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Letting Off Steam

Objectives:  Students will learn about transpiration or water loss in plants.

Grade Level: 3-6

Groupings: Small groups

Materials:  (per group) Small plastic mirror; three pieces of flexible plastic tubing (18-24 inches long and 1/4 inch diameter); two plant stems, one with leaves and one without leaves; clay; clear plastic bags; rubber bands; heavy duty tape; permanent marker.

Time Allotment:  20 minutes for initial set up, 15 minutes at the end of the day for observation and conclusions.


This experiment works best if set up on a bright, sunny morning.

1. Ask the class whether they have ever seen their breath. Divide the class into small groups and pass each group a small plastic mirror. Explain that one of the students in each group will hold the mirror in front of his or her face for several seconds. What do they see on the mirror? (Condensed water vapor.) Where does it come from? Explain that people constantly give off water vapor, along with carbon dioxide, as they breathe. This process called respiration is the necessary exchange of gases of an oxygen breathing organism with the environment.

2. Show the class the collection of leafy plant stems. Do they think plants also 'breathe' or exchange gases with the environment? Explain that they will set up some experiments to see whether plants give off water vapor as part of this exchange.

3. Give each group three pieces of clear plastic tubing. Have them fill the tubes with water, making certain air bubbles are removed. Ask them to place a plant stem with leaves in one end of the first tube, and seal any opening in that end with clay. Have them cover the plant with a plastic bag and secure the bag to the tubing with a rubber band. Have them bend this tubing into a U-shape and securely tape it to a sunny window, marking the present water level at the open end with a permanent marker.

4. Ask them to place a plant stem without leaves in a second piece of tubing, seal the opening with clay, cover it with a plastic bag, and secure it with a rubber band. They then bend and attach the tubing to the window in a similar fashion to the first, marking the present water level. What do they think will happen to the inside of these plastic bags? What will happen to the water level in the tubes? Ask them to record their predictions.

5. Explain that a control, or a standard for comparing is needed for this experiment to prove that any changes that occur inside the plastic bags or tubing are related to the plants. Ask for their ideas on how they could set up the third tube as a control. Using their ideas, direct them to fill the tube with water, seal one end with clay, cover this end with a plastic bag and attach the tubing to the window in a similar manner, marking the present water level.

children observing the experiment

6. Ask the groups to monitor their experiments at one to two hour intervals and note and record changes. What, if anything, happens to the inside of the plastic bags? (The bags are covered with condensed water vapor.) Are there any differences in the bags of the plants with leaves and the ones without leaves? (There will be more water in the bags of the plants with leaves.) Where is the water coming from? (Most of the water is given off by the leaves, ultimately coming from the water the plants absorb.) Explain to the students that transpiration is the process of giving off water vapor through the leaves. (See Up, Up and Away!, for further details.)

7. Have the groups measure the amount of water given off through transpiration by comparing the water level in their tubes at the end of the day to the initial levels. Do they notice a change in all three tubes? How do they explain the differences?


Have the students conduct the same experiment in the dark. Do they think the transpiration rate will differ? Have them record their predictions. Have them vary the experiment again, this time with the plastic bags removed and a fan blowing on their plants. What do they think will happen? How do they explain the results? Ask the students to think of other variables that might alter transpiration rates and set up new experiments to test their ideas.

UP, UP, and AWAY!

On a cold day you have probably seen your own breath as small clouds of water vapor given off when you breathe. Through a process called transpiration, plants give off water vapor too - more than 17 times the amount we do! In fact, an acre of pasture grass may lose more than 1500 gallons of water during one hot midsummer day! Ninety percent of all the water taken up by plant roots is recycled back into the atmosphere as plants transpire. Transpiration is the principle force that lifts water to the tops of tall trees. As water is lost through the leaves, more is absorbed by the roots to replace it. The stomata control the amount of water lost through transpiration. Stomata are small openings on the underside of the leaves which are surrounded by two guard cells. These cells change in size and shape, thus changing the size of the opening and regulating the gas and water exchange. Various factors affect the guard cells and hence control transpiration rates, including temperature and humidity, water availability, carbon dioxide concentrations and light. So despite sun, wind, and rain, the perfect balance is always maintained.

* This material has been used with permission from Shelburne Farms, Copyright © 1995.

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