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Measurement of Transpiration Rate using Ganong's Potometer

Experiment video

What you will learn

Measure the rate of water absorption and transpiration in a leafy shoot using Ganong's Potometer under varying environmental conditions (still air, fan wind, bright sunlight).

Materials needed

Ganong's Potometer apparatus with graduated capillary tube and water reservoir
Ganong's Potometer apparatus with graduated capillary tube and water reservoir
₹50
What is it?
Ganong's glass potometer apparatus with graduated capillary tube and water reservoir.
Specifications
Capillary Bore
1.0 mm internal diameter precision bore
Scale
0 – 20 cm graduated in 1 mm
Reservoir
Integral glass reservoir with ground glass stopcock
Mounting
Mounted on polished wooden base with spring clips
Fresh leafy shoot (Coleus / Hibiscus / Nerium) cut underwater
Fresh leafy shoot (Coleus / Hibiscus / Nerium) cut underwater
₹95
What is it?
Freshly harvested leafy twig of Hibiscus rosa-sinensis or Coleus blumei.
Specifications
Shoot Type
Woody stem with 15 – 20 broad healthy leaves
Preparation
Harvested under boiled cooled water at 45° angle
Colored water / beaker with water
Colored water / beaker with water
₹140
What is it?
Environmental simulation accessories: table fan, 100W incandescent lamp, clear polythene hood.
Specifications
Electric Fan
3-speed adjustable laboratory table fan
Lamp
100 W tungsten incandescent spotlight
Stopwatch
Digital 0.01s stopwatch
Stopwatch
Stopwatch
₹185
What is it?
Standard laboratory apparatus used in Measurement of Transpiration Rate using Ganong's Potometer.
Specifications
Category
Biology Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Electric Table Fan & 100W Incandescent Lamp (for variable testing) set
Electric Table Fan & 100W Incandescent Lamp (for variable testing) set
₹230
What is it?
Standard laboratory apparatus used in Measurement of Transpiration Rate using Ganong's Potometer.
Specifications
Category
Biology Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Vaseline / Petroleum jelly (for airtight seals) tub
Vaseline / Petroleum jelly (for airtight seals) tub
₹275
What is it?
Standard laboratory apparatus used in Measurement of Transpiration Rate using Ganong's Potometer.
Specifications
Category
Biology Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard

Tick items as you collect them. Tap + to see the product, specifications and buy options.

Stay safe

Cut the shoot underwater at a 45° angle to prevent air entering xylem vessels and blocking water uptake.
The entire apparatus must be completely airtight; any leakage prevents bubble movement.
Introduce only a single, compact air bubble into the capillary tube.
Ensure leaves are dry before commencing measurements.

Steps

0 of 7 steps done

1Fill the entire Ganong's potometer apparatus with water, ensuring zero trapped air bubbles in the capillary.
Observation
Cut healthy leafy shoot of Coleus or Hibiscus under water with sharp razor to prevent air entry into xylem vessels.
Safety
Always cut shoot under water to prevent xylem cavitation / air embolism.
2Cut the base of a leafy shoot underwater at a slant with a sharp razor blade to prevent air embolism in xylem vessels.
Observation
Fit shoot tightly into potometer rubber stopper under water; seal joint with molten paraffin wax or vaseline.
Safety
Apparatus must be 100% airtight; any tiny air leak halts bubble movement.
3Insert the cut shoot tightly into the vertical rubber cork of the potometer.
Observation
Introduce single air bubble into capillary tube by briefly lifting bent end out of water beaker for 2 seconds.
Safety
Control air bubble size to approximately 2-3 mm length.
4Apply petroleum jelly around all cork joints to ensure the entire apparatus is 100% airtight.
Observation
Record bubble movement across calibrated capillary scale for 10 minutes under normal room conditions.
Safety
Ensure bubble moves smoothly without sticking to tube glass walls.
5Dip the bent end of the capillary into a beaker of colored water. Lift it briefly for a few seconds to introduce a single air bubble, then re-immerse.
Observation
Reset bubble to zero using reservoir stopcock; expose shoot to electric fan breeze and record increased rate.
Safety
Keep water reservoir stopcock tightly closed during measurement intervals.
6Record the time taken by the air bubble to travel 5 cm along the graduated capillary in normal room conditions.
Observation
Compare rates across wind, light, humidity, and dark; graph comparative transpiration dynamics.
Safety
Keep lamp at least 30 cm from shoot to prevent thermal scorching of leaves.
7Reset the bubble using the reservoir stopcock and measure bubble movement under fan breeze and lamp illumination.

Tap + on a step to see what you will observe and any safety tips.

Record your observations

Environmental Condition Tested	Bubble Travel in 10 mins (cm)	Rate of Bubble Travel (cm/min)	Transpiration Rate (mm³/min)	Physiological Response
Normal Lab (Still air, 24°C)	3.5 cm	0.35 cm/min	2.75 mm³/min	Baseline stomatal transpiration
High Wind (Electric fan at 1m)	7.8 cm	0.78 cm/min	6.13 mm³/min	Wind sweeps boundary vapor layer away; rapid rate
Bright Light (100W lamp at 30cm)	6.2 cm	0.62 cm/min	4.87 mm³/min	Light triggers stomatal opening; increased transpiration
High Humidity (Polythene cover)	0.8 cm	0.08 cm/min	0.63 mm³/min	Vapor pressure deficit drops; transpiration stalls
Darkness (Dark room box)	1.2 cm	0.12 cm/min	0.94 mm³/min	Stomata close in darkness; cuticular transpiration only

Tap a highlighted cell to write your own observation.

Result and conclusion

Bubble travel velocity is fastest under bright light and breeze (0.85 cm/min) and slowest in humid shade (0.15 cm/min), demonstrating stomatal transpiration dynamics.

Transpiration Rate R = (π · r² · d) / t = (3.1416 × 0.5² mm² × 60 mm) / 10 min = 4.71 mm³/min

Observation	What it means
Water Absorption Couples Transpiration	Potometer measures rate of water absorption by cut leafy shoot, which closely parallels foliar transpiration rate.
Wind Velocity Acceleration	Air movement over leaf surfaces blows away stagnant humid boundary layers, steepening vapor pressure gradient (2.2× rate increase).
Light-Dependent Stomatal Response	Bright light stimulates guard cell proton pumps, opening stomata and increasing water loss by 77% over baseline.
Humidity Suppression	Enclosing shoot in a humid polythene bag lowers vapor gradient between leaf mesophyll and air, reducing transpiration by 77%.

Transpiration Rate under Diverse Environmental Regimes

Sample readings. Replace with your own results.

Experiment photos

Before the experimentDuring the experimentResult