Transpiration ICSE Class 10: Stomata, Potometer, Factors and Experiments

What Transpiration is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Transpiration is in the 2. Plant Physiology (ii) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Transpiration: types and significance

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

Types:

• Stomatal: through stomata on leaves; the largest share of water loss.
• Cuticular: directly through the cuticle of leaves and stems; small.
• Lenticular: through lenticels (pores in the bark of woody stems); very small.

Significance (advantages):

• Creates transpirational pull, the main force for the ascent of sap.
• Helps carry minerals dissolved in water up to the leaves.
• Cools the leaves by evaporation, protecting them from heat.
• Keeps the cells' water moving, so they receive fresh water and minerals.

It is sometimes called a 'necessary evil': useful, but excessive loss causes wilting and can harm the plant in dry conditions.

Worked example

A friend says transpiration only wastes water. Give two ways in which it actually benefits the plant.

Try it first, then show the solutionSolution

1. It produces transpirational pull, which lifts water and minerals.
2. Evaporation cools the leaf surface.

Answer: Ascent of sap (with minerals) and cooling.

Mechanism of stomatal opening: potassium ion exchange theory

Each stoma is a pore bordered by two kidney-shaped guard cells. The inner wall of each guard cell (next to the pore) is thick and less elastic; the outer wall is thin and elastic. Guard cells have chloroplasts.

Opening (in light):
1. Potassium ions (K⁺) move actively from the neighbouring (subsidiary/epidermal) cells into the guard cells.
2. The guard cell sap becomes more concentrated, so water enters them by osmosis from neighbouring cells.
3. The guard cells become turgid. Their thin outer walls stretch more than the thick inner walls, so the guard cells bow outwards and the pore opens.

Closing (in darkness or water shortage):
1. K⁺ ions move out of the guard cells.
2. Water leaves by osmosis; the guard cells become flaccid.
3. They straighten and the pore closes.

Worked example

Stomata open in the morning light. Name the ion whose movement starts this, and explain how it opens the pore.

Try it first, then show the solutionSolution

1. Potassium ions are pumped into the guard cells in light.
2. This draws in water by osmosis.
3. Turgid guard cells curve apart, opening the pore.

Answer: Potassium (K⁺).

Factors affecting the rate of transpiration

External factors:

• Light: stomata open in light, so transpiration is faster in light.
• Temperature: higher temperature increases evaporation: faster.
• Humidity: in humid air the diffusion gradient is small: slower; in dry air: faster.
• Wind: moving air removes the water vapour near the leaf: faster (very strong wind may close stomata).
• Atmospheric pressure: lower pressure (high altitudes) speeds evaporation.
• Water supply in the soil: if roots cannot get water, guard cells lose turgor, stomata close: slower.

Internal factors: number, size and position of stomata; total leaf area; thickness of cuticle; presence of hairs.

Worked example

Why do clothes dry slowly, and plants transpire slowly, on a humid day?

Try it first, then show the solutionSolution

1. Humid air already contains much water vapour.
2. The difference in water vapour concentration between the leaf (or cloth) and the air is small.
3. So water vapour diffuses out slowly.

Answer: A small diffusion gradient in humid air.

Ganong's potometer and its limitations

Ganong's potometer measures the rate of water uptake by a leafy shoot, which is taken as approximately equal to the rate of transpiration.

Set-up: a freshly cut leafy shoot (cut under water to avoid air entering the xylem) is fixed airtight into the apparatus filled with water; a graduated horizontal capillary tube contains an air bubble; a reservoir with a stopcock lets you push the bubble back to the start. As the shoot absorbs water, the bubble moves along the capillary; the distance moved per minute gives the rate.

Limitations (not precautions):

• It measures water absorbed, not water lost; some absorbed water is used in photosynthesis and to keep cells turgid.
• A cut shoot behaves differently from a whole plant with roots, and it does not stay healthy for long.
• It is hard to keep the apparatus completely airtight, and the air bubble may expand or contract with temperature.

Precautions (a different list): cut the shoot under water; make all joints airtight with Vaseline; keep the capillary horizontal.

Worked example

Why can a potometer reading not be taken as an exact measure of transpiration? Give two reasons.

Try it first, then show the solutionSolution

1. It measures the rate of absorption, not transpiration; some water is retained or used.
2. A cut shoot does not behave exactly like an intact plant (and may wilt).

Answer: As above.

Experiments on transpiration

(a) Loss in weight: water a potted plant and cover the pot and soil with a polythene bag tied around the base of the stem, so no water evaporates from the soil. Weigh it, leave it in sunlight for a few hours and weigh again. The loss in weight is due to transpiration from the shoot. (Alternative: a leafy shoot in a test tube of water with a layer of oil on top to stop evaporation; weigh at intervals.)

Bell jar demonstration: cover such a potted plant with a bell jar; droplets of water collect on the inside of the jar.

(b) Cobalt chloride paper (dorsiventral leaf): dry cobalt chloride paper is blue and turns pink with moisture. Clip a piece of dry blue paper to the upper and another to the lower surface of a leaf (covered with glass slides to keep out air moisture). The paper on the lower surface turns pink faster, showing that the lower surface transpires more, because a dorsiventral leaf has more stomata on its lower surface.

