What it shows
The leaves of the touch-me-not plant fold up within seconds of being touched and open again after a few minutes. This shows that plants also respond to stimuli, but through changes in water pressure in cells rather than through nerves.
The science
At the base of each leaflet there is a swelling called the pulvinus. When the leaf is touched, a signal passes and water moves out of the cells on one side of the pulvinus, so they lose firmness (turgor) and the leaflet folds. Later the cells take up water again and the leaf reopens. The response is quick, but it is not a nerve reflex because plants have no nerves, and it is called a touch movement.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A touch-me-not (lajwanti, Mimosa pudica) plant in the garden or a pot
- A stopwatch or phone
- A soft paintbrush
- A notebook and pencil
- A small cardboard shade
Safety
For this project: Mimosa stems have small prickles, so touch the leaves gently with the brush and wash hands after the work. Wear closed shoes in the garden and take an adult along.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Find a healthy touch-me-not plant with fully open leaves in the morning, and do not touch it before you begin.
Touch the tip of one leaf lightly with the soft brush and start the stopwatch. Note the time the leaflets take to close fully.
Watch the leaf until it reopens fully and note this recovery time in minutes.
Repeat on a different leaf three times and record the results in a table. Leave each leaf to recover fully before touching again.
Test a stronger touch by tapping the stem lightly and compare the number of leaves that fold.
Cover the plant with the shade for 10 minutes and test again. Compare the time with the first reading.
Test in the afternoon and at dusk to check whether the response speed changes with the time of day.
Draw the open and the closed leaf with labels for leaflet, pulvinus and stem.
What you should see
Leaflets close within 1 to 3 seconds of a touch and reopen after about 5 to 15 minutes, and repeated touching in quick succession gives a slower and weaker response.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes the leaf fold?
Water leaves the cells of the pulvinus, and these cells lose their stiffness.
2Is it the same as a nerve reflex in animals?
No, plants have no nerves; the signal passes cell to cell through chemical and electrical changes.
3What is the use of this movement to the plant?
It may scare off insects and grazing animals and reduce water loss.
4Why wait before touching the same leaf again?
The cells need time to take up water and reset.
Take it further
Check whether a drop of cold water or a shadow also makes the leaf close, and compare with the touch response.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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