The Role plants play in the Water Cycle

To introduce students to transpiration. Students measure the amount of water transpired by a leaf on a tree during a 24 hour period. Connect the role that plants play in the water cycle.

Plastic sandwich bags (not ziplock)
Twist ties
Small pebbles
Metric balances capable of measuring to a hundredth of a gram
Small graduated cylinders

Teachers Notes:

1. Discuss meaning of transpiration relate to water cycle.
2. Give each student plastic sandwich bag, small pebble and twist tie.
3. Have students measure mass of bag, pebble and twist tie to the nearest hundredth gram.
4. Have each student place the pebble in the bag and tie the bag and pebble around a single leaf of a deciduous tree. The pebble weights the bag so it hangs down and the transpired water will collect in the bottom of the bag rather than running out. The bag should be secured tightly on the stem of the twig. Allow the bag to remain undisturbed for 24 hours.
5. At the beginning of the next period, ask the students to observe the condensation of moisture on the interior of the bag. Have them remove the bags carefully so that none of the water is spilled. Use the twist tie to close the bag tightly. Weigh the bag, pebble, twist tie and water. The difference in mass is the amount of water transpired in 24 hours.
6. Have the students record the difference in weight, remind them that one gram of water under standard temperature and pressure (STP) has a volume of one milliliter. If enough water has been collected you may want them to use graduated cylinders to measure the volume of the water collected.

Additional Activities:

1. Math / Social Studies: Count the number of leaves on a small tree and estimate how much water that tree will transpire. Large and mature trees may have upwards of 100,000 leaves and there may be as many as 200 fully grown trees on a single acre. By some estimates 40,000 square miles of rain forest are being cleared each year in tropical South America. A square mile contains 640 acres of land. Calculate the amount of water vapor that would be returned to the atmosphere by 40,000 acres of trees per day if transpiration conditions were the same as those of your experiment.
2. Make the connection between global warming, deforestation and the greenhouse effect.
3. Conduct this experiment on different types of trees, day time vs. night time, cloudy days vs. sunny days.
4. Cell biology: make cross sections of plant stems. Examine under microscope. Look at leaf under microscope and examine stomata of leaf.

*from "Water Wisdom" , Massachusetts Water Resource Authority (MWRA).


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