Watching gas bubble up from a pondweed as it photosynthesises can be a great demonstration or student practical. When placed closer to a light source, the rate of bubbling will speed up, and as the pondweed is taken further away, the bubbles will slow down again - an instant and visual indicator of the importance of light intensity in photosynthesis. Our video demonstrates how best to use this protocol with your students in the lab.
The bubbles can be counted and the rate of bubbling can serve as an indication of the rate of photosynthesis, or the gas can be collected in a pipette or microsyringe and the amount measured. Students can investigate the effects of either light intensity or the wavelength (colour) of light on photosynthesis.
This resource includes student sheets with 4 different investigations, technical notes and full teachers' notes.
Cabomba is no longer available in the UK, due to the recent invasive plant directive.
Cabomba caroliniana is no longer available in the EU, due to new regulations on the propagation of invasive species. We have trialled various alternatives but we have had varied results and though we recommend trying the species below, unfortunately nothing has proved as reliable as Cabomba caroliniana. In addition, you may wish to consider alternative practicals to look at photosynthesis, such as the ‘algal balls’ practical.
Note: When buying from pet stores in particular, the staff may not be able to identify the type of pondweed they are selling but we hope that this guide proves helpful. We welcome any feedback on pondweed varieties and tests are still ongoing.
Cabomba aquatica (yellow cabomba) is not as easy to grow as C. caroliniana but in our tests produced either a stream of small bubbles or a slower stream of larger bubbles. Available online and from some pet shops and aquarium suppliers.
Cabomba furcata (red cabomba) produced a good stream of bubbles in our tests. It is available online and from some pet shops and aquarium suppliers.
Ceratophyllum demersum (hornwort) A British native species, suitable for growing in a school pond, or in an indoor tank. This is a robust, fast-growing, easily-propagated plant but may be too woody during winter. Available online.
Egeria densa (formerly Elodea densa). This is a non-native species that can tolerate relatively cold temperatures and low light levels. The short distance of stem between leaf junctions means that it’s harder to cut the stem cleanly. However, we would not recommend growing this in school ponds, as it is an invasive species that can cause harm to invertebrates and fish. Widely available from aquarium suppliers online, and from Blades Biological.
Egeria najas is an easy growing species from South America. It can tolerate relatively cold temperatures and low light levels. However, with high light levels it will grow rapidly. The short distance of stem between leaf junctions means that it’s harder to cut the stem cleanly. Available online and from pet shops and aquarium suppliers.
Elodea species. There are two Elodea species found in the UK, both of which are non-native and potentially invasive: Elodea canadensis and Elodea nuttallii. They are already widespread but should not be introduced to school ponds. They produce large, slow bubbles. Available online.
Myriophyllum scabratum (note – do not use other Myriophyllum species, as these can be invasive). This is a fast growing aquarium plant, which appreciates bright light and an aquarium tank at room temperature. Widely available from aquarium suppliers online, and from Blades Biological.
Download the resources from the link on the right.
