BiologyICSE Class 10 · Transpiration

Ganong's Potometer, Factors Affecting Transpiration and Experiments (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

Ganong's potometer measures how fast a cut leafy shoot takes up water, which is roughly its rate of transpiration: an air bubble moves along a graduated capillary as water is drawn in. Transpiration rises with light, temperature, wind and dry air, and falls with high humidity or dry soil. Cobalt chloride paper shows the lower leaf surface transpires more.

As the shoot transpires it pulls water in, so the bubble drifts toward the shoot; the distance moved in a fixed time shows the uptake rate.

Illustration drawn for QuestionBuddy.

Understand Ganong's Potometer and Factors Affecting Transpiration

Measuring transpiration. Water vapour cannot easily be collected, so in the laboratory we measure the water taken up by a shoot. Since most absorbed water is transpired, the rate of uptake is taken as roughly the rate of transpiration.

Ganong's potometer: parts and working.

A wide-mouthed vessel filled completely with water, holding a freshly cut leafy shoot fixed airtight through a cork.
A graduated horizontal capillary tube connected to the vessel, containing a small air bubble as a marker.
A reservoir with a stopcock, used to push the bubble back to the start of the scale.
Working: as the shoot transpires, it pulls water from the apparatus, so the air bubble moves along the capillary towards the shoot. The distance moved in a fixed time gives the rate of water uptake. By changing one condition at a time (light, fan, humidity) and repeating, the effect of each factor can be compared.

Precautions (how to set it up well).
1. Cut the shoot under water so that air does not enter the xylem and break the water column.
2. Make every joint airtight with Vaseline or grease.
3. Keep the capillary horizontal and the apparatus free of air except the one marker bubble.
4. Let the shoot settle before taking readings, and take several readings.

Limitations (what the apparatus cannot do even when used well).
1. It measures the rate of water absorption, not loss; some water is used in photosynthesis or kept in cells.
2. A cut twig may not stay alive or behave like a whole rooted plant for long.
3. Introducing the air bubble into the capillary is difficult.
4. Air temperature changes make the bubble expand or contract, altering its position.

Factors affecting the rate of transpiration.

Factor	Effect	Reason
Light	increases	stomata open in light
Temperature	increases	faster evaporation; air holds more vapour
Humidity of air	high humidity decreases	smaller vapour gradient between leaf and air
Wind	increases (gentle breeze)	removes moist air near the leaf
Atmospheric pressure	lower pressure increases	evaporation is faster
Soil water	scarcity decreases	guard cells lose turgor; stomata close
Leaf features	many stomata, thin cuticle, large area increase	more surface and pores

Experiment (a): loss in weight. Water a potted plant and enclose the pot and soil in a polythene bag tied round the base of the stem. Weigh, keep in sunlight for some hours and weigh again. The loss in weight is due to transpiration from the shoot alone. Alternative: a leafy shoot in a test tube of water with a thin oil layer on the water surface to stop evaporation. (A bell jar placed over such a plant collects visible droplets on its inner wall.)

Experiment (b): cobalt chloride paper. Dry cobalt chloride paper is blue and turns pink when moist. Place dry blue squares on the upper and lower surfaces of a dorsiventral leaf (still on the plant) and cover each with a glass slide, clipped in place, to keep out room moisture. The paper on the lower surface turns pink first, because a dorsiventral leaf has more stomata on its lower surface.

Flow chart: using the potometer. Cut shoot under water → fix airtight → introduce air bubble → note start mark → time the bubble over a set distance → reset with reservoir → change one factor → compare rates.

Draw-and-label checklist: Ganong's potometer. Leafy shoot in a cork; water-filled vessel; horizontal graduated capillary with an air bubble; reservoir with stopcock above the capillary; stand; arrow showing the bubble moving towards the shoot. Labels: leafy shoot, cork (airtight), water, air bubble, graduated capillary tube, reservoir, stopcock.

Ganong's Potometer and Factors Affecting Transpiration: key facts

Rate from a potometer: $rate of uptake = \frac{distance moved by bubble}{time taken}$; Compare rates under different conditions, one change at a time.
Limitations versus precautions: Limitations: measures absorption not loss; cut twig may not stay alive; air bubble hard to introduce; temperature shifts the bubble | Precautions: cut shoot under water; airtight joints; capillary horizontal; Examiners penalise precautions given as limitations.
Factors at a glance: Increase rate: light, high temperature, wind, dry air, low pressure | Decrease rate: darkness, high humidity, still air, dry soil; Always give the reason with the effect.
Cobalt chloride test: Dry paper blue
Moist paper pink
Lower surface of dorsiventral leaf turns pink first (more stomata); Glass slides keep out room moisture.

Dry paper	blue
Moist paper	pink
Lower surface of dorsiventral leaf	turns pink first (more stomata)

How to answer Ganong's Potometer and Factors Affecting Transpiration questions

ICSE asks for limitations of the potometer, the effect and reason for each factor, readings from potometer tables, and the observation and conclusion of the cobalt chloride and weight-loss experiments.

