Transpiration Lab for High School Biology: Simple Plant Transpiration Experiment

Plant transpiration can be a challenging concept for high school biology students, especially when it is only taught through notes or diagrams. This simple plant transpiration lab helps students understand what plant transpiration is by observing water loss in leaves through an easy, hands-on classroom experiment. Designed for high school biology courses, this transpiration lab makes plant water movement visible, measurable, and meaningful.

I often find that my students are woefully lacking in their knowledge of plants. When time runs short at the end of the school year, plant units are usually the first to be cut. Because I enjoy teaching plants, I make it a priority to leave time each year for a plant unit.

This version of a plant transpiration lab is incredibly easy to set up, clean up, and take down. It uses a simple materials list and includes all the components I want in a lab activity, including data collection, graphing, critical thinking, problem solving, data analysis, and drawing conclusions. Plus, it is fun, and students always enjoy doing it.

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What is Plant Transpiration?

Plant transpiration is the loss of water vapor from a plant through the leaves. As the stomata open to allow carbon dioxide to enter the leaf for photosynthesis, water vapor escapes the leaf and enters the atmosphere.

For trees and other tall plants, moving water from the roots to the top of the plant is a challenge. The major force in water transport is provided by the evaporation of water from the leaves during transpiration. As water is lost, the movement of water out of the leaf pulls water upward through the xylem all the way down to the roots. This process is known as transpiration pull.

Traditional Plant Transpiration Lab Using a Potometer

Traditionally, biology teachers used potometers to measure water loss from leaves during transpiration. Does this image bring back memories of a failed lab? Potometers are notoriously difficult to set up because it is hard to create a tight seal around the plant stem and rubber tubing. Without a good seal, results are unreliable, and students often crush the delicate stem during setup.

Transpiration Lab for AP Biology and High School Biology

This simple plant transpiration lab is easy to set up, clean up, and take down, making it ideal for high school biology classrooms. Go to your local nursery or Walmart and buy some small bedding plants. I like to use begonia plants. The begonia is a good choice because it has a thick and fleshy leaf. 

Place the plants in small (250 mL) beakers. Water them thoroughly. In this experiment, you want to measure the amount of water lost from the leaves, so you have to ensure that no evaporation occurs from the soil in the beaker. Place a plastic sandwich bag around the beaker and wrap it very tightly around the stem of the plant. A bit of tape may be needed to keep the sandwich bag wrapped tightly around the stem. Only the leaf should be sticking out of the plastic bag.

The only way water is getting out of this beaker is through the leaves of this plant!

Determine the amount of water lost from the leaves by massing the entire setup. Be sure to get an initial mass before the experiment begins. In the photo shown, the initial mass was 195.57 grams.

Testable Variables in a Plant Transpiration Experiment

Now for the testable variables. What factors will cause an increase or decrease in the rate of transpiration? This lab will test 3 different environmental factors.

• One plant will be placed in continuous light for 24 hours. 
• One plant will be placed in front of a blowing fan for 24 hours.
• One plant will be placed inside a plastic bag that has been spritzed with water to create high humidity.
• The fourth plant will serve as a control.

Every experiment needs a control. In this transpiration lab, the control plant is not exposed to any experimental environmental factors. While the control plant is still transpiring, it does so under normal conditions, allowing students to compare results accurately.

Allow the plants to sit for 24 hours. After this time period, each plant is massed again. Students are often amazed at how much water has been lost through the leaves. All final masses should be compared to the control to determine whether the environmental factor tested caused an increase or decrease in the rate of transpiration.

There are also some uncontrolled variables in this experiment. The plants are not identical, and each has a different total leaf surface area, which can affect transpiration rates. Even so, students collect clear data showing the effects of environmental conditions on plant transpiration.

Adapting the Transpiration Lab for Different Grade Levels

Elementary: At the elementary level, it may be enough to demonstrate that water is absorbed by roots and released through leaves. Students can make predictions, form hypotheses, and practice steps of the scientific method.

Middle Grades: Middle school students can calculate the rate of water loss per minute, test additional variables such as darkness or temperature, or compare transpiration rates among different plant species.

High School Biology and AP Biology: High school students can remove leaves to calculate surface area and determine water loss per unit area. Students may also research mechanisms of transpiration such as cohesion, adhesion, capillary action, and transpiration pull.

After the experiment, I take the plants home and place them in my flower beds.

For additional plant kingdom resources, consider short video segments from The Private Life of Plants by David Attenborough. Many clips are available on YouTube and are typically three to five minutes long. Students always enjoy them.

Frequently Asked Questions About Plant Transpiration Labs

What is plant transpiration?
Plant transpiration is the process by which water moves from a plant’s roots, through the stem, and out of the leaves as water vapor. This water loss helps regulate temperature and supports the movement of water and nutrients throughout the plant.

What is a simple plant transpiration experiment for high school biology?
A simple plant transpiration experiment uses a live plant placed in a sealed container so that water loss occurs only through the leaves. By measuring mass before and after the experiment, students can observe and quantify transpiration.

How does this transpiration lab work for AP Biology?
This transpiration lab works well for AP Biology by allowing students to analyze variables such as light, humidity, air movement, and leaf surface area. Students can collect quantitative data, graph results, and connect observations to concepts like transpiration pull and water potential.

Why is a control important in a transpiration lab?
A control plant provides a baseline for comparison. By comparing experimental plants to a control, students can determine whether specific environmental factors increase or decrease the rate of transpiration.

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