Ganong’s Potometer

A Ganong’s Potometer is a device used to measure how much water a plant absorbs through its shoot during transpiration. It is also called a transpirometer. Ganong’s potometer is commonly used in laboratories to measure transpiration rates. The potometer consists of a bent glass tube and a wide-mouthed glass cylinder. A capillary tube is placed in a horizontal glass bar attached to a reservoir. At the front end of the apparatus, there is a wide-mouthed glass cylinder with a rubber cork and a hole where a freshly cut twig is inserted.

The horizontal bar is marked with graduated readings and has a bent end with a nozzle opening. A beaker with colored water is positioned below the horizontal bar, and the bent end is inserted into it. The reservoir is used to store water, and the entire set-up is placed on a flat surface.

Procedure

• A few drops of eosin oil are added to the water to make it coloured.
• A freshly cut twig is placed on the mouth of the glass cylinder.
• A single air bubble has to be kept at the zero reading of the horizontal bar. Lifting the bent end allows the air bubbles to enter and get trapped in the horizontal bar.
• When the experiment begins, it is observed that the air bubble at the zero reading starts to move.

As transpiration takes place, a transpirational pull is created by the shoot to make up for the loss. Due to this, the horizontal bubble starts moving in the direction of the twig. The transpiration rate is calculated by taking the distance covered by the air bubble in a certain time period. Several readings were noted by repeatedly adjusting the air bubble to zero. The average of these readings gives the transpiration rate.

Limitations Of Ganong’s Potometer

• The Ganong’s Potometer does not measure the transpiration rate accurately because not all the water taken up by the plant is used for transpiration. It usually measures the rate of water uptake.
• The twig placed on the glass cylinder might not stay alive for a longer time.
• The introduction of an air bubble in the horizontal bar is very difficult and time-consuming.
• A slight change in the atmospheric temperature might affect the position of the air bubble.