Potometer

Quick Answer: A potometer is a biology apparatus that measures how fast a cut leafy shoot takes up water, which is used as an estimate of its transpiration rate. It consists of a water-filled tube or reservoir sealed to the shoot, with a capillary tube along which an air bubble or water front moves.

How a Potometer Works

Water evaporating from the leaves pulls a continuous column of water up the xylem of the shoot. In the apparatus, the cut end of the stem sits in a sealed, water-filled system, so every drop the shoot draws up is replaced from a graduated capillary tube. An air bubble introduced into the capillary moves towards the shoot as water is taken up, and the distance it travels in a set time gives the rate.

Because the capillary is narrow, a tiny volume produces a clearly measurable movement. Multiplying the distance moved by the cross-sectional area of the bore converts it into a volume per minute. A reservoir with a tap, or a syringe, lets the bubble be pushed back to the start, so repeated readings can be taken under changing conditions such as bright light, a fan, a plastic bag over the leaves or a cooler room.

Several designs share the name, including simple bubble types and Ganong and Farmer types with a reservoir. The listing does not state which design this is, the capillary bore or the parts supplied.

Getting Reliable Readings

• Cut the shoot under water and fit it to the apparatus under water, so no air enters the xylem.
• Seal every joint with petroleum jelly and dry the leaves before starting; wet leaves transpire less.
• Let the shoot settle in the new conditions for several minutes before timing, then take at least three readings.
• After the practical, rinse the tubing with clean water and let it drain; handle the capillary gently, since thin glass breaks easily.

Transpiration Practicals

• Measuring transpiration rate in secondary and pre-university biology practicals
• Investigating the effect of wind, light, humidity and temperature on water uptake
• Comparing shoots of different species, or with leaves partly coated in petroleum jelly, to show the role of stomata
• Demonstrating the cohesion-tension explanation of water movement in the xylem

Specifications

Why Choose LabEquip

Biology departments buy these in sets so that groups can run the transpiration practical side by side and pool their data. LabEquip includes the potometer in its biology lab products, and many teachers teach it next to the Dutrochet osmometer when covering water movement in plants. Order enquiries go through the contact page.

Frequently Asked Questions

Does a potometer measure transpiration directly?

No. It measures water uptake by the shoot. Most of that water is lost by transpiration, but a small share is used in photosynthesis and to keep cells turgid, so uptake is an estimate of transpiration rather than an exact measure of it.

Why must the shoot be cut under water?

Cutting in air lets an air bubble enter the xylem vessels. The bubble breaks the continuous water column that transpiration relies on, and the shoot then takes up little or no water, giving false low readings.

Which plant shoots give clear results?

Leafy shoots with firm, solid stems seal well and give a clear result; many teachers use shoots of hibiscus, privet or other hedge plants. Soft or hollow stems are hard to seal, and shoots with very few leaves move the bubble too slowly.

How do I reset the bubble in the capillary?

On a design with a reservoir, open the tap briefly to push water through and send the bubble back to the start. On simpler designs, a syringe connected to the tubing does the same job. Close the tap again and let the bubble settle before timing.

How do I calculate the rate of water uptake?

Measure how far the bubble moves in a fixed time. Multiply the distance by the cross-sectional area of the capillary bore, found from its radius as pi times r squared, to get a volume, then divide by the time to express the rate per minute.

Why does the bubble move faster in front of a fan?

Moving air carries away the humid layer of air around the leaves, keeping the water vapour gradient between the leaf spaces and the outside air steep. Water evaporates faster from the leaves, so the shoot draws water up faster and the bubble speeds up.

Last Updated: September 2026