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A simple potometer

A video of how to build a simple potometer
ALEX JENKIN: A potometer is a piece of apparatus used to measure the rate of water loss from a plant, transpiration. Though this potometer setup may not look like the classic potometer diagrams, it’s functionally exactly the same and is easier and more reliable to set up. The low cost of these potometers also makes a class set of the more affordable. And a class set means that each small group of students can find out how the potometer is set up and works. They can also fully engage with the practical, providing them with a concrete meaningful firsthand experience of investigating factors that affect the rate of transpiration. The rate of transpiration can be estimated in two ways.
Firstly, indirectly, by measuring how the water level drops in the graduated pipette over a measured length of time. It’s assumed that this is due to the cutting taking in water, which in turn is necessary to replace an equal volume of water lost by transpiration. Secondly, it can be measured directly, by measuring the change in mass of the potometer over a period of time. Here it’s assumed that any change in mass is due to transpiration. The potometer is made using the following equipment.
A small glass jar; a rubber band to fit the jar; with two 4 millimetre and one 2 millimetre hole drilled in it; a 1 centimetre cubed graduated pipette– a plastic pipette makes this pedometer much safer; –10 centimetre cubed or 20 centimetre cubed plastic syringe; paper towel; scissors or secateurs; a sink or a large bowl of water; washing up liquid; a light source; woody plant cuttings at least 15 to 20 centimetres long with a stem diameter of approximately 5 millimetres. We recommend using leaves which don’t have thick waxy cuticles. So those without thick glossy leaves. Each shoot should be placed in a container of water immediately upon being cut and kept in water until it’s used.
So to set up the potometer, using some washing up liquid as lubricant insert the pipette into one of the 4 millimetre holes, pushing it all the way through the bung taking care to make sure that the jar can still be sealed without the pipette hitting the bottom of the jar. Then insert the syringe nozzle into the 2 millimetre hole.
Again, lubricating with washing up liquid, insert a plant stem through the other 4 millimetre hole so that at least 3 centimetres protrudes from the bottom of the bung.
Place the rubber bang and glass jar in a bowl or sink filled with water. One of the advantages of this potometer setup is that only the final assembly needs to be done under water. Use the plunger to fill the syringe with water and recut the plant stem under the water. This avoids airlocks in the xylem vessels.
You only need to cut about one centimetre off.
Making sure the jar is full of water insert the bung, take care as water will shoot out the end of the pipette.
You can use the syringe to reset the potometer for different investigations. The meniscus in the pipette should be stable and not visibly dropping. If it is then the potometer has a leak and you should try another piece of plant material. Your potometer is ready to go. Students can work in groups to test different variables. For example, in and out of a sealed plastic bag, light versus dark, and windy conditions. Care should be taken that if a hair dryer was used for this that it is on a cool setting. The plant should be given time to adjust to its new condition before starting an investigation. This will be about 10 to 15 minutes.
In this video you will find out how to set up and use a simple potometer. As a safety precaution, it is suggested that a technician should set up the potometers in advance, although students will enjoy inserting the assembled bungs into the glass jars in bowls of water at the start of their investigation.
This simple piece of apparatus allows students to work individually or in pairs to measure the rate of water loss (transpiration) in a plant shoot within a one hour lesson. This low-cost potometer gives your students opportunities to investigate first-hand the effects of different environmental factors on the rate of transpiration.
Additional information about the equipment is available in the protocol.


List three questions in the comments below that you would you ask your students to check their understanding during this practical.
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