Use models as representations of the real world—mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues.
This is another NOS that provides many links to the prescribed syllabus in Topic 9.1. In fact, you could teach many of the content using the potometer as a demonstration model, and then have students individually use their own and design their own independent variables etc.
- Prescribed Practical 7: Measurement of transpiration rates using potometers
- Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing;
- Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.
Potometers can be as simple or technical as you like them to be. A standard set-up might look something like this:
The general plant requirements are to use a woody stemmed-branch and that the leaves have a thin waxy cuticle.
The key to success is ensuring that there are no air bubbles in the tubing, as air bubbles will prevent the transpiration stream from working effectively.
If you have access to Vernier data loggers (or something similar) you can use a gas pressure sensor to record the rate of transpiration. This can provide a more reliable quantitative measurement of the rate of transpiration and could be a good option for an Individual Investigation (IA).
Here are some pictures of the ones we set-up:
In the bottom right-hand corner you can see that the pressure in the tube is decreasing, indicating that transpiration is taking place. We did some simple independent variables – removing leaves and changing the light intensity. You could use a fan to try and stimulate a windier environment and small plastic bags on the leaves to increase humidity. The advantage with a data logger is the ease of collecting the data and then analysing it, allowing for quick calculations of rate and other statistics.
“LabBench Activity.” Design of the Experiment – Potometer, Pearson Education Inc., http://www.phschool.com/science/biology_place/labbench/lab9/design.html. Web. Accessed Feb 12, 2018.
“Measuring Rate of Water Uptake by a Plant Shoot Using a Potometer.” Practical Biology, Nuffield Foundation, http://www.nuffieldfoundation.org/practical-biology/measuring-rate-water-uptake-plant-shoot-using-potometer.Web. Accessed Feb 12, 2018.