ICSE Class 10 Biology Chapter 05 Transpiration

Transpiration

Syllabus: Transpiration, process and significance; experimental work includes the loss in weight of a potted plant and a leaf shoot in a test tube, the use of cobalt chloride paper. Ganong's potometer and its limitations. The effect of external conditions on the rate of water loss should be stressed.

Scope of syllabus: Mechanism of stomatal transpiration must be explained so that concept of the process is clear. Adaptations in plants to reduce transpiration to be discussed. A brief idea of guttation and bleeding should be given.

Transpiration is a very useful process for plants for two reasons: one, creating suction force in the stem to enable the roots to absorb water and mineral nutrients, and two, for cooling the plant in hot weather. The process is demonstrated and studied by a number of experiments, which are very interesting to perform.

5.1 Transpiration

Transpiration is the evaporative loss of water from the aerial parts (leaves and stem) of the plant.

OR

Transpiration is the loss of water in the form of water vapour from the leaves and aerial parts of the plant.

All plants continuously absorb water through their roots. This water is conducted upwards through the stem and is distributed to all the aerial parts including the leaves. Only a small quantity of this water (about 2%) is used by the plant in photosynthesis and other activities. The rest of it is almost lost to the atmosphere as water vapour during transpiration.

5.2 Demonstration Of Transpiration

Experiment 1: Take a medium-sized well watered potted plant. Cover the plant with a transparent polythene bag and tie its mouth around the base of the stem (Fig. 5.1). Leave the plant in sunlight for an hour or two. Drops of water will soon appear on the inner side of the bag due to the saturation of water vapour given off by the leaves (the water vapours condense only if the outside temperature is cool enough). A similar empty polythene bag with its mouth tied and kept in sunlight will show no drops of water. This is a control to show that plants transpire water in the form of vapour. If tested with dry cobalt chloride paper, the drops will be confirmed as water only if the blue paper turns pink.

Experiment 2: (Fig. 5.2). Arrange three set-ups A, B and C as follows:

Set-up A. Take a small well-watered potted plant, preferably one with broad leaves. Enclose the pot completely within a polythene bag and tie the mouth of the bag firmly around the base of the stem. This would prevent the escape of water vapour from the pot. Now cover the entire plant under a bell jar as in (A).

Set-up B. Arrange another similar plant and cover it with a bell jar exactly in the same manner as the first one, except that here you also keep a piece of dry cobalt chloride paper by the side of the plant inside the bell jar (B). The paper may be pinned to a wooden stick or to a strip of cork sheet.

Set-up C. Take a third bell jar without the plant, but still containing a similar piece of cobalt chloride paper (C). Now, keep all the three bell jars together in the sun.

After about half an hour we observe that,

The first bell jar (A) would show water vapour condensing on its inner walls.

The second bell jar (B) would also show a similar condensation and at the same time, the initially blue cobalt chloride paper in it would turn pink.

The blue colour of the cobalt chloride paper in the third bell jar (C) does not change at all and there are no water drops on the jar's inner walls either. The third bell jar in this experiment is a control which proves that there was no moisture in the air due to transpiration as there was no plant in it.

By taking the three bell jars as described above, there is a double visual proof of transpiration: (1) condensation of water vapour into droplets and (2) change of colour in cobalt chloride paper. You may simplify the experiment either by taking (B) and (C) only, or by taking the bell jar A and another empty bell without the plant or the paper.

Cobalt chloride paper is an indicator of moisture:

Blue when dry

Pink when exposed to moisture

5.3 Measurement Of Transpiration

There are a number of methods for measuring transpiration. Some of these are:

1. Weighing method. A small light weight potted plant can be weighed before and after the end of a certain period of time. The soil surface and the pot should be fully covered to prevent evaporation from the surfaces other than the plant. The loss in weight by the plant during that time is due to the loss of water by transpiration.

An improvement in the weighing method can be made by using a glass bottle with a graduated side tube, filled with water and a tube fitted into it as shown in Fig. 5.3A. The water level in the side tube falls to demonstrate loss of water through transpiration from the leaves.

This would indicate the volume of water loss that can be compared with the loss in weight with the help of a weighing machine (B) or by converting cc into grams (1 cc water weighs 1g).

Another weighing experiment can be made by using a test-tube filled with water and inserting a leafy shoot (no roots) in it and pouring some oil on the surface to prevent loss of water by evaporation (Fig. 5.4). Place the test-tube in a small beaker and weigh them together. Remove the intact test-tube and keep it straight in the test-tube stand for a few hours.

Weigh it again by keeping it in the same beaker. Any difference in weight will indicate loss of water by the shoot (due to transpiration). Since there are no roots to actively absorb water, the water loss through transpiration will be much less.

2. Potometer method. Potometer means a device that measures the rate of water intake by a plant (potom: drink, meter: measure), and this water intake is almost equal to the water lost through transpiration.

Note: Potometer's are of various types designed by various scientists to study and measure the rate of transpiration, namely, Farmer's potometer, and Ganong's potometer, which helps to measure the rate of water intake by a plant. Darwin's potometer helps to demonstrate the suction force created due to transpiration and Garreau's potometer to demonstrate unequal transpiration from the two surfaces of a dorsiventral leaf. Given below is the commonly used Ganong's potometer and its working.

Ganong's Potometer:

A twig of some suitable plant (e.g. coleus) cut with a sharp knife is fixed in an apparatus as shown in Fig. 5.5. The entire apparatus is filled with water so that no air spaces are present. An air bubble is introduced into the horizontal graduated capillary tube which is dipping into the beaker containing water.

This is done by lifting the bent capillary tube above the coloured water so that air may be sucked in due to suction pull and is again dipped into the water. As transpiration proceeds, i.e., as the water is lost from the twig, a suction force is set up which pulls the water from water beaker and the bubble in the capillary tube moves along. The readings on the capillary tube would give the volume of water lost in a given time. The air bubble can be brought back to its original position by releasing some more water from the reservoir into the capillary tube by opening the stop cock.

Potometers do not measure the water lost during transpiration but measure the water uptake by the cut shoot (L. potos: drink). Some of the water is used by the cells to carry out other processes, for example, manufacture of food (Photosynthesis). The potometer should be made completely water-tight and the twig should be cut obliquely (to allow larger surface for the water intake) and under water to avoid suction of an air bubble into the twig which will stop the absorption of water into the xylem.

Limitations in the use of potometer

(i) Introducing the air bubble is not very easy.

(ii) The twig may not remain fully alive for a long time.

(iii) Any changes in the outside air temperature may affect the position of the air bubble in the capillary tube.

Teacher's Note

When you water your plants in the morning, notice how the soil dries out faster on hot, windy days - this is transpiration at work, helping the plant regulate its temperature while moving nutrients throughout its body.

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