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Purpose:
In this experiment, students study the movement of water
within a plant, measuring a change in pressure as the plant
transpires.
Equipment:
- Put the barb end of a quick-release connector into one
end of the plastic tubing. Use a drop of glycerin for lubrication
if needed.
 
- Cut the stem of the plant seedling 2-3 cm above the soil.
Immediately submerge the cut end into water, and leaving
it submerged, shave the freshly cut end to a 45-degree angle.
- Fill the tubing with water so that there is a 2-3 cm air
gap at one end: raise the end with the quick-release connector
and allow excess water to spill out the other end.
- Placing your thumb over the free end of the tubing, put
the tubing under water and insert the cut plant stem into
the tubing. Avoid creating any air bubbles in the tubing;
pull the tubing away from the stem if air bubbles form.
- Spread the petroleum jelly around the end of the tube
to create an airtight seal between the top edge of the plastic
tubing and the plant stem.
- Secure the plant in an upright position with the clamp
to the lab stand.
- Mount the Pressure Sensor to the lab stand’s support
rod using the other clamp. The pressure port should be 5-7
cm above the cut end of the plant stem to prevent water
from entering the Pressure Sensor.
- Align the quick-release coupling on the end of the plastic
tubing with the pressure port of the Pressure Sensor. Push
the coupling onto the port, and then turn the coupling clockwise
until it clicks (about 1/8 of a turn). Make sure no water
enters the sensor.
-
PASPORT users: Connect the pressure
sensor / plant assembly to the USB link or Xplorer; choose
"Launch DataStudio". A graph display (Pressure
vs. Time) will appear.
-
Double-click the Digits display icon,
or drag it to the Relative Pressure data channel, to open
a Digits display of Pressure (kPa).
 Users
of ScienceWorkshop 500 or 750 interface click
here.
Back to
top
Data Collection Procedure:
-
Click the Start button ( 
) to begin collecting data.
-
Collect data for approximately 10 minutes,
then click the Stop ( 
) button.
Note: If the pressure does not change or starts
to increase, it is likely that your system has developed
a leak. Try reseating the plant in the tube and applying
more petroleum jelly around the end of the tube to create
a seal.
-
Set up the electric fan at least one meter
away from the plant. Turn the fan on to a low setting
so that it blows a light breeze over the plant.
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Record another data run, again for approximately
10 minutes.
-
Scale the axes to fit the data using
the Scale to Fit button ( 
) in the Graph toolbar.
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Use the Smart Tool ( 
) to find the pressure (P 1) at 20 seconds (t 1)
and the pressure (P 2) at 475 seconds (t 2)
for Run #1 without the fan, and record the values in the
data table below.
-
Repeat the above process for Run #2 with
the fan.
-
Calculate the change in time and change
in pressure for each data run, and record the values in
the data table.
- Divide
 P
by t
to calculate the transpiration rate for each run.
Data Table:
|
Transpiration
Run
|
P1
|
P2
|
P
(kPa) |
t1
|
t2
|
t
(s) |
Rate of Transpiration
(kPa/min)
|
|
No Fan
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With Fan
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Conclusions and Extensions:
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What was the rate of pressure change
in the tubing? How does the change in pressure correspond
to an increase or decrease in water loss through the seedling’s
stomates? Explain.
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Did the fan affect the rate of pressure
change? Explain how the fan affects transpiration. What
natural phenomenon does the fan mimic?
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Describe some adaptations that plants
have to enable them to minimize water loss from their
leaves.
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