What is a Ganong's Potometer?
A Potometer is an apparatus which is used to measure the amount of moisture absorption by the leafy shoot during transpiration. It is also referred to as a transpirometer. The reasons for moisture uptake are transpiration and photosynthesis. The Ganong's Potometer finds its purpose in the measurement of the transpiration rate in a cut twig in lab settings.
Ganong's Potometer is a glass tube that is bent twice and consists of a broad-mouthed glass cylinder, a horizontal glass rod with a capillary tube inserted within and a reservoir connected with the horizontal rod.
The broad-mouthed glass cylinder is present towards the front end of the entire setup. The opening of the cylinder is attached with a single-holed cork (rubber). The newly cut twig for the experiment is placed via this hole in the rubber cork.
The horizontal rod of the setup is a long glass tube within which a capillary tube is placed and has graduated readings marked on it. This horizontal rod has a bent end containing a nozzle mouth. This bent side is placed in a beaker that consists of colored liquid (water).
As we move towards the front end of the horizontal rod, a vertically placed reservoir is present that is attached with the horizontal rod in which water can be accumulated. A pinch cock or stop cock is placed at the bottom of the reservoir to enable or halt the flow of water from reservoir to the horizontal rod. The entire apparatus is mounted on a flat surface like the table with the help of a stand.
A Typical Ganong's Potometer follows the Below-Mentioned Design Principle:
Length of Capillary Tube - A bubble is forced into the capillary. As the water is absorbed by the plants, the bubble starts moving. By spotting regular markings on the tube, it is possible to calculate the amount of water absorbed.
Reservoir – It is generally a funnel with a tap attached to it. Rotating the tap on the reservoir adjusts the bubble. Some other designs make use of a syringe instead.
Tube for Handling the Leafy Shoot - The leafy shoot should be kept in contact with the water. In addition, the surface of the water is not supposed to be exposed to the air. Or else, evaporation will interfere with the calculations. A rubber bung lubricated with petroleum jelly is sufficient for the setup.
Transpiration
Transpiration is the mechanism where there is loss of moisture or water in the form of water vapor from the aerial organs of the plants, particularly via the leaves. In every green plant, the process of transpiration takes place via the stomata of the leaves, in most cases. The rate of transpiration relies on different climatic factors such as temperature, intensity of sunlight, humidity, velocity of wind, availability of water to the plant, atmospheric pressure, and many more.
There are various types of transpiration which are:
Lenticular transpiration
Cuticular transpiration
Stomatal transpiration
Water loss mechanism
Procedure
The rate of transpiration can be calculated in the following ways:
Directly – By calculating the weight of the Ganong's Potometer after a definite duration of time. In this case, it is assumed that any loss that takes place is because of transpiration.
Indirectly – By calculating the path the water level drops in the graduated tube after a measured period of time. In this case, it is assumed that this phenomenon takes place because of the taking in water that, in turn, is important to substitute an equal amount of moisture or water lost due to transpiration.
The entire setup or apparatus is full of colored water. The colored water is made by pouring a couple of drops of Eosin oil into the water. A newly cut twig is placed with the help of the single-holed rubber cork to the opening of the glass cylinder. The entire setup or apparatus is supposed to be mounted on a flat surface like a table under bright sunlight.
Prior to the beginning of the experiment, a lone air bubble is supposed to be kept at zero reading in the capillary tube of the horizontal glass rod. To achieve this, the bent side of the horizontal rod is supposed to be shifted from the container or the beaker. Doing this will enable a couple of air bubbles to get held in the capillary tube of the horizontal glass rod. However, we need to observe that there must be only a single air bubble to get held at the zero reading.
To do this modification, we need to take off the stopcock of the reservoir to enable water from the reservoir to flow through the horizontal rod. The water now compels the extra air bubbles to move out from the nozzle. We need to do numerous trials until a single air bubble gets modified at the zero reading.
After a certain time duration post beginning of the experiment, we observe that the air bubble from the zero reading begins to move. As the moisture or water is transpired from the aerial regions of the leafy shoot, a transpirational pull is generated by the leafy shoot to compensate for the loss of moisture. Because of this force, the air bubble begins to move from the zero reading of the horizontal rod towards the newly cut twig.
