Have you considered introducing your students to carnivorous plants? They capture your pupils' imagination with a wide range of cunning tactics to deceive and entrap their prey. A variety of digestive processes can remove the available nitrogen compounds from the smallest gnat to a full grown rat. An arsenal of carnivorous plants makes for a fascinating, environmentally safe, insect control regime in your greenhouse or school lab. In addition a variety of experiments can be performed on the remarkable digestive physiology of these plants.
These make great activities for a STEM science club, or as a an activity for a gardening club when it rains. If students succeed in propagating carnivorous plants, selling them can also be a useful source of funds for the club.
Darwin was particularly interested in carnivorous plants, and students can follow in his footsteps, carrying out their own investigations into Drosera (see accompanying files, available to download from the links on the right - note that the Powerpoint is a large file due to the number of images).
If these are some of the most bizarre and interesting plants to be found, why do all schools not grow them? The belief that carnivorous plants are expensive and difficult to cultivate has put off many teachers. Several species are easily available, inexpensive and will thrive in school science laboratories given a few basic horticultural requirements. The plants dealt with here are specimens trialled in schools and found to be robust and hardy to 'minor student abuse'.
Plants grow well from April to October and should be watered with rain or distilled water only (the plants are calcifuges). During the rest of the year keep in cool conditions (but protect from frost) and keep compost moist only. The dormant period promotes vigorous growth next season.
The plants do not require any feeding and will catch prey on their own. Fertiliser will kill them quickly.
Here are some of the easy to grow/useful specimens:
- Venus Fly Trap (Dionaea muscipula). This has an active - rapid response 0.1 to 0.5 s. Culture conditions: full sun 0-30 C; compost - moss peat:sharp sand 1:1; water from below using rain or distilled water to keep wet during growing season.
- Cape Sundew (Drosera capensis). This has an active but slow response. Culture conditions: full sun 0-30 C; compost - moss peat:sharp sand 1:1; water from below using rain or distilled water to keep wet during growing season.
- Huntsmans Cap (Sarracenia purpurea). Culture conditions: full sun 0-30 C; compost - moss peat:sharp sand:live sphagnum:perlite 4:2:2:1; water from above and below using rain or distilled water to keep wet during growing season.
Drosera capensis (Cape sundew) and Dionaea muscipula (Venus fly trap) are easy to grow from leaf cuttings taken in April or May. If good lighting and high humidity can be maintained the plants will root quickly in the normal peat/sand 1:1 mix. Sarracenia purpurea (Huntsmans Cap) can be propagated by dividing the rhizome. Small specimens do well in large eco-column units.
Some ideas for investigations
Can a fly trap count? Does it have a memory?
Strange questions to ask about a plant? Examine 10 open fly traps. Do not touch them! Look for the trigger hairs. How many are there on each side of the trap? Is there the same number of trigger hairs in all 10 traps?
One of the most interesting questions about the fly trap is this. How does the trap know when to close and when not to close? For example, it will not close when a raindrop or a piece of debris falls on a trigger hair but it will when an insect does. It seems as if the trigger hairs must be touched more than once.
Use a very fine pin or needle and a stop watch to investigate this interesting problem. Here are some suggestions:
- Touch 1 hair once and note the response.
- Touch 1 hair twice in quick succession (e.g. 1 second)
- Touch 1 hair twice with a longer interval (e.g. 1 minute)
- Touch 1 hair once and then another hair on the same side once. And so on.
Fly trap digestion
Sometimes a fly trap will close on something which is not suitable 'food' e.g. a soil particle. If this happens it will open again without trying to digest it. How does it know what to digest?
You can investigate this by placing small quantities of different substances in closed traps and seeing if they open.
What sort of substances are likely to be effective? Remember that fly traps grow in soils deficient in one particular mineral.
Demonstrating digestion by sundews
If an insect lands on a sundew (Drosera) leaf it is held by the glandular hairs which slowly curve round it and push it towards the central glands that secrete the enzymes to digest it. You can watch this happen over the course of 24 hr either by adding a small insect or some other form of protein- perhaps a tiny piece of jelly. The plant growth regulator, auxin, speeds up the growth of the outer cells to aid the curling of the leaf and after digestion does the same to the inner cells to help unfurl the leaf again.
Protein in the layer of gelatin on the non-shiny surface of exposed photographic film can also demonstrate its digestion by a Drosera leaf. Use paperclips to hold the film close to the leaf and leave it overnight. A wonderful outline appears in the morning showing where every gland has exuded its enzymes.
Another demonstration of protein digestion is to set some jelly (any flavour you like!) in the bottom of a Petri dish and cover with small discs of filter paper that have been wiped over the surface of Drosera leaves. 1-2 days later, removal of the discs will expose holes where the enzymes, adsorbed onto the filter paper, have “eaten” the jelly.
Acknowledgements: Nigel Stevens and Jenny Edrich