temperature, relative humidity, and transpiration rate were recorded for each 1-hour trial. The results are shown in the table below.

Trial	Distance from fan ( $\text{m}$ )	Time of day	Temperature ( $^\circ\text{C}$ )	Relative Humidity ( $\%$ )	Transpiration rate ( $\text{mL/m}^2/\text{hr}$ )
1	$0.5$	10:00 AM – 11:00 AM	$22$	$60$	$4.2$
2	$1.0$	11:00 AM – 12:00 PM	$25$	$50$	$4.8$
3	$1.5$	12:00 PM – 1:00 PM	$28$	$42$	$5.1$
4	No fan	1:00 PM – 2:00 PM	$30$	$35$	$3.9$

Based on these results, the students concluded that a wind speed corresponding to a distance of $1.5\text{ m}$ from the fan maximizes the rate of transpiration in oak saplings. Which of the following statements identifies the primary flaw in this conclusion?

A
The students failed to control the dependent variable (distance from the fan), which should have been kept constant while the independent variable (transpiration rate) was measured.
The sequential execution of the trials allowed ambient temperature to rise and relative humidity to fall over time, both of which independently increase transpiration rates and obscure the true effect of wind speed.Answer
C
Trial 4 cannot serve as a control group because a true control group requires the fan to be set to a baseline low speed rather than being shut off completely.
D
Because transpiration rate is inversely proportional to temperature, the higher temperature in Trial 3 should have decreased transpiration, proving that wind speed was the sole driver of the observed increase.

Answer

The correct answer states that the sequential execution of the trials allowed ambient temperature to rise and relative humidity to fall over time, both of which independently increase transpiration rates and obscure the true effect of wind speed.

The correct option is correct because conducting the trials sequentially throughout the day allowed ambient temperature to rise and relative humidity to fall. Both higher temperatures and lower relative humidity increase the rate of transpiration by increasing the rate of water evaporation from the leaves. Because these environmental factors changed systematically alongside the distance from the fan, they act as confounding variables, making it impossible to determine whether the differences in transpiration rate were caused by the change in wind speed or the changes in temperature and humidity.

Step-by-Step Solution

Identify the independent variable and the dependent variable in the experiment.

The independent variable is the distance from the fan (representing wind speed). The dependent variable is the measured transpiration rate of the oak saplings.

Establishing the variables is the first step in assessing the validity of an experimental design.

Examine the environmental conditions (temperature and relative humidity) recorded across the trials to see if they were controlled.

The temperature systematically increased from

22^\circ\text{C}

in Trial 1 to

30^\circ\text{C}

in Trial 4, while relative humidity systematically fell from

60\%

35\%

To isolate the effect of the independent variable, all other variables that could influence the outcome must be kept constant.

Determine how these uncontrolled environmental variables affect the dependent variable.

Higher temperatures and lower relative humidity increase the atmospheric vapor pressure deficit, which accelerates evaporation and increases transpiration rates in plants.

This establishes that the environmental changes act as confounding variables that favor higher transpiration rates in later trials, regardless of the wind speed.

Evaluate the validity of the students' conclusion that transpiration is maximized at

1.5\text{ m}

Because temperature and humidity were not controlled and varied in a way that increases transpiration, the peak transpiration observed in Trial 3 (

1.5\text{ m}

) cannot be uniquely attributed to the wind speed.

A scientific conclusion is invalid when confounding variables are allowed to vary systematically alongside the independent variable.

Key Concept

Identifying Confounding Variables

Estimated Time:2m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic