Transpiration

ICSE Class 10 Biology • Chapter 05 • Detailed Master Notes

Transpiration is a highly useful process for plants primarily for two reasons: one, it creates a suction force in the stem to enable the roots to absorb water and mineral nutrients, and two, for cooling the plant in hot weather. Only about 2% of the water absorbed is used by the plant in photosynthesis and other activities; the rest is lost to the atmosphere.

5.1 What is Transpiration?

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

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

5.2 Demonstration of Transpiration

Transpiration can be easily demonstrated by several simple experiments.

Experiment 1: Bell Jar Experiment

Take a small, well-watered potted plant, preferably one with broad leaves. Enclose the pot completely within a polythene bag and tie it securely around the base of the stem (this prevents the escape of water vapour from the soil). Cover the entire plant under a glass bell jar and keep it in sunlight for an hour or two.

Observation: Drops of water appear on the inner surface of the bell jar. This water vapour has condensed from the air inside, which came from the leaves due to transpiration.

Control Setup: A similar empty bell jar without the plant should be set up side-by-side. No water drops will appear in it.

Experiment 2: Cobalt Chloride Paper Test

Set up the same bell jar experiment, but this time, keep a piece of dry Cobalt Chloride paper alongside the plant inside the bell jar. Dry cobalt chloride paper is blue in colour. When exposed to moisture, it turns pink.

Observation: The blue cobalt chloride paper turns pink, chemically proving that the droplets appearing on the bell jar are indeed water.

Fig. 5.1 & 5.2: Demonstration of Transpiration using a bell jar and Cobalt Chloride paper

5.3 Measurement of Transpiration

The rate of transpiration can be measured using different methods:

1. Weighing Methods

Method A: A small lightweight potted plant can be weighed before and after a certain period of time. The soil surface is covered to prevent direct evaporation. The loss in weight corresponds to the amount of water lost by transpiration.
Method B: Using a test tube filled with water and a leafy shoot. A few drops of oil are added on the water surface to prevent evaporation. The test tube is weighed. The difference in weight after a few hours indicates transpiration loss.

2. Potometer Method (Ganong's Potometer)

Potometer: A device that measures the rate of water uptake by a cut shoot. Since water uptake is almost equal to the water lost through transpiration, it is used to measure the rate of transpiration.

In Ganong's potometer, a twig is fitted into a water-filled apparatus. An air bubble is introduced into the horizontal graduated capillary tube. As water is lost by transpiration, the shoot absorbs water, pulling the air bubble forward. The volume of water lost is measured by reading the movement of the air bubble on the scale.

Fig. 5.4: Ganong's Potometer for measuring the rate of water uptake

Limitations of Ganong's Potometer (ICSE Frequent Question)

Introducing the air bubble is not very easy.
The twig may not remain fully alive for a long time.
Any slight change in the outside air temperature may affect the position of the air bubble in the capillary tube.

5.4 Kinds of Transpiration

Transpiration from the aerial parts of a plant occurs in three ways:

Stomatal transpiration: Occurs through the stomata present mostly on leaves. It accounts for the maximum water loss (about 90%).
Cuticular transpiration: Occurs directly from the surface of the leaves and stems through the cuticle.
Lenticular transpiration: Occurs from the lenticels, which are minute openings on the surface of old woody stems and twigs.

Mechanism of Stomatal Transpiration

Leaves have numerous minute openings called stomata on their lower surface (in dicots). Inside the leaf, there is a large amount of spongy mesophyll tissue with interconnected intercellular spaces.

Water absorbed by roots rises up through the xylem and reaches the mesophyll cells. Water evaporates from the damp surfaces of these cells into the intercellular spaces as water vapour. From here, the vapour diffuses out into the atmosphere through the stomata.

Fig. 5.5: Vertical section of a leaf showing the mechanism of stomatal transpiration

Stomatal Regulation (Opening and Closing)

The stomata are regulated by two bean-shaped guard cells. The inner walls of the guard cells (facing the pore) are thick and stiff, while the outer walls are thin and elastic.

During the day: Guard cells photosynthesise, produce glucose, and their cell sap becomes hypertonic. They absorb water from adjacent cells by endosmosis and become turgid. The thin outer walls bulge out, pulling the thick inner walls apart, thereby opening the stoma.
During the night: Glucose is used up or converted to starch, the cell sap becomes hypotonic, and water leaves the guard cells by exosmosis. They become flaccid, and the inner walls straighten, thereby closing the stoma.

Fig. 5.6: Stomatal apparatus showing open and closed states of the stomata

Experiment: Unequal Transpiration in a Dicot Leaf
If you tie pieces of dry cobalt chloride paper on both the upper and lower surfaces of a dicot leaf using glass slides and clips, the paper on the lower surface turns pink much faster. This proves that transpiration is more rapid on the lower surface because there are more stomata on the lower epidermis of a dicot leaf.

