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)
Metric balances capable of measuring to a hundredth of a gram
Small graduated cylinders
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.
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