Transpiration Made Simple!

I always find that my students are woefully lacking in their knowledge about the plants. If a unit has to be left out because time is running out in the school year, plants are usually the first to get cut. I like teaching plants, so I make it a priority each year to leave enough time at the end of the school year for my plant unit. 

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

TL;DR and want to see the activity in my TPT store? Click here.

Background Info:

Transpiration is the loss of water 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, it is tricky business to get the water up to the top of a very tall tree. 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 the water out of the leaf "pulls" water upward through the xylem all the way down to the roots. This is called transpiration pull.

The Old Way:

Traditionally, we biology teachers used potometers to measure the water loss from the leaves during transpiration. Does this image bring back bad memories of a failed lab? What a horrid mess it was to get it to work! When using potometers, it is very hard to get a good seal around the plant stem and the rubber tubing. No seal means no results. Also, students often crushed the delicate stem as they cut it to place it in the rubber tubing.

The Easy Way:

Go to your local nursery or Wal Mart 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. Since you want to measure the amount of water lost from the leaves, 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 unit. Be sure to get an initial mass before the experiment begins. In my photo, the initial mass was 195.57 grams.


Now for the testable variables!  What factors will cause an increase or decrease in the rate of transpiration? My 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 previously been spritzed with water (high humidity inside the bag). 
• The fourth plant will serve as a control.

Every experiment needs a control. This lab uses a plant that is not exposed to any of the above environmental factors. The control plant is still transpiring, but under normal conditions.  

Let the plants sit for 24 hours. Each plant is massed again after a 24 hour period of time.  Students will be amazed at how much water has been lost through the leaves. All masses should be compared to the control to see if the factor being tested causes an increase or decrease in the rate of transpiration.

There are some "uncontrolled" factors. For one, the plants are not identical to each other. Each of the plants has a different leaf surface area that will affect the results. Even so, students will collect data that will show the effect of these environmental conditions on leaf transpiration.

Here are some ideas of how this might be used at different age levels:

Elementary: It may be enough in the lower grades to just show that water is taken up by the roots and escapes from the leaf. Students can be asked before the experiment to make predictions as to what they think the outcome will be. They can form a hypothesis and go through the steps of the scientific method.

Middle Grades may add the following: Have students calculate how much water is lost per minute in each plant. Test an additional factor, such as complete darkness, or various temperatures. Test different types of plants to see if the transpiration rate is the same in all types of plants.

High School Grades may add the following:  Remove the leaves and determine the surface area of each leaf. Determine how much water is lost in a given amount of surface area.  Have students research the 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!

Here are some additional resources for those of you teaching the Plant Kingdom:

Look for video segments from "The Private Life of Plants"by David Attenborough.  There are quite a few of these on YouTube.  Each is very short - about 3 to 5 minutes - and my students really enjoy
them.

Plant Kingdom Complete Teaching Unit Bundle

Photosynthesis Complete Teaching Bundle

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