How to measure the rate of transpiration of a cut twig with Ganong's Potometer

Transpiration is a physiological process of plants where the plant looses water in the form of water vapor through the aerial parts of the plant. In this article I have described an experiment to find out the rate of transpiration in a cut twig by using an instrument Ganong's potometer. Along with the experiment some important questions related to the experiment were given below and they are helpful to the students who are appearing for practical as well as various Board's and university final theory exams.


Transpiration is the loss of water in the form of water vapor through the aerial parts of the plants especially through the leaves. In all green plants transpiration occurs mostly through the stomata of the leaves. The rate of transpiration depends upon various climatic factors like intensity of sunlight, temperature, humidity, velocity of wind, atmospheric pressure, availability of water to the plant etc. Potometers are the instruments used to measure the rate of transpiration in a cut twig under laboratory conditions in a plant. Usually Ganong's potometer is used to measure the rate of transpiration in a cut twig under laboratory conditions.

Here is Experiment on Transpiration in plants with dicot leaves

Aim of the experiment

To find out the rate of transpiration in a cut twig with the hep of Ganong's Potometer.


A freshly cut twig, Ganong's Potometer, water, Eosin oil, beaker.

Ganong's Potometer description:-

Ganong's potometer is a twice bent glass tube having a wide mouthed glass cylinder, a horizontal glass bar with an inserted capillary tube inside and a reservoir attached to the horizontal bar. Towards the front end of the apparatus a wide mouthed glass cylinder is present. The mouth of the cylinder is fitted with a one holed rubber cork. Freshly cut twig for the experiment is arranged through this hoe in the rubber cork. The horizontal bar of this apparatus is a long glass tube inside which a capillary tube is fitted and is having a graduated readings marked above it. This horizontal bar is a having a bent end having a nozzle opening. This bent end is kept inside a beaker containing colored water. Towards the front end of the horizontal bar a vertically arranged reservoir is present connected to the horizontal bar where water can be stored in it. There is a stop cock or pinch cock is present below the reservoir to allow or stop the flow of water from reservoir to horizontal bar. The whole apparatus stands on flat surface like table with the hep of a stand.

Ganongs Potometer experiment

Procedure for the experiment

The whole apparatus is filled with colored water. Colored water is prepared by adding few drops of Eosin oil to water. A freshly cut twig is arranged with the help of one holed rubber cork to the mouth of the glass cylinder. The whole apparatus has to be placed on flat surface like table under bright sunlight. Before starting of the experiment a single air bubble has to be arranged at zero reading in the capillary tube of horizontal glass bar. For this the bent end of the horizontal bar has to be lifted from the beaker. This will allow some air bubbles to get trapped in the capillary tube of horizontal glass bar. But we have to see there should be only one air bubble to get trapped at zero reading. To make this adjustment we have to open stop cock of the reservoir to allow water from reservoir into horizontal bar. Now the water forces the excess air bubbles to flow out from the nozzle. We have to make several trails till one air bubble get adjusted at zero reading.

After some time after starting of the experiment we see the air bubble from the zero reading start moving. As water is transpired from the aerial parts of the leafy shoot a transnational pull is created by the leafy shoot to make up the loss of water. Due to this force the air bubble starts moving from zero reading of the horizontal bar towards the twig. The rate of water uptake can be calculated by taking the distance covered by the air bubble in a fixed set of time and several readings were taken by readjusting the air bubble to zero reading. The average of these readings will give the rate of transpiration. To reset the air bubble to zero reading and to repeat the experiment water in the reservoir is to be used.

Here is How Oxygen is evolved by green plants in an experiment on Photosynthesis

Questions and answers related to the experiment

  • What is the aim of the above experimental set up?
    Ans. To find out the rate of transpiration with the help of Ganong's potometer.

  • How air bubbles to be introduced into the apparatus?
    Ans. By lifting the horizontal bar from the beaker.

  • What is the role of air bubble in this experiment?
    Ans. The rate of movement of the air bubble indicates the rate of transpiration.

  • What is the role of reservoir in the apparatus?
    Ans. By allowing water from the reservoir with the help of stop cock we can remove excess air bubbles from the horizontal bar and to adjust one air bubble at the zero reading before starting of the experiment. Reservoir is also helpful to readjust the air bubble to zero reading for restarting of the experiment.

  • Why the twig taken for this experiment to be cut under water from the plant?
    Ans. If we cut the plant under water air may not get trapped in xylem vessels of the twig which may curtail transpiration process and our experiment may fail.

  • Why the twig used in this experiment to be cut in an oblique fashion but not in a horizontal fashion?
    Ans. If we cut the twig in a oblique fashion it provide large cut surface area to absorb more water by xylem vessels and so the rate of transpiration will be more.

  • Why colored water is used in this apparatus?
    Ans. By using colored water in the apparatus measuring the movement of air bubble becomes easier.

  • What happens to the movement of air bubble if the apparatus is kept in dark, in bright sunlight and in front of a fan?
    Ans. If the apparatus is kept in dark the rate of transpiration will be less, if the apparatus is kept in bright sunlight rate of transpiration will be more and it will be more even when the apparatus is in front of a fan.

  • Here is An experiment to show chlorophyll is necessary for plants to carry out photosynthesis
  • Mention any two cares to be taken for the success of this experiment?
    1. A freshly cut twig only to be used for this experiment.
    2. Vaseline should be applied at various joints of the potometer and at the place of rubber cork to make the whole apparatus air tight otherwise change in air pressure will affect the movement of air bubble.

  • Mention any two limitations in doing this experiment with Ganong's potometer?
    1. The twig may not be alive for long duration.

    2. Introducing single air bubble at zero reading is not an easy task.

Here is a video on finding the rate of transpiration:-


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