Transpiration Lab Activity
Introduction to Lesson:
This lesson is a preparation for the next day’s lesson on
transpiration. Students will be setting up their own demonstration of the
process of transpiration. It will take the entire period for the students to
create an environment for transpiration to occur so that the students can
see what exactly transpiration is. This is a fun activity and the students will
be amazed to see transpiration in action!
Objectives:
Students will
observe transpiration occurring in a plant placed into a
simple potometer that they will construct in this activity.
Students will connect what they see with the process of water movement
into and out of a plants’ tissues.
Materials and Resources:
1. Two plastic cups per student
2. Square piece of cardboard for between the cups
3. Plant shoot (must be provided by teacher or students can bring in but this
will not be in the learning trunk)
4. Vaseline or similar brand of petroleum jelly
5. Window sill or other light source
6. Water
7. Scissors (warn students to use these carefully especially if cutting woody
stems)
Note: This lesson plan is based on an exercise at:
http://www.ucar.edu/learn/1_4_2_18t.htm
Anticipatory Set: Teacher will hand out an “admit/enter” slip to students
asking them to write what they know about how and why humans sweat and
plants lose water from their leaves. If necessary, review the basics of water
movement in plants with the students: Water enters the plant from soil
mostly through the _____________. The tissues through which water
moves within the plant are called __________________ . The water then
evaporates and moves out of the plant through holes on the undersides of
the leaves called _________________ whose size is controlled be
specialized cells called ___________________ . Evaporation rates depend
on temperature, wind and humidity levels among other factors.
Sequence Instruction Day 1:
1. Using the scissors CAREFULLY, make a small hole (just big enough for
the plant stem) in the center of the piece of cardboard.
2. Take shoots of a healthy broad-leaved plant (e.g. maple, oak, marigold,
whatever). If comparisons are to be made between different treatments,
try to match type and size of the shoots so that the results can be
meaningfully compared. Cut the stem on an angle immediately before
use, since an angle cut exposes a larger area of the stem for water
intake.
3. Pull the plant stem through the hole and seal around the hole with
petroleum jelly.
4. Fill the bottom cup with a KNOWN amount of water (BE SURE THAT
STUDENTS MEASURE THE VOLUME USED AND THAT NO WATER
IS LOST AFTER THAT) and place the stem with the cardboard collar
into the cup. Cover with the clear plastic cup as shown. Record the
volume of water in the bottom cup in the table provided.
5. Create a “control” terrarium that is identical to that with the living plant
but which does not contain the plant itself. To do this the students could
simply cut a hole in a second sheet of cardboard and cover that with
petroleum jelly as in the living plant “treatment” and place that into a
terrarium just like that. Alternately, a “fake plant” – either silk or plastic
could be inserted in a similar way as the living plant, and the terrarium
set up identically to the living plant “treatment”.
6. Again, fill the bottom cup with a KNOWN amount of water (BE SURE
THAT STUDENTS MEASURE THE VOLUME USED AND THAT NO
WATER IS LOST AFTER THAT) and place the stem with the cardboard
collar into the cup. Cover with the clear plastic cup as shown. Record
the volume of water in the bottom cup in the table provided.
7. Put the small terrariums in the sun or under a lamp.
8. In fifteen minutes, you should begin to see droplets of water on the
sides of the clear inverted cup over the living shoot but not over the
control. More moisture will accumulate in the top cup of the terrarium
with the live plant with time, but little to none should be seen in the top
cup in the control terrarium.
9. If possible, leave the terrarium cups set up in the classroom for several
days and measure the amount of water transpired (it’s usually easiest to
do this by measuring what’s lost from the bottom cup rather than trying
to measure what evaporates into the top one).
TABLE FOR RECORDING WATER VOLUMES USED IN THE VARIOUS
TERRARIUMS
Treatment
Volume of
Water in
bottom cup at
start of
experiment
Volume of
Water in
bottom cup at
end of
experiment
Volume of
water lost
Regular Living
Branch
Control
Extension
Treatment (if
used write in
what was done)
Anchor Activity (if lesson goes shorter than planned)
Discuss the scientific method with the students in the context of the
experiment. What was the purpose of the control? Why would it be
important if this was a real experiment to have more than one REPLICATE
of each treatment when building conclusions from the results of the
experiment? If living plants DO NOT play a role in speeding up evaporation
via transpiration, what would be the prediction about the amount of water in
the top cup in the control versus the treatment (same amount, more or less
water in the control versus the treatment)? If living plants DO play a role in
speeding up evaporation via transpiration, what would be the prediction
about the amount of water in the top cup in the control versus the
treatment? What is the effect of water loss in transpiration for the amount of
water in the bottom cup? What would this mean for plants in soil in the real
world? How would transpiration be expected to affect the water levels in
the soil in which the plant is growing?
Closure
Have students write for themselves a hypothesis about what they think will
occur with the water levels in the control and treatment cups over the next
few days.
Homework
1. For each of the following scenarios, would you expect transpiration to
occur faster or slower rate than happened in your own experiments in
your own classroom? In each case, explain WHY?
a. If the shoots were placed in a very windy spot.
b. If the shoots were placed in a very humid environment.
c. If the shoots were placed in a classroom that was much warmer than the
one in your school.
Sequence Instruction Day 2:
1. Retrieve the terrariums that you made in your previous class.
2. Measure the volume of water remaining in the lower cup of each
terrarium as accurately as possible (use volumetric flasks, measuring
beakers or similar lab equipment if possible).
3. Fill in the results in the table used to record the initial volumes
4. Calculate the volume of water lost in each case and write this number in
the appropriate column in the table provided.
5. Have students compare results of similar treatments. Do all of the
replicates from similar treatments show the same results? If not, what
might account for the variability that you see between replicates?
6. How do the results from the different treatments compare? Which lost
the most water? Which lost the least?
7. Review the hypothesis that you made at the end of your last class. Do
your results support your predictions? Why or why not?
8. If the extension activities were carried out, what can be concluded
about which surface of the leaf is the source of the majority of the water
loss through transpiration? How does this relate to the distribution of
stomatal pores on the leaf surface?
Assessment: Use a rubric such as the one at
http://www.uen.org/Rubric/rubric.cgi?rubric_id=25 to assess student's participation in the
activity and their subsequent write up of the experiment and its results.