Ganong’s potometer

Transpiration may be defined as a phenomenon where the plant loses water in the form of vapour. Plants lose most of their water through their aerial parts, especially their leaves.
The apparatus used for measuring the total amount of water absorbed by a plant is called Ganong’s potometer. In short, Ganong’s potometer is employed in calculating the rate of transpiration. Ganong’s potometer is also called a transpirometer.

Construction of Ganong’s potometer

The Ganong potometer consists of a twice bent glass tube and glass cylinder. The glass cylinder is located toward the front side of the potometer and has a wide opening. A horizontal glass bar is connected to the reservoir into which the capillary tube is inserted. The glass cylinder is covered with a rubber cork containing a small hole. This small hole is used to place the freshly cut twigs of the plant. There are graduated readings on the horizontal bar of the potometer. The horizontal bar is fitted with a nozzle at the bent end. A beaker with coloured water is placed beneath the horizontal bar, and the bent end is inserted into it. Connected to this horizontal bar is a reservoir for storing water.

Design principles of Ganong’s potometer

The classic Ganong’s potometer follows three design principles - Length of the capillary tube, tube to handle the shoot, and a reservoir.

1. Length of the capillary tube - The length of the capillary tube plays a significant role in the determination of the transpiration rate of the plant under study. The capillary tube is used to trap the first air bubble, which gives readings as transpiration begins. The air bubble is forcibly introduced into the capillary tube and placed at the zero reading. As the plant begins to transpire, the air bubble also starts moving from the zero reading. The bubble stops moving further once the plants stop transpiration. The reading at which the bubble stops determines the rate of transpiration.

2. Tube to handle the shoot - For the experiment to be performed accurately, it is essential for the leafy shoot to be in touch with the water. Additionally, the entire water surface needs to be covered to ensure that no part of it is exposed to air. If exposed to air, evaporation of the water may take place. Therefore, the end calculations of transpiration rate will be affected. A rubber bung or a small tube is necessary to ensure that the leafy shoot is in contact with the water.

3. Reservoir - The reservoir is generally a funnel and is usually connected to the tap. The tap, along with the reservoir, helps in adjusting the position of the bubble. A few other designs of potometers use syringes as reservoirs instead of a funnel. However, both are convenient to use.

How to measure the rate of transpiration using Ganong’s potometer?

Place the freshly cut twigs in the glass cylinder.
Add a few drops of eosin oil to colour the water.
You need to ensure that there is one air bubble at the zero reading of the horizontal bar of the potometer.
Air bubbles will enter the horizontal bar as you lift the bent end.
Within seconds of the experiment beginning, the bubble present at the zero reading begins to move. The start of transpiration is marked by this.
The movement of the air bubble takes place in the direction of the twig. This is primarily because of the transpirational pull.
The amount of distance travelled by the bubble in the horizontal bubble represents the transpiration rate.
Repeat the process several times. The average of all the readings obtained gives the rate of transpiration.

Limitations of Ganong’s potometer

Ganong’s potometer offers the following limitations.

The procedure involves the placing of a cut twig on the glass cylinder. The twig may not be alive until the experiment finishes. Therefore, the results also may not be accurate if the twig dies.
Ganong’s potometer helps in measuring the overall water uptake by the plant. This procedure does not give a direct indication of transpiration rate because the entire water absorbed may not be used for transpiration. Some of it may be used for other processes.
Even a slight change in the overall atmospheric temperature influences the position of the air bubble in the horizontal bar. Moreover, maintaining a constant atmospheric temperature condition is not possible.
This procedure involves the introduction of a single air bubble into the horizontal bar. Forcing the air bubble to enter the horizontal bar is difficult, time-consuming, and stressful.