Worked example

In the cobalt chloride paper experiment, why is the pot's soil not relevant, but the papers must be covered with glass slides?

Try it first, then show the solutionSolution

1. The papers detect water lost from the leaf only.
2. Glass slides stop moisture from the surrounding air reaching the papers, so any colour change is due to the leaf.

Answer: Slides protect the paper from atmospheric moisture.

Adaptations, guttation and bleeding

Adaptations to reduce transpiration (in dry habitats): sunken stomata; fewer stomata, mostly on the lower surface; a thick waxy cuticle; hairs on leaves; leaves reduced to spines (cactus) with photosynthesis in the green stem; rolling or folding of leaves; shedding leaves in the dry season.

Guttation: loss of water as liquid droplets from special pores called hydathodes at the tips or margins of leaves. It occurs early in the morning or on humid nights, when root pressure is high and transpiration is low (grass, tomato, strawberry, garden nasturtium).

Bleeding: the oozing of sap from a cut or injured part of a plant, due to root pressure, for example when the stem of a plant is cut near the base, or when palm trees are tapped for their sap.

Worked example

Drops of water appear along the edges of tomato leaves at dawn. Through which pores do they come, where exactly are these pores, and how does this differ from transpiration?

Try it first, then show the solutionSolution

1. Hydathodes are at the tips and margins of leaves.
2. Guttation loses water as liquid drops, through hydathodes, when transpiration is low.
3. Transpiration loses water vapour, mainly through stomata.

Answer: At leaf tips and margins; liquid versus vapour, hydathodes versus stomata.

Transpiration in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Transpiration: Types, Significance and Guttation
• Ganong's Potometer and Factors Affecting Transpiration
• Opening and Closing of Stomata: Potassium Ion Exchange

Transpiration: key facts

Stomatal opening chain

$K^{+}\text{ in}\to \text{water in (osmosis)}\to \text{guard cells turgid}\to \text{stoma opens}$

Reverse for closing.

Rate from a potometer

$\text{rate}=\frac{\text{distance moved by bubble}}{\text{time}}$

Approximates water uptake.

Cobalt chloride test

$\text{blue (dry)}\to \text{pink (moist)}$

Shows water loss.

Factors that increase the rate

$\text{light}\uparrow ,\text{temperature}\uparrow ,\text{wind}\uparrow ,\text{humidity}\downarrow$

Each speeds transpiration.

Transpiration: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Transpiration

The question patterns to expect, and the method that works for each.

Tricks to remember Transpiration

Short memory aids. Say each one aloud once, then use it on a question the same day.

• K⁺ is the Key

  Potassium is the Key to opening stomata.

  Use it when: Ion in the mechanism.

• Blue Dry, Pink Wet

  Cobalt chloride: Blue when Bone-dry, Pink when Wet.

  Use it when: Colour change.

• LTW up, H down

  Light, Temperature, Wind increase the rate; Humidity decreases it.

  Use it when: Factors.

• Guttation = Gutters

  Like water dripping from gutters at the edges of a roof, guttation drips from leaf margins.

  Use it when: Guttation vs transpiration.

Transpiration: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Transpiration

The slips examiners see most often, and what to write instead.

• ✗Mistake: Osmosis for guttation

  CISCE examiners' comments, 2024: for water lost from the margins of strawberry leaves, candidates wrote osmosis instead of guttation.

  Wrong working

  Water drops at leaf margins: osmosis.

  Right working

  Guttation, through hydathodes.

  ✓Do this: Liquid at margins = guttation.

  Usually costs: 1 mark.

• ✗Mistake: Magnesium for potassium

  CISCE examiners' comments, 2024: for the element that helps in the opening of stomata, candidates wrote magnesium.

  Wrong working

  Magnesium ions open stomata.

  Right working

  Potassium (K⁺) ions.

  ✓Do this: K⁺ is the Key.

  Usually costs: 1 mark.

• ✗Mistake: Irrelevant advantages

  CISCE examiners' comments, 2024: for advantages of transpiration, candidates wrote irrelevant points such as 'removes excess water from plants'.

  Wrong working

  Transpiration removes excess water.

  Right working

  Creates transpirational pull for ascent of sap; cools the leaves; helps distribute minerals.

  ✓Do this: Name a benefit to the plant's functioning.

  Usually costs: 1 mark per point.

• ✗Mistake: Precautions instead of limitations

  CISCE examiners' comments, 2024: asked for limitations of Ganong's potometer, candidates wrote precautions.

  Wrong working

  Cut the shoot under water.

  Right working

  It measures absorption, not transpiration; a cut shoot is not like a whole plant.

  ✓Do this: Limitation = what the apparatus cannot do; precaution = what you do to get a good result.

  Usually costs: 1 mark per point.

• ✗Mistake: Vague location of hydathodes

  CISCE examiners' comments, 2025: candidates wrote only 'leaves' for hydathodes, or confused them with stomata.

  Wrong working

  Hydathodes are on leaves.

