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Fig 1: Darwin Potometer
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A potometer is a device which measures the amount of water a plant shoot takes in over a set period of time.


There are four main types of bubble potometer which are used in school biology. All work in a similar way but are slightly different in appearance. The types are listed below and a photograph of each is shown above.

Darwin Potometer (See Fig 1)
Farmer Potometer (See Fig 2)
Ganong Potometer (See Fig 3)
Thoday Potometer (See Fig 4)
Simple mounted potometer (See Fig 5)

In all bubble potometers, a small bubble of air is introduced into the graduated capillary tube. As water is taken up by the plant, the bubble will move and the distance it moves over time can be recorded using the marked graduations. All have some kind of reservoir or syringe which acts as a resetting mechanism by allowing little water into the capillary tube to clear the bubble so recording can begin.

The plant shoot is held securely in a connected tube. The shoot must be in contact with the water and an air tight seal is essential to ensure the water surface does not come into contact with the surrounding air as evaporation may interfere with the results. Vaseline and/or a tightly fitted bung with hole may be used to create this seal.

Setting up the potometer takes a few simple but essential steps. (If you have instructions for your particular device which contradict these steps, follow them instead).

Firstly put the whole unit under water to fill with water ensuring there are no bubbles of air anywhere within the tubing. Take a cut stem from a plant and insert it into the bung or tube making sure it is secure. The bung may have to be greased with Vaseline in order to get an air tight seal. Close the tap if your device has one and take the unit out of the water. Hold the capillary tube out of the water until a bubble forms and then place the device where you want to conduct the experiment and place a beaker under the free end.

Another type of potometer is the 'Mass Potometer' This consists of a plant shoot held securely in a sealed beaker. The bottom of the shoot must be in contact with the water. The whole device is weighed at fixed intervals. The loss in weight represents the water lost through transpiration and not specifically the water taken up by the shoot.

When taking a cutting from a plant to use in a potometer, immediately hold the cut end under water and cut it again under water before securing it into the tube. This prevents any air getting into the xylem vessels of the shoot and interfering with the experiment.

Also once the experiment is set up it should not be moved into a different environment as temperature changes may interfere with the results.


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