1Read the question word: 'limitation' (fault of the method) or 'precaution' (care taken).
2For a factor, state the effect (increase or decrease) and the reason (stomata or vapour gradient).
3For a table, divide distance by time to get each rate and compare.
4For experiments, write aim, set-up, observation, conclusion; name the control where there is one.
5Mention the oil layer or polythene cover whenever evaporation from water or soil must be excluded.

Ganong's Potometer and Factors Affecting Transpiration in real life

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

Drying clothes and transpiring leaves

Suppose on a sunny, breezy day your washing dries in two hours, but on a still, sticky monsoon day it stays damp all afternoon.

Use this to predict when a plant transpires fastest and slowest, with reasons.

Try it first, then show the solutionSolution

Evaporation from cloth and from leaf cells depends on the same conditions.
Sun and warmth speed evaporation; in light, stomata are also open, so transpiration is fastest on sunny, breezy days.
Wind removes the moist air next to the surface, keeping the vapour gradient steep.
On humid, still days the air is nearly saturated, so the gradient is small and transpiration is slowest.

Answer: Fastest on sunny, warm, dry, breezy days; slowest on humid, still days or in darkness.

Memory hook: Good drying weather is good transpiring weather.

Tricks to remember Ganong's Potometer and Factors Affecting Transpiration

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

Limits are faults, precautions are care
Limitation = what goes wrong anyway; precaution = what you do to avoid errors.
Use it when: Not mixing up the two lists.
Blue to pink = drink
Cobalt chloride turns pink when it 'drinks' moisture.
Use it when: Recalling the colour change.
LTWH
Light, Temperature, Wind up; Humidity down.
Use it when: Listing factors with their direction.

Ganong's Potometer and Factors Affecting Transpiration: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Potometer limitations · 2 marks · Short answerBoard level

A student sets up Ganong's potometer with every precaution taken. Give two weaknesses that still remain in the method when it is used to show water uptake by a leafy shoot. (The 2024 paper asked for limitations of the potometer.)

Try it first, then show the solutionSolution

Introducing the air bubble into the capillary is difficult (1 mark).
The cut twig may not remain alive for long; or changes in air temperature alter the position of the bubble (1 mark).

Answer: Air bubble hard to introduce; twig may not stay alive (or temperature shifts the bubble).

Example 2: Cobalt chloride observation · 2 marks · Short answerBoard level

Dry cobalt chloride paper is fixed on the upper and lower surfaces of a dorsiventral leaf. State the observation after 15 minutes and explain it.

Try it first, then show the solutionSolution

The paper on the lower surface turns pink faster than that on the upper surface (1 mark).
The lower surface has more stomata, so more water vapour escapes from it (1 mark).

Answer: Lower-surface paper turns pink first because the lower surface has more stomata.

Example 3: Reading a potometer table · 3 marks · Short answerHarder board level

Suppose a potometer bubble moved 10 mm in 5 minutes in still air, 22 mm in 5 minutes in front of a fan, and 6 mm in 5 minutes under a humid bell jar. (a) Calculate the rate in each case in mm per minute. (b) Explain the result in front of the fan and under the bell jar.

Try it first, then show the solutionSolution

(a) Still air 10 ÷ 5 = 2 mm/min; fan 22 ÷ 5 = 4.4 mm/min; bell jar 6 ÷ 5 = 1.2 mm/min (1 mark).
(b) The fan blows away moist air near the leaves, keeping the vapour gradient steep, so the rate rises (1 mark).
Under the bell jar the air becomes humid, the gradient falls, so the rate drops (1 mark).

Answer: (a) 2, 4.4 and 1.2 mm/min. (b) Wind increases and humidity decreases transpiration.

Example 4: Why cut under water · 2 marks · Short answerHarder board level

Why must the shoot for a potometer be cut under water, and why must the joints be greased?

Try it first, then show the solutionSolution

Cutting in air lets air enter the xylem vessels and break the water column, blocking uptake (1 mark).
Greased joints keep the apparatus airtight so that the bubble moves only because of water taken up by the shoot (1 mark).

Answer: To keep air out of the xylem; to keep the apparatus airtight.

Example 5: Two factors in a greenhouse · 3 marks · Case-basedHOTS / competency

Suppose a greenhouse owner switches on misting sprayers at noon and finds the potometer reading drops sharply, though light and temperature are unchanged. (a) Which factor changed? (b) Explain the drop. (c) Suggest one reason the owner might want this.

Try it first, then show the solutionSolution

(a) Humidity of the air rose (1 mark).
(b) Humid air reduces the difference in water vapour between the leaf air spaces and the outside, so less vapour diffuses out (1 mark).
(c) To reduce water loss and wilting of seedlings or cuttings on a hot day (1 mark).

Answer: (a) Humidity. (b) Smaller vapour gradient. (c) To prevent wilting of young plants.

Example 6: Absorption is not loss · 2 marks · Short answerHOTS / competency

A potometer shows the shoot took up 30 mm³ of water in 10 minutes. A student concludes that 30 mm³ was transpired. Is she right? Give two reasons.

