A potometer is a device for measuring transpiration in plants. If you have not read Leaves (Part Two), then you probably should before finishing this post.
Water evaporates from leaves through the stomata. Since water molecules are attracted to one another by tiny forces, this draws new water molecules in through the roots. This process of evaporation and transport is called transpiration. The apparatus shown below can measure the rate at which transpiration occurs under different environmental conditions. It can also be used to compare different plants/leaves.
The apparatus is filled with water and a tiny air bubble allowed to enter the capillary tube. A capillary tube has a very narrow lumen. As water evaporates from the surface of the leaves, water within the apparatus is absorbed. This causes the bubble to move from right to left. The distance travelled and the time taken can be used to produce a rate of transpiration in cm/minute.
Potometer apparatus can be bought as a kit from science supply companies. It is notoriously fiddly to set up because everything has to be put together underwater to make sure that no air bubbles enter the plant stem and block the xylem. Of course, it is also possible to design your own potometer once you understand how the idea works. The following video shows a simple home made design being constructed.
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Questions…
- What word means, the empty space inside a pipe or tube?
- For what process within plants is water a raw material?
- Suggest a nutrient also absorbed from the soil with the water. For what does the plant use it?
- Why is the stem cut under water before being fitted to the potometer?
- How would blowing a fan at the plant affect the rate of transpiration?
- Would you expect light intensity to affect the rate of transpiration? If so, in what way?
- In times of drought, plants may need to reduce the rate of transpiration. How can they do this?
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