Biology Lab Investigation: Transpiration
How do environmental factors (light intensity, temperature, wind, and humidity) and
anatomical features (leaf surface area) affect the rate of transpiration in plants?
Introduction. Plants just like other organisms must be able to transport materials from
one part to another. Plant transport systems consist of two large tubes made of vascular
tissue that basically run from the roots through the shoots (stems) and to the leaves of
the plant. Sugars produced through the process of photosynthesis are transported
through plants from leaves to roots via the vascular tissue known as the phloem. Cells
use these sugars to produce the energy needed for the rest of the plants functions.
Sugars move through the plant because they are in greatest concentration in the leaves,
where photosynthesis takes place, and in least concentration in the roots. Many plants
will store excess sugars in specialized root structures called tubers.
Water is transported in plants from the roots to the leaves through the vascular tissue
known as the xylem. The water then enters the leaves and is either used in the process of
photosynthesis or is released back into the atmosphere through stomata in the process
of transpiration. About 1% of the water taken up by plants through their root system is
used for metabolic functions such as photosynthesis. The other 99% exits the stomata
without ever being used by the plant. In a tree, such as the giant redwood of California,
water must ascend over 300 feet to reach the highest leaves. The water moves through
the plant via osmosis because the concentration of water is greatest in the roots of a
plant and the least in the leaves.
Transpiration, or the evaporative loss of water from the stomata (singular: stoma) of
leaves, helps to create a lower concentration of water (or lower osmotic potential) in the
leaf. The differences in water concentration are also responsible for the movement of
water from the xylem to the mesophyll layer of the leaves and subsequently out of
stomata to the atmosphere in the hydrologic cycle. (See Figures 1 and 2).
Figure 1. The structure of a leaf featuring the major tissues; the upper and
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
1
lower epidermis, the palisade and spongy mesophyll and the guard cells of the
stoma. Vascular tissue (veins), made up of xylem and phloem are also shown.
The light green circles within cells represent chloroplasts and indicate which
tissues undergo photosynthesis.
Figure 2: The Hyrdrologic (Water Cycle)--Water vapor is released into the atmosphere at
the leaf surface through stomata. This process, known as transpiration, is ecologically
vital. It is one of the most important ways that water travels from the soil and back into the
atmosphere, where it can then return to earth as life-giving precipitation.
Stomata also play an important role in photosynthesis and the carbon cycle. When
stomata are open, carbon dioxide enters the plant from the atmosphere to drive
photosynthesis, and oxygen is released back into the atmosphere as a by-product of
photosynthesis. (See Figure 3)
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
2
Figure 3: The Carbon Cycle—Carbon dioxide is removed from the atmosphere by terrestrial
plants and marine phytoplankton as a necessary reactant for photosynthesis. Terrestrial
plants take in carbon dioxide through open stomata in their leaves.
The transpiration rate of a plant (or how quickly water is lost from the leaves due to
evaporation) is influenced by a number of environmental factors (such as light intensity,
temperature, wind, and humidity) as well as anatomical factors (leaf surface area).
Your Task: Determine if there is a relationship between one of these variables and the
plant’s transpiration rate.
The guiding question of this investigation is:
How does _______________________ affect the rate of transpiration in plants? (Your lab group
will be assigned the variable you will test.)
Materials. You may use any of the following materials during your investigation.
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Calibrated/Graduated Pipettes
Plastic Tubing
Parafilm or Vaseline
Ring Stand
Clamps
Plant Samples
Water
Lamp
Various Wattage Light Bulbs
Desk Fan
Large Ziplock Bags
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Twist Ties or Rubber bands
Plastic Tubs
Various Lab Glassware
Scissors
Ruler
Graph Paper
Microscope Slides
Clear Packing Tape
Electronic Balance
Safety Precautions. Follow all normal lab safety rules.
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
3
Getting Started. In order to answer this guiding question you will need to design and
conduct an experiment. To accomplish this task, you must be able to measure the
transpiration rate of a plant and determine if stomata are open or closed. To do this,
you can use the equation and the experimental method detailed below for constructing
and using a potometer and making stomatal impressions.
Transpiration Rate =
Original amount of water in pipette – Amount of water left in
pipette
Minutes
Potometer Setup
Method for Using a Potometer
1. Collect a stem of the appropriate size to fit your potometer. Try to select a stem
with whole, undamaged leaves.
2. Place the stem in the plastic tub and cut the stem underwater, keeping the cut
portion submerged.
3. Place the potometer (plastic tubing with graduated pipette attached) under the
water and fill it up with water. Remove any air bubbles from the plastic tubing.
4. Keeping the potometer and the cut end of the branch submerged, place the end
of the branch into the plastic tubing. Be careful not to tear the exterior of the
stem. If the stem is too large, recut it underwater and try inserting it into the
tubing again.
5. Attach the potometer and branch to the ring stand with clamps as demonstrated
by your instructor (see fig. 1). Make sure the tubing is not being constricted by
the clamps. If there are leaks where the branch and tubing meet, wrap this area
with parafilm or smear with Vaseline.
6. Let the branch rest for 5 minutes.
7. After 5 minutes, record in Table 1 the time and the amount of water in the
graduated pipette. This is your start time.
8. Record the amount of water in the graduated pipette every five minutes for a total
of at least 30 min.