Try it first, then show the solutionSolution

Not exactly: the potometer measures water absorbed, not water lost (½ mark).
Some water is used in photosynthesis and some stays in cells to keep them turgid; also a cut shoot may not behave like an intact plant (1½ marks).

Answer: No; uptake slightly exceeds loss because some water is used or retained, and a cut shoot is not a whole plant.

Where students lose marks in Ganong's Potometer and Factors Affecting Transpiration

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

Mistake: Writing precautions as limitations
CISCE examiners' comments (2024): asked for two limitations of Ganong's potometer, candidates wrote precautions instead; the expected points were difficulty of introducing the bubble, the twig not staying alive, and temperature affecting the bubble.
Wrong working
Cut the shoot under water; make the joints airtight.
Right working
The air bubble is difficult to introduce; the cut twig may not remain alive for long.
Do this: Learn the two lists separately.
Usually costs: 2 marks.
Mistake: Factor without reason
Writing only 'wind increases transpiration' earns half the credit; the reason is expected.
Do this: Add 'because it removes humid air near the leaf'.
Usually costs: 1 mark.
Mistake: Wrong colour change
Students reverse it to pink-to-blue.
Do this: Dry is blue, wet is pink.
Usually costs: 1 mark.

Exam tips

Underline 'limitation' or 'precaution' in the question before answering.
Show rate working as distance ÷ time with units.
In the cobalt chloride test, give the reason: more stomata on the lower surface.
Change one factor at a time when describing a comparison.

Practice questions on Ganong's Potometer and Factors Affecting Transpiration

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

Q1 · 1 mark · MCQBoard level
Ganong's potometer measures:
(a) rate of photosynthesis (b) rate of water uptake (c) rate of respiration (d) root pressure
Hint
Bubble moves as water is drawn in.
Answer
(b) rate of water uptake.
Q2 · 1 mark · Very short answerBoard level
What is the colour of dry cobalt chloride paper?
Hint
It turns pink when wet.
Answer
Blue.
Q3 · 2 marks · Short answerHarder board level
State the effect of (a) high humidity and (b) bright light on the rate of transpiration, with reasons.
Hint
Gradient and stomata.
Answer
(a) Decreases: the vapour gradient between leaf and air is small. (b) Increases: stomata open in light.
Q4 · 2 marks · Short answerHarder board level
In the weight-loss experiment, why is the pot covered with a polythene bag? What would happen to the result if it were not?
Hint
Soil evaporation.
Answer
To stop water evaporating from the soil and pot. Without it, the loss in weight would include soil evaporation and overstate transpiration.
Q5 · 1 mark · Very short answerHarder board level
Give the function of the reservoir in Ganong's potometer.
Hint
Resetting.
Answer
It lets water in to push the air bubble back to the start of the scale for repeat readings.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Transpiration is slow when the soil is very dry, even in bright sunlight.
Reason (R): Guard cells lose turgor when water is scarce, so stomata close.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Soil water as a factor.
Answer
(c).
Q7 · 2 marks · Short answerHarder board level
Why are glass slides placed over the cobalt chloride paper in the leaf experiment?
Hint
Atmospheric moisture.
Answer
To prevent moisture in the room air from turning the paper pink, so that any colour change is due only to water vapour from the leaf.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a potometer bubble moves 30 mm in 10 minutes at 25 °C. The room then warms to 35 °C and the bubble moves 30 mm in 6 minutes.
(a) Calculate both rates. (1)
(b) Give the biological reason for the change. (1)
(c) Which limitation of the potometer might also have affected the second reading? (1)
Hint
Distance ÷ time.
Answer
(a) 3 mm/min and 5 mm/min. (b) Higher temperature increases evaporation from leaf cells, so transpiration and uptake rise. (c) The air bubble expands with temperature, which can shift its position independently of uptake.

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.Which way does the potometer bubble move?Show answer
Towards the shoot, as water is drawn in.
2.Which surface of a dorsiventral leaf transpires more?Show answer
The lower surface.
3.What is added to the water surface in a test-tube shoot experiment?Show answer
A thin layer of oil.
4.Does humidity raise or lower transpiration?Show answer
High humidity lowers it.
5.Name one limitation of Ganong's potometer.Show answer
It measures absorption, not loss (or bubble hard to introduce; twig may die; temperature shifts the bubble).

Ganong's Potometer and Factors Affecting Transpiration: frequently asked questions

Why does a potometer not measure transpiration directly?

It measures water drawn into the shoot. Transpiration is almost equal to this, but a little water is used or stored by the shoot.

What is the difference between a limitation and a precaution?

A precaution is care taken during the experiment, such as cutting under water. A limitation is a weakness of the method that remains, such as measuring uptake rather than loss.

Does wind always increase transpiration?

A gentle breeze does. Very strong wind may cause stomata to close and so reduce it.

Why is a dorsiventral leaf used in the cobalt chloride experiment?

Its two surfaces differ in the number of stomata, so the experiment shows unequal transpiration.