The rate of water absorption can be calculated by measuring the distance traveled by the air bubble in a certain period of time and numerous measurements are recorded by adjusting the air bubble to the zero reading again. The average or mean of these measurements will provide us with the rate of transpiration. To adjust the air bubble to the zero reading and to perform the experiment again, the water stored in the reservoir is to be utilized.
Precautions
The following precautions must be taken before using Ganong's Potometer:
When a twig is freshly cut from a plant, make sure that it is immediately placed under water (only the cut region). After that, a tiny portion is cut while it is still under water. Doing this avoids the entrance of the air into the vessels of the xylem.
The state of the Potometer, except the modification that is being measured, must not be manipulated during a test, as external factors such as temperature, and pressure can affect the water absorption.
Each and everything should be absolutely water-tight in order to avoid any leakage.
Limitations of Ganong's Potometer
The Ganong's Potometer comes with the following limitations:
The newly cut twig positioned on the glass cylinder may not stay alive for a significant period of time.
The forcing of an air bubble into the horizontal rod is not an easy task and takes a lot of time.
The Ganong's Potometer does not calculate the rate of transpiration with the required accuracy as not the entire water absorbed by the plants is utilized for transpiration. The device generally estimates the rate of moisture uptake.
A little alteration in the atmospheric temperature can have some impact on the position of the air bubble.
FAQs on Ganongs Potometer
1. From where can students study about Ganong's Potometer?
From Vedantu, which is an online learning platform students can study and learn about the Ganongs Potometer. The well-researched content is available on this website in the form of Pdf file. With the help of this article, students will get to know about the meaning of the Ganongs Potometer, its structure, its mechanism, its limitations and precautions taken while using this instrument. All these key points regarding the instrument are written by skilled teachers. Therefore, students must read this article thoroughly to understand the Ganongs Potometer. They can also ask their queries from the experts through the comment section.
2. Describe the appearance of the Ganong's Potometer.
Ganong's Potometer is an instrument or device used to measure the amount of water taken by shoot during the phenomena of transpiration. This device has a glass tube and a glass cylinder. The glass tube is bent twice and the glass cylinder has a wide mouth. A reservoir is attached to the capillary tube in the horizontal glass bar. The glass cylinder is present at the front end of the apparatus and its mouth is closed with a cork. A hole is present on the cork in which a twig is kept. The reservoir helps in storing the water. The whole apparatus is placed on a flat surface.
3. Which phenomenon is associated with Ganong's Potometer?
Transpiration is the phenomenon associated with Ganong's Potometer. Transpiration is defined as the excretion process in plants through which excess water is removed from the aerial parts of the plants. The plants use only a small amount of water for their development and growth. The rest is evaporated through the process of transpiration. There are various types of transpiration. These are:
Cuticular Transpiration
Stomatal Transpiration
Lenticular Transpiration
The factors that affect transpiration in plants are listed below:
Cellular factors
Humidity
Light
Temperature
Pressure
Wind speed
The surface area of leaves
4. What is the procedure of experimenting with Ganong's Potometer?
The method of experimenting with the Ganong's Potometer is discussed below:
Firstly, make the water coloured by adding a few drops of eosin oil to it.
Then, put a small and fresh twig on the mouth of the glass cylinder.
For writing the readings, the single air bubble should be kept at the zero reading of the horizontal bar.
The air bubbles will get trapped in the horizontal bar by lifting the bent end.
The air bubbles will start moving from zero reading. This is the stage where readings can be taken.
5. How to prepare the topic of "Ganong's Potometer" for the practical examination?
Practical examinations are conducted to assess the practical knowledge of the students. These examinations are part of the internal assessment exams. Thus, students have to prepare well to score good marks. To prepare the topic of "Ganong's Potometer" for practicals, students must know the principle on which it works. After that, they should see and understand the functions of each part of the instrument. After learning about the device and its working, students should solve the viva questions given in the practical book. This will help them in comprehending the mechanism of the instrument more easily.
