The amazing concertina cactus

Do you have a dried up cactus lurking in your classroom? Treasure it – it is a valuable educational resource.

A class of Year 3 and Year 4 pupils visited the Cambridge University Botanic Gardens. In the cactus house they speculated about highly indented cacti. Perhaps the shape enabled it to swell up quickly to make the most of any rain in the desert? We wondered what would happen if we watered our dried up cactus in the classroom. You might like to investigate this:

1. Before watering the cactus, make some measurements which will allow you to record any changes in the shape and size of the cactus.

2. Make a series of angle cards in steps of 5 degrees, between 45 and 90 degrees.

3. Estimate the angle of the indentations, using these cards and testing for the best fit.

4. Water the cactus by soaking it in a large bowl of water for 24 hours.

5. Re-measure the cactus each day for the next 7 or 10 days and record the results.

Questions

• Where should you measure the cactus to record any changes?
• How can you be sure to take the measurements in the same place each day?
• What would you expect to happen? Make a note of your ideas.
• How soon did you see any changes in the cactus?
• What happened?
• Did what happened agree with what you thought would happen?
• What mathematical ideas are introduced by this experiment?

We thank Veronica Hanke of Dry Drayton School, Cambridge for this idea and Linda Gray for the illustrations.

More about the amazing concertina cactus and some data collected by pupils

Last summer I took a class of year 3 and 4 to the Cambridge University Botanic Gardens. In the cactus house, they speculated about highly indented cacti. Perhaps the shape enable them to swell up quickly to make the most of the rain in the desert? Andrew asked “Would it happen if we watered our dried up classroom cactus?”

Before watering, we did some measuring. We used a tape to measure its girth at three places: at its widest point, near the top and near the base. The children then made a set of angle cards in steps of 5 degrees, between 50 and 85 degrees, and estimated the angle of indentation by testing for the best fit. The children ensured that the same fold was always measured by identifying two sites on the cactus with red and black marker pens. On Monday morning we put the cactus into a washing up bowl of water and left it for 24 hours.

We re-measured the cactus at the same time every day for just over a week. We all found the results unexpected and fascinating.

The data the children collected told the story of how the cactus was reacting to a changed environment. They learned that angles and centimetres could be used to explain the world around them. It seemed that their theory could be valid. First the cactus swelled up at the widest point, and then its folds widened. The woody base stayed much the same. At no point did the cactus split.

We took the work to show at the Cambridge Natural History Society Conversazione where it generated a lot of interest. Cactus experts and scientists of all sorts talked to the children about their work. They gradually realised, with increasing excitement, that what they had done was original, interesting and valid enough to be part of the real world of science.