Transpiration Experiments: In the article, we will learn about transpiration in plants’ experiments through jar bells and polythene bags, etc. Plants absorb the water through roots, and the xylem transports the water to the stem, leaves, and other parts of the plant. The leaves utilise only about 2 % of the absorbed water in photosynthesis. The leaf surface contains tiny pores called stomata, through which the water transpired into the atmosphere. The loss of water through the surface of leaves is called transpiration.

Several experiments have been performed to demonstrate the process of transpiration. The experiment related to transpiration is based on the concept of loss of water in the form of droplets. Certain weighing experiments have also been performed to demonstrate the loss in volume of water from the source during transpiration. Let’s read the article to study the experiments related to transpiration in detail.

What is Transpiration?

The loss of water in the form of water vapour through the aerial parts of the plants is called transpiration. A large amount of water is lost through the leaf surfaces. Leaves contain several minute pores called stomata on their upper and lower surfaces. The stomatal pores open into intercellular spaces of the leaf and provide an uninterrupted path from the interior of the leaf to the external environment.

Important Facts About Transpiration

Transpiration is, however, concerned with the loss of water in the plants through stomata but serves as a necessary evil for the plant. This can be discussed as follows:

1. Transpiration creates a suction pull or force on the roots to absorb more water from the soil.

2. Transpiration pull facilitates the upward movement of water through the xylem.

3. The cohesive property of water can be easily determined due to the process of transpiration.

4. It regulates the temperature of the plant parts and prevents the wilting of the leaves through the continuous pull of water.

5. Transpiration contributes to the cyclic flow of water in the atmosphere and therefore maintains the temperature of the surroundings.

6. The excess accumulation of water may cause waterlogging in the roots of the plants. Transpiration prevents the water from logging in the roots and therefore contributes to the survival of plants to some extent.

Experiment Related to Demonstration of Transpiration

1. Polythene bag or Bell jar Experiment
Aim: Demonstration of transpiration through the stomatal pores of the leaves.
Materials required: A well-watered potted plant, a polythene bag or a bell jar, greasy substance.
Procedure: The demonstration of transpiration  can be done by following the below-mentioned steps in a sequential manner:
I. A well-watered potted plant is taken, and the plant is covered with a transparent polythene bag. The bag is tied up to make the setup airtight. A bell jar can also be used instead of a polythene bag, and the grease or vaseline can be used to make the experimental setup airtight.
II. The covered plant is further placed in the sunlight for about two to three hours.

Observation: A few drops of water can be observed on the inner surface of the polythene bags.

Conclusion: The water droplets that appear on the inner surface is due to the condensation of water vapour into liquid water. It proves the loss of water through the surface of leaves during transpiration.

Fig: Polythene bag experiment to demonstrate transpiration (Transpiration Experiment Plastic Bag)

2. Cobalt chloride Experiment:
Aim: Demonstration of the transpiration by using cobalt chloride paper.
Materials required: A well-watered potted plant, a polythene bag, a bell jar, cobalt chloride paper.

Procedure: The experiment can be demonstrated in the following steps:
I. A plant with broadleaf is preferably taken to perform the experiment. The plant is supplied with sufficient water and further covered with the polythene bag to escape water vapour from the pot.
II. The entire setup is placed into the bell jar. A piece of cobalt chloride paper is also placed in the bell jar along with a polythene-wrapped plant.
III. Another control setup is designed as a control experiment in which a cobalt chloride paper is alone placed in a jar without the plant.

Fig: Cobalt chloride paper experiment to demonstrate transpiration

Observation:

I. The observation is marked by the change in the colour of cobalt chloride paper from blue to pink because of the reaction of cobalt chloride with water.
II. While the cobalt chloride paper of another setup remains blue and does not show any colour change as there is no water for the reaction with cobalt chloride paper.

Conclusion: The change in the colour of the cobalt chloride paper ensures the occurrence of transpiration in the plant.

3. Four-Leaf Experiment:
Aim: Demonstration of transpiration through stomata.
Materials required: four leaves, vaseline, string.
Procedure:
I. The four fresh leaves are taken and labelled as A, B, C, D, and E.
II. Leaf A is coated with Vaseline on both surfaces.
III. The lower surface of leaf B is coated with Vaseline. On the contrary, the upper surface of leaf C is coated with Vaseline.
IV. The leaf D kept uncoated.
V. Vaseline closes the stomata and therefore prevents the loss of water through the stomata.
VI. The leaves are tied in a similar sequence on the string. The setup is then placed in the sunlight for about one or two days.

Observation: The following observation has been found:
I. Leaf A is fully coated with vaseline and remains fresh and green.
II. The leaf B shrivelled a little.
III. The leaf C shrivelled comparatively more.
IV. The leaf D is completely wilted.

Conclusion:

I. Leaf A shows that no transpiration has occurred through leaf A as stomata were closed due to the application of vaseline.
II. The leaf B shows a little transpiration as the lower surface contains fewer stomata, and the upper surface is coated with vaseline.
III. The leaf C shows comparatively more transpiration as its upper surface is exposed to transpiration.
IV. The leaf D shows the maximum rate of transpiration.
It can be concluded that transpiration occurs through stomata as the fully vaseline coated leaf remains fresh due to the absence of transpiration, while those partially coated show a little transpiration hence become shrivelled. The non-coated leaf completely wilts as transpiration through stomata occurs to its maximum extent.

