Sensitive plants: who says plants don’t have feelings?
by Drucilla James
credit: Keith Simmons
The sensitive plant, Mimosa pudica, is a hit with children of any age thanks to its intriguing ability to move its leaves. When touched, even by water, the leaves fold up and the stems droop. The plant has no muscles, so how does it do it and why? There is so much for children to learn from these fascinating plants and they’re easy to grow, inexpensive and safe (just watch out for the small thorns that appear as they mature).
Things you’ll need
1. Mimosa pudica seeds
2. 10cm flower pots
3. Seed compost
4. Polythene bags
5. Elastic bands to fit the pots
Step by step
1. Sow the seeds in moist, good quality seed compost, pressing them gently into the surface.
2. Cover the pots with polythene bags held in place with elastic bands.
3. Put the pots in a warm (18-20°C), dark place until the seeds germinate – this can take anything from a few days to a month.
4. As soon as the seeds have germinated, remove the polythene bags, move the pots to a bright position out of direct sunlight and keep at a temperature of about 18°C. Keep the compost moist.
5. Transplant as required and as the plants grow, apply a liquid feed each week.
Kids will soon be able to discover what happens when they touch one of the leaves – it will quickly fold up and droop. Some minutes later it will open up again but will immediately close once more if touched.
This basic nervous system cleverly protects the plant from damage by foraging animals or bad weather.
So how do they do it?
It’s all to do with the biology of plant cells. A plant cell has two basic parts: the PROTOPLAST and the CELL WALL. At the centre of the protoplast is a membrane filled with fluid (the vacuole), just like a water balloon. The protoplast is enclosed within a rigid cell wall made of cellulose (this is what supports the cell and ultimately the plant, the equivalent of a skeleton). In trees these cell walls are thick, rigid and strong, in leaves they are thin and flexible.
When the vacuole is full of water, it presses against the cell wall, making it rigid and giving support. When the cell loses water from this space, the rigidity is lost and the plant wilts. The plant can pump water in and out of the cell and so can control how rigid it is.
In the Mimosa pudica plant, there are very special cells, called PULVINUS cells, located at the base of each leaflet. When these cells are filled with water they become very rigid and push the leaflets open; when they lose water they become flaccid and the leaves fold in. The pulvinus cells therefore act like mini pumps.
How do the pulvinus cells do it? There is a signal that tells the pulvinus cells to pump water in or out. When the leaves are open these cells contain a lot of potassium ions relative to the outside which draws water in and makes them rigid. When the leaf is touched, it creates an electric action potential or wave of depolarisation (which is also how neurons conduct impulses around our nervous system) and this opens up channels through which the potassium flows out of the pulvinus cells, taking water with it. The cells lose their rigidity and the leaf closes up. As soon as the signal has stopped, the pulvinus cells pump potassium back into the cells and the leaf opens up again.