  Right working

  Hydathodes are at the tips and margins of leaves.

  ✓Do this: Say where on the leaf.

  Usually costs: 1 mark.

• ✗Mistake: Not naming three phenomena

  CISCE examiners' comments, 2025: for a tree picture labelled at three places, candidates could not identify the three phenomena.

  Wrong working

  Leaves: photosynthesis; stem: support.

  Right working

  Leaves: transpiration; stem: ascent of sap; roots: absorption.

  ✓Do this: Follow the water from soil to air.

  Usually costs: 1 mark each.

Practice questions on Transpiration

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The rate of transpiration decreases when:
   (a) temperature rises
   (b) wind speed increases
   (c) humidity increases
   (d) light intensity increases

   Hint

   Diffusion gradient.

   Answer

   (c)

2. Q2 · 1 mark · MCQBoard level

   Which structures release water as liquid drops on humid nights?
   (a) stomata
   (b) lenticels
   (c) hydathodes
   (d) guard cells

   Hint

   Guttation, at leaf tips and margins.

   Answer

   (c)

3. Q3 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Cobalt chloride paper on the lower surface of a dorsiventral leaf turns pink faster.
   Reason (R): The upper surface of a dorsiventral leaf has a thinner cuticle.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Stomata count.

   Answer

   (c): the lower surface has more stomata; the upper cuticle is usually thicker.

4. Q4 · 3 marks · Very short answerBoard level

   Name: (a) the pores in the bark through which some transpiration occurs; (b) the cells that regulate the stomatal pore; (c) the apparatus used to measure water uptake by a shoot.

   Hint

   One term each.

   Answer

   (a) Lenticels (b) Guard cells (c) Ganong's potometer

5. Q5 · 2 marks · Short answerHarder board level

   Give four adaptations in desert plants that reduce transpiration.

   Hint

   Leaf, cuticle, stomata.

   Answer

   Leaves reduced to spines; thick waxy cuticle; sunken stomata; fewer stomata (also hairs, rolled leaves, leaf fall).

6. Q6 · 2 marks · Short answerHarder board level

   Why is transpiration called a 'necessary evil'?

   Hint

   Benefit and cost.

   Answer

   It is necessary because it drives the ascent of sap and cools leaves; it is an 'evil' because excessive loss of water can cause wilting and harm the plant when water is short.

7. Q7 · 3 marks · Short answerHarder board level

   A potometer bubble moves 6 cm in 3 minutes in still air and 6 cm in 1.5 minutes under a fan. Calculate both rates and explain the difference.

   Hint

   cm per minute.

   Answer

   Still air: 6 ÷ 3 = 2 cm/min. Fan: 6 ÷ 1.5 = 4 cm/min. Wind removes humid air near the leaves, steepening the diffusion gradient, so transpiration (and uptake) doubles.

8. Q8 · 3 marks · Short answerHarder board level

   Explain why stomata close when a plant is short of water, using the potassium ion exchange theory.

   Hint

   K⁺ out.

   Answer

   When water is short, K⁺ ions move out of the guard cells; water follows by osmosis; the guard cells become flaccid and straighten, closing the pore and reducing water loss.

9. Q9 · 3 marks · Case-basedHOTS / competency

   Two identical plants are placed, one in a sealed polythene bag and one in open air, both in sunlight. After some hours, the bag is misty inside. Which plant loses more water, and why? What is the mist?

   Hint

   Humidity builds up.

   Answer

   The open-air plant loses more: humidity inside the bag rises and slows transpiration. The mist is transpired water vapour condensing on the bag.

10. Q10 · 3 marks · Short answerHOTS / competency

    A gardener says guttation and bleeding are the same because both release liquid. Give one similarity and two differences.

    Hint

    Root pressure in both.

    Answer

    Similarity: both are due to root pressure (liquid exudation). Differences: guttation is through hydathodes at leaf tips and margins, bleeding from cut or injured parts; guttation occurs naturally in an intact plant (early morning or humid nights), bleeding only when the plant is wounded or cut.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define transpiration.Show answer

   Loss of water as water vapour from the aerial parts of a plant.

2. 2.Through which structures does most transpiration occur?Show answer

   Stomata.

3. 3.Which ion moves into guard cells to open stomata?Show answer

   Potassium (K⁺).

4. 4.Colour of dry cobalt chloride paper?Show answer

   Blue (turns pink when moist).

5. 5.Effect of high humidity on transpiration?Show answer

   Decreases it.

6. 6.Define guttation.Show answer

   Loss of water as liquid droplets through hydathodes at leaf tips and margins.

7. 7.Define bleeding.Show answer

   Oozing of sap from a cut or injured part of a plant due to root pressure.

8. 8.Name two adaptations to reduce transpiration.Show answer

   Sunken stomata; thick cuticle (also leaves as spines, hairs, rolled leaves).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain stomatal opening in three steps?
• ☐Can you list four factors and how each changes the rate?
• ☐Can you give two limitations (not precautions) of the potometer?
• ☐Can you describe the cobalt chloride experiment and its result?
• ☐Can you distinguish transpiration from guttation and bleeding?

Transpiration: frequently asked questions