Transpiration

ICSE Class 10 Biology • Chapter 04

1. Definition & Significance

Transpiration: The loss of water in the form of water vapour from the aerial parts (leaves and stems) of a living plant.

Significance:

Cooling Effect: Evaporation reduces the temperature of the leaf surface, preventing it from sun damage.
Suction Force: Generates a 'Transpirational Pull' which is responsible for the ascent of sap (upward movement of water and minerals) in tall trees.

2. Types of Transpiration

Type	Site	Amount
Stomatal	Stomata mainly on leaves.	80-90% (Maximum).
Cuticular	Cuticle (waxy layer) on leaves/stem.	3-10% (Occurs day & night).
Lenticular	Lenticels on woody stems/fruits.	Minute (0.1%).

3. Experiments

A. Demonstration (Bell Jar)

Setup: A potted plant covered with a bell jar. Soil pot is covered with polythene (to prevent evaporation from soil).

Observation: Water droplets appear on inner walls of the jar.

Conclusion: Plants lose water as vapour.

Control: Empty bell jar without plant (No droplets).

B. Proof (Cobalt Chloride)

Dry Cobalt Chloride Paper: Blue.

Observation: Paper attached to leaf turns Pink.

Dorsiventral Leaf: Lower surface paper turns pink faster (more stomata).

4. Measurement: Ganong's Potometer

Action Principle: "The rate of water intake is approximately equal to the rate of transpiration."

[IMAGE PLACEHOLDER: GANONG POTOMETER]

Diagram of Ganong's Potometer setup. Show: Cut capillary tube, graduated scale, air bubble, reservoir, cut twig. Arrow showing bubble movement towards the twig.

Important Precautions:

The apparatus must be completely water-tight (use vaseline).
The twig should be cut obliquely under water (to prevent air bubble entry in xylem).
The air bubble should be introduced carefully.

Limitation: It measures water uptake, not exactly transpiration (some water is used for photosynthesis).

Practice Q1: Reasoning

BOARD QUESTION Why is the twig cut under water in Ganong's Potometer experiment?

Ans: To prevent air bubbles from entering the xylem vessels, which would block current water transport (transpiration pull).

5. Factors Affecting Transpiration

Factor	Effect	Reason
Sunlight	Increases	Stomata open in light.
Temperature	Increases	Rate of evaporation increases.
Humidity	Decreases	High external moisture lowers the diffusion gradient.
Wind	Increases	Removes saturated air around the leaf.
CO2	Decreases	High CO2 causes stomatal closure.

6. Adaptations (Xerophytic)

Adaptations to reduce excessive transpiration:

Sunken Stomata: Stomata hidden in pits (e.g., Nerium).
Thick Cuticle: Impermeable waxy layer (e.g., Banyan, Peepal).
Reduced Leaves: Leaves modified into spines (Cactus) or needles (Pine).
Narrow Leaves: Reduces surface area (Nerium).

7. Guttation vs Bleeding

Feature	Guttation	Bleeding
Definition	Loss of water as droplets from intact leaf margins.	Loss of sap from injured parts.
Cause	Root Pressure (high in morning).	Root Pressure / Injury.
Pores	Hydathodes (Water Stomata).	Cut vessels.

Exam Practice Questions (PYQ Trends)

Year: 2017

BOARD QUESTION Name the chemical used to test for the presence of water (transpiration) on a leaf.

Ans: Cobalt Chloride Paper (Turns Blue to Pink).

Reasoning

THINKING During the day, transpiration and photosynthesis are interlinked. How?

Ans: Stomata are open during the day to allow CO2 entry for photosynthesis. This simultaneously allows water vapour to escape (transpiration). Thus, transpiration is the "price paid" for photosynthesis.