9. Graph your data (you may begin graphing as soon as you start collecting data).
Use the slope of a straight, best-fit line through the data points to calculate the
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
4
rate of water loss (ml/min) (Hint: remember “rise over run”).
Method for Determining Surface Area of Leaves
Note: If your group chooses leaf surface area as the variable you are investigating, you will
need to use one of the methods below for measuring the surface are of the leaves on your
stem.
Method 1:
1. Cut all the leaves (not stems) off the plant and determine their mass using a
balance.
2. Estimate the total leaf surface area in square centimeters for the plant by cutting
out a section of leaf 2cm × 2 cm.
3. Determine the mass for this leaf section and divide by 4 cm2 to find the mass of l
cm2 of leaf.
4. Divide the total mass of the leaves by the mass of l cm2 to find the total leaf
surface area.
OR
Method 2
1. Cut all the leaves (not stems) off the plant and place them on a sheet of
centimeter grid graph paper.
2. Measure the surface area of the leaves using the graph paper. Count the total
number of square centimeters the leaves occupy on the graph paper. Partial
boxes will require some estimation.
3. Think about ways your group may reduce measurement error due to estimation.
Note: Every group will need to determine if stomata were open or closed during the
experiment. The procedure is detailed in the steps below.
Method for Producing Stomatal Impressions
To determine whether the stomata were open or closed, follow the procedure for
stomatal impressions detailed in the steps below.
1. Remove a leaf from your stem.
2. Paint a thin layer of clear nail polish on a patch of the underside of the leaf.
3. Allow the nail polish to dry completely. 4. Tape a piece of clear cellophane tape to the dried nail polish patch. (The tape must
be clear. Clear carton-sealing tape works well.) 5. Gently peel the nail polish patch from the leaf by pulling on a corner of the tape
and peeling the fingernail polish off the leaf. This is the leaf impression you will
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
5
examine.
6. Tape your peeled impression to a very clean microscope slide. Use scissors to trim
away any excess tape.
7. Examine your slide under a compound light microscope to determine if stomata
are open or closed.
Now you must determine what type of data you will need to collect, how you will collect
it, and how will you analyze it for your actual investigation. To determine what type of
data you need to collect, think about the following questions:
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What type of measurements will you need to record during your investigation?
To determine how you will collect your data, think about the following questions:
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What will serve as a control (or comparison) condition?
What types of treatment conditions will you need to set up and how will you do
it?
How many trials will you need to do?
How often will you collect data and when will you do it?
How will you keep track of the data you collect and how will you organize it?
In order to determine how you will analyze your data think about the following
questions:
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How will you determine if there is a difference between the treatment conditions
and the control condition?
What type of calculations will you need to make?
What type of graph could you create to help make sense of your data?
Investigation Proposal Required:
x Yes
No
Connections to Crosscutting Concepts and the Nature of Science and Scientific
Inquiry. As you work through your investigation, be sure to think about:
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The importance of identifing the underlying cause for observations
What is and what is not relevelent at different scales or time frames
How matter moves within or through a system
How structure determines function in living things
The difference between observations and inferences in science
The different methods scientists can use to answer a research question in science.
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
6
Presentation Session: Once your group has
finished collecting and analyzing your data,
prepare a whiteboard that you can use to share
your initial argument. Your whiteboard should
include all the information shown Figure 3.
The Research Question:
Our Claim:
Our Evidence:
Our Justification of
the Evidence:
Since each lab group tested a different variable,
we will conduct ourselves a little differently than
we normally do with ADI. To share your argument Figure 3. Information needed on a Whiteboard
with others, your group will make a presentation
of your whiteboard to the rest of the class. Each group member should participate in
the presentation and be prepared to answer questions from the audience.
The goal of this presentation session is to share your experimental findings with the
scientific community (your classmates). Your classmates and teacher will ask questions
to identify errors or instances of faulty reasoning in the arguments so these mistakes
can be fixed.
Members of the audience will evaluate the content of the claim, the quality of the
evidence used to support the claim, and the strength of the justification of the evidence
included in each presentation. Members of the audience, therefore, may need to ask the
presenters one or more follow up questions such as:
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How did you collect your data? Why did you use that method?
What did you do to make sure the data you collected is reliable? What did you
do to decrease measurement error?
What did you do to analyze your data? Why did you decide to do it that way? Did
you check your calculations?
Is that the only way to interpret the results of your analysis? How do you know
that your interpretation of your analysis is appropriate?
Why did you decide to present your evidence in that manner?
What other claims did your group discuss before you decided on that one? Why
did your group abandon those alternative ideas?
How confident are you that you claim is valid? What could you do to increase
your confidence?
Once the presentation session is complete, you will have a chance to meet with your
group and revise your original argument. Your group might need to gather more data
or design a way to test one or more alternative claims as part of this process.
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
7
Remember, your goal at this stage of the investigation is to develop the most valid or
acceptable answer to the research question!
Report. Once you have completed your research, you will need to prepare an
investigation report that consists of three sections. Each section should provide an
answer for the following questions:
1. What question were you trying to answer and why?
2. What did you do during your investigation and why did you conduct your
investigation in this way?
3. What is your argument?
Your report should answer these questions in 2 pages or less. This report must be typed
and any diagrams, figures, or tables should be embedded into the document. Be sure to
write in a persuasive style; you are trying to convince others that your claim is
acceptable or valid!
The development of this laboratory activity was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant
R305A100909 to the Florida State University
8