Fig: Four-leaf experiment to demonstrate transpiration

Experiment Related to the Measurement of Transpiration

1. Weighing Experiment I:
Aim: Demonstration of loss in the volume of water due to transpiration.
Materials required: Small potted plant, weighing machine.
Procedure:
I. A small potted plant is supplied with a sufficient amount of water and then weighed on the weighing machine.
II. Further, the plant is kept in the sunlight for about an hour. The soil surface and the pot should be covered to prevent the evaporation of water from any other surface.
III. After an hour, the plant is again weighed on the weighing machine.

Observation: It has been observed that the plant weight is reduced after placing in the sunlight due to the loss in the volume of water.

Conclusion: The loss in plant weight occurs due to the loss of water. It shows that transpiration occurs through the aerial parts of the plant since all the possible ways for evaporation have already been checked.

2. Weighing Experiment II:
Aim: Measurement of transpiration by a gradual decrease in the water level.
Materials required: Leafy shoot, water, test tube, oil, weighing machine.
Procedure:
I. A test tube filled with water is taken.
II. A leafy shoot is immersed in the test tube so that the roots remain submerged in the water and the shoot and leaves remain out from the water.
III. A small amount of oil is now added to the test tube. The oil forms a thin film over the water surface and thereby prevents water loss by evaporation.
IV. The experimental setup is then weighed, and the weight is noted.
V. The setup is further placed in the sunlight to allow transpiration.
VI. The plant is again weighed after some time.

Observation: A difference in the weight is observed. The water level in the test tube also falls down.

Conclusion: The difference in the weight and water level in the test tube ensures transpiration through the plant since the evaporation is already interrupted.

Fig: Weighing method to measure the transpiration

3. Potometer Experiment: Potometer is a device that measures the amount of water absorbed by the plant. The amount of water intake is almost equal to the amount of water loss through transpiration. There are a number of potometer that have been designed to measure the rate of transpiration. These are named as follows:
I. Garreau’s Potometer is used to demonstrate transpiration from both the surfaces of the leaves.
II. Darwin’s Potometer is used to demonstrate the suction force created by transpiration.
III. Farmer’s Potometer and Gangong’s Potometer is used to measure the amount of water intake.
Aim: Measurement of the rate of transpiration through Ganong’s potometer.
Materials required: small plant twig, water, Ganong’s potometer.
Procedure:
I. A small twig of a plant is cut obliquely to larger the surface for water intake.
II. The apparatus is filled with water, and the twig is placed in the vertical arm of the potometer and fixed with the help of a one-hole cork.
III. Grease is applied to all the joints to make the apparatus airtight to escape the loss of water through evaporation.
IV. An air bubble is inserted in the horizontal graduated tube of the apparatus by slighting lifting the bent capillary tube.
V. The initial reading of the bubble on the graduated tube is noted.
VI. The experimental setup is then placed in the different surrounding conditions.

Fig: Measurement of transpiration through Ganong’s potometer

Observations:
I. In the experimental setup, if placed in the sunlight, the reading in the graduated tube is the maximum, i.e., the distance travelled by the bubble is the maximum due to the greatest suction force that is created by the maximum water loss. Moreover, in a windy atmosphere along with the sunlight, the rate of water loss even exceeded more.
II. In the experimental setup, if placed in the shade, the reading on the scale is the minimum, i.e., the distance travelled by the bubble is the minimum due to the least suction force applied to the twig.
III. In the experimental setup, if placed in the complete dark, the distance travelled by the bubble is negligible since the loss of water through the twig is nullified.

Conclusion: The distance travelled by the bubble and the suction force created for the absorption of water determines the rate of transpiration through the leaves in the different surrounding conditions. The more the loss of water, the more is the absorption of water.

Limitations of potometer:
I. It is difficult to insert a single air bubble in the capillary tube.
II. The twig may not remain alive for a longer period of time to demonstrate the change in the rate of transpiration in different environmental conditions.

Summary

Transpiration is the loss of water through the aerial parts of the plant. It is an essential process as it creates a suction force to form a continuous column of water from the roots to the leaves of the plant and therefore prevent the wilting of the leaves. A number of experiments have been performed to demonstrate transpiration in plants and to measure the rate of transpiration in plants. Since the leaves are responsible for the maximum water loss, therefore transpiration can be determined by placing a polythene covered plant in the sunlight.

Moreover, a four-leaf experiment is also performed to determine the occurrence of transpiration, where the transpiration through the upper and the lower surface of the leaf is controlled by Vaseline coating.
The rate of transpiration is almost equal to the rate of absorption of water. Based on this concept different scientists contribute to demonstrate the rate of transpiration in accordance with the rate of water absorption with the help of a device called the potometer.

FAQs on Transpiration Experiments

Below are the most frequently asked questions on Transpiration Experiments:

Q.1: Why did we tie the bag around the leaves of the potted plant?
Ans: The potted plant covered with the polythene bag is placed in the sunlight. The water lost through the leaves becomes condensed, and the water droplets that appear on the inner surface of the polythene prove the occurrence of transpiration.

Q.2: Why is a potometer used?
Ans: A potometer is used to measure the rate of transpiration that is equal to the amount of water absorbed by plants.

Q.3: What is the role played by the bubble in Ganong’s potometer?
Ans: The bubbles travel a distance in the capillary tube along with the suction of water and therefore determine the rate of transpiration by the plant.

Q.4: How is the air bubble introduced in the Ganong’s potometer?
Ans: Air bubbles are introduced in the Ganong’s potometer by slightly lifting the bent tube.

Q.5: What is the main limitation of Gangong’s potometer?
Ans: It is difficult to insert a single bubble in the tube.

We hope this detailed article on transpiration experiments is helpful. If you have any queries drop them in the comment section below and we will revert with answers.

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