Fig. 5.8: Experiment to show unequal transpiration from the two surfaces of a dorsiventral leaf

5.5 Factors Affecting Transpiration

A. External Factors

Factor	Effect on Transpiration Rate	Reason
Sunlight	Increases	Stomata open in light and close in darkness. High light intensity also increases leaf temperature.
Temperature	Increases	Higher temperature increases the rate of evaporation of water from cell surfaces.
Velocity of Wind	Increases	Wind blows away water vapour surrounding the leaves, preventing air saturation and speeding up evaporation.
Humidity	Decreases	If the outside air is already humid (saturated with moisture), it cannot easily hold more water vapour.
Carbon dioxide	Decreases	An increase in $CO_2$ concentration in the outside air causes stomata to close.
Atmospheric Pressure	Increases (at low pressure)	As atmospheric pressure decreases (like at high altitudes), the rate of evaporation and transpiration increases.

B. Internal Factors

Water content of the leaves: If the water content decreases due to insufficient absorption by roots, the leaves wilt, stomata close, and transpiration drops.

5.6 Adaptations in Plants to Reduce Transpiration

Plants living in dry climates (Xerophytes) evolve special adaptations to conserve water:

Sunken stomata: Stomata are hidden in pits (e.g., Nerium) preventing direct exposure to wind.
Fewer stomata: The absolute number of stomata is reduced.
Narrow leaves: Leaves become narrower (e.g., Pine) or turn into spines (e.g., Cactus) to reduce surface area.
Thick cuticle: Leaves are covered by a thick waxy cuticle (e.g., Banyan).
Loss of leaves: In some cases, leaves drop off, or are entirely absent, and the stem becomes flattened, green, and photosynthetic (e.g., Cactus).

5.7 Significance of Transpiration

Cooling Effect: Evaporation reduces temperature. The latent heat of evaporation provides a cooling effect to the plant body, preventing heat injury under the hot sun.
Suction Force (Transpiration Pull): As water evaporates from leaves, a suction force is created at the top of the plant. This "transpiration pull" acts as a major force drawing water and minerals upwards from the roots (Ascent of sap).
Distribution of Water and Minerals: It helps distribute water and dissolved mineral salts uniformly to all parts of the plant.

Transpiration vs Evaporation
Evaporation is a purely physical process that occurs from any free water surface and cannot be controlled.
Transpiration is a physiological process that occurs only from living plants and is controlled by internal factors like the opening and closing of stomata.

5.8 Guttation and Bleeding

Guttation

In certain plants (like tomato, grass, banana, nasturtium), the root pressure is very high and transpiration is very low (especially during early, humid mornings or nights). The excess water is forced out in the form of liquid droplets along the margins of the leaves.

These droplets escape from special pore-bearing structures called Hydathodes. This process of losing water in liquid form is called Guttation.

Fig. 5.10: Guttation showing water droplets at the margins of a leaf

Bleeding

The exudation of plant sap from any cut or injured part of a plant (like a broken stem) is called Bleeding. It happens due to high root pressure pushing the sap upwards.

Exam Practice Questions (ICSE PYQ Trends)

Diagram Based: Ganong's Potometer

ICSE FAVOURITE Based on the Ganong's Potometer diagram:

What does the apparatus measure?
Ans: It measures the rate of water uptake by a cut shoot.
Why is a cut shoot used?
Ans: To allow direct continuous uptake of water from the apparatus to observe the movement of the air bubble.
How is the air bubble introduced into the capillary tube?
Ans: By lifting the bent capillary tube above the coloured water for a moment to let a small amount of air enter, and dipping it back.
What is the function of the reservoir?
Ans: Opening the stopcock of the reservoir allows water to flow into the capillary tube, pushing the air bubble back to its starting position for a new reading.

Give Reasons

REASONING Answer the following:

Why do some plants wilt during midday even when soil has plenty of water?
Ans: During midday, the rate of transpiration exceeds the rate of absorption of water by the roots. The cells lose turgidity, causing wilting. They recover in the evening when transpiration reduces.
Why is it better to water plants in the evening?
Ans: Transpiration is low in the evening, so water absorbed by roots helps the plant retain turgidity overnight.
Why is a drop of oil placed over water in the test tube experiment for transpiration?
Ans: To prevent any physical evaporation of water directly from the surface, ensuring the loss in weight is solely due to physiological transpiration by the shoot.

Differentiate

DIFFERENCES Differentiate between the following pairs:

Transpiration and Guttation: Transpiration is loss of water as vapour, occurs mostly through stomata during the day. Guttation is loss of water as liquid droplets, occurs through hydathodes at leaf margins, mainly during night or early morning.
Stomata and Lenticels: Stomata are found mainly on the epidermis of leaves and open/close based on turgidity of guard cells. Lenticels are minute permanent openings found on older woody stems and never close.