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Measuring Rate of Transpiration

Measuring the Rate of Transpiration

To measure the rate of transpiration we use a piece of equipment called a potometer. A potometer measures how factors such as light, temperature, humidity, light intensity and wind will affect the rate of transpiration. The main type of potometer is the ‘bubble’ potometer shown in the figure below. The potometer measures the amount of water lost from a leafy shoot by monitoring the rate at which an air bubble moves along the narrow tube as the leafy shoot sucks up water to replace the water lost by the transpiration of the plant.


A potometer provides an indirect measurement of the transpiration rate – it measures how fast water is absorbed, which is related to how fast water vapour is being lost. It cannot measure how fast water vapour is being given off directly.

As the leafy twig transpires, the air bubble moves to towards the plant. The quicker the air bubble moves, the faster the leafy twig is transpiring.

Potometer measures the rate of transpiration.

Investigation: Determining the effect of environmental conditions on transpiration rate using a potometer


To determine the effect of environmental conditions on transpiration rate using a simple potometer.


  • drinking straw or clear plastic tubing
  • soft green leafy shoot
  • Vaseline
  • marking pen
  • play dough / putti/ Prestick
  • plastic bag
  • elastic band
  • ruler


A potometer measures the rate of transpiration by measuring the movement of water into a plant. The following experiment uses a simple hand made potometer.

Learners will be divided into four groups as each group will investigate a different factor and then all the results can be shared at the end of the investigation.

Perform the following steps under water:

  1. Cut the stem of the leafy shoot (at an angle to increase the surface area) under water . The reason we cut it under water is to prevent air bubbles entering the xylem vessel. You must use a very sharp knife or new scalpel and cut at an angle in order to increase surface area for water uptake in the xylem. Florists who cut plants before immersing them in water follow the same procedure for this reason.
  2. Test to make sure the stem of the leafy twig will fit snugly into the top of the straw.
  3. Remove the leafy shoot from the straw and set aside, keeping the stem submerged, and the leaves above water.
  4. Fill the straw with water. Place your finger over one end of the straw to stop the water from running out.
  5. Put the leafy shoot into the open end and seal it with play dough/ putti/ Prestick while removing it from water KEEPING YOUR FINGER ON THE STRAW! Perform the following steps above water.
  6. Seal with Vaseline. Make sure it is air tight and water tight. If not, all the water will run out when you take your finger off the straw.
  7. Mark the water level on the straw.
  8. Place your potometer under one of the following conditions for one hour:
    • as is, in a warm, sunny place (no wind)
    • as is, in a warm, windy place
    • with a plastic bag tied around the leaf, in a warm, sunny place
    • a shady place.
  9. Every 10 minutes use a marking pen to mark the change in water level on the straw. Continue taking measurements for 1 hour.
  10. Measure the distance the water moves during each time interval.


Each of the four groups that investigated different environmental conditions should contribute their results for the final analysis.

  • Draw a table and record the class’ results.
  • Plot a bar graph to compare the total distances the water moved in the different straws in 1 hour under the four different environmental conditions.
  • At the end of the experiments, all students must plot the following line graphs:
    • the effect of temperature on the rate of transpiration
    • the effect of light intensity on the rate of transpiration
    • the effect of relative humidity on the rate of transpiration
    • the effect of wind on the rate of transpiration


Record your observation from the table, bar graph and line graphs.


  • What can you conclude from this investigation?
  • Give two ways in which you can improve your experimental results.


  1. Why is it important to cut the stem at an angle under the water?
  2. Which part of the stem does the straw represent?
  3. Which four factors are you investigating?
  4. Under which condition is the highest rate of transpiration?
  5. Name one possible error that could have occurred in your investigation.
  6. What are the potential limitations of this investigation?

Investigation: Determining the effect of light intensity on transpiration


To determine the effect of light intensity on transpiration.


  • plants
  • plastic bag
  • piece of string
  • graduated measuring cylinder


  1. Use at least three plants of the same species and as close to the same size as possible (think of why this might be important).
  2. Ensure that all three plants are exposed to the same amount of light.
  3. Use clear plastic bags to completely cover all the leaves of each plant.
  4. Tie the bottom of the plastic around the main stem of the plant, allowing the water lost from the plant to collect inside the bag. Try not to crush the leaves of the plant with the bag.
  5. Place the bags on the plants early in the morning. Leave the bags on all day and check for signs of water drops inside. If there are water drops, shake the bag so that the water drops to the bottom of the bag.
  6. At the end of the day, carefully remove the bags to ensure that you do not lose any water. It will help if you tilt the plant slightly while removing the bags.
  7. Collect the water inside a measuring cylinder and measure how much water the plant has lost.
  8. Tie a new plastic bag around the plant and leaver overnight.
  9. The following morning, collect and measure the water that was released by the plant overnight.


Record the amount of water lost during the day and during the night.

Using the three plants, figure out the average water loss for each time period.

Plot a bar graph comparing the average amount of water loss in the day and night.


Write down anything you observed about the plants, the plastic bags and the rate of water loss from the plant.


What can you conclude regarding the rate of transpiration at different light intensities? Was there higher or lower water loss when you left the plant overnight compared to when you monitored it throughout the day?


How can you improve this experiment to determine the effects of different light intensities on transpiration?

In this experiment what are the key variables we are controlling for? Have we properly controlled for these?


Table comparing the advantages and disadvantages of transpiration

Advantages of transpirationDisadvantages of transpiration
Cools the plant downExcessive water loss causes the plant to wilt
Assists in the transport of water from the soil 
Important for transport of water through the xylem 
Regulates the concentration of cell sap 
Distribution of salts and minerals in the plant 

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