Plant Physiology — Transpiration
Introduction
Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant. In ICSE Class 10 Biology, you study the types of transpiration, factors affecting it, experimental demonstrations, and the mechanism of stomatal opening and closing.
Types of Transpiration
| Type | Part involved | Percentage contribution |
|---|---|---|
| Stomatal transpiration | Through stomata (leaves) | 80−90% |
| Cuticular transpiration | Through cuticle (leaf surface) | 5−10% |
| Lenticular transpiration | Through lenticels (bark) | 1−5% |
Stomatal Mechanism
Stomata are minute pores on the leaf surface, each surrounded by two guard cells.
Mechanism of Opening and Closing
| Condition | Guard cell state | Stomatal pore |
|---|---|---|
| Light, sufficient water | Turgid (water enters by osmosis) | Opens |
| Dark, water scarcity | Flaccid (water leaves by osmosis) | Closes |
How it works:
- In sunlight, guard cells produce glucose by photosynthesis.
- Increased solute concentration in guard cells draws water from neighbouring cells by osmosis.
- Guard cells become turgid and bend, opening the stoma.
- In darkness, glucose is converted to starch (osmotic concentration decreases).
- Water leaves guard cells, they become flaccid, and the stoma closes.
Factors Affecting Transpiration Rate
| Factor | Change | Effect on transpiration |
|---|---|---|
| Temperature | Increases | Increases (more evaporation) |
| Humidity | Increases | Decreases (air is already moist) |
| Wind speed | Increases | Increases (removes water vapour) |
| Light intensity | Increases | Increases (stomata open wider) |
| Soil water availability | Decreases | Decreases (plant conserves water) |
| Number of leaves | More leaves | Higher transpiration |
Experiments on Transpiration
Potometer Experiment (Measuring Transpiration Rate)
Setup: A leafy shoot is attached to a potometer (a device with a water-filled tube and a capillary).
Procedure:
- The apparatus is set up underwater to prevent air bubbles.
- The rate of water uptake is measured by the movement of an air bubble in the capillary.
- The rate of transpiration is calculated as distance moved by bubble per unit time.
Precaution: All joints must be airtight.
Cobalt Chloride Paper Test
Principle: Cobalt chloride paper is blue when dry and turns pink on absorbing moisture.
Procedure:
- Dry blue cobalt chloride paper is attached to both surfaces of a leaf.
- After some time, the paper on the lower surface turns pink first (more stomata on the lower surface in most leaves).
- This demonstrates that transpiration occurs through stomata.
Adaptations to Reduce Transpiration (Xerophytes)
- Thick cuticle (e.g., cactus).
- Reduced/absent leaves (spines in cactus).
- Sunken stomata (e.g., pine, oleander).
- Leaves folded/rolled (e.g., marram grass).
- Hairy leaves (trap moisture, reduce air movement).
Significance of Transpiration
- Creates transpiration pull for ascent of sap.
- Helps in absorption and transport of water and minerals.
- Cools the plant (evaporative cooling).
- Maintains turgidity of cells.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Thinking transpiration is the same as evaporation | Transpiration is a physiological process through plant parts |
| Confusing transpiration and guttation | Transpiration: water vapour; Guttation: liquid water from hydathodes |
| Assuming transpiration only occurs through leaves | Also occurs through stem (lenticels) and cuticle |
| Forgetting that light affects stomatal opening | Stomata open in light and close in darkness |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: types of transpiration, factors, potometer experiment, cobalt chloride test, stomatal mechanism, xerophytic adaptations.
Marks Blueprint: Types/factors — 2 marks, Experiments — 2 marks, Stomatal mechanism — 1 mark.
Self-Test Questions
-
Define transpiration. Name the three types of transpiration.
-
Describe an experiment using a potometer to measure the rate of transpiration.
-
How would you use cobalt chloride paper to demonstrate transpiration from leaves?
-
Explain the mechanism of stomatal opening and closing.
-
List four factors that increase the rate of transpiration.
-
What adaptations do xerophytes have to reduce water loss?
