Mimosa

What you need

A mimosa plant (Mimosa Pudica). Mimosa is grown as an annual or indoor plant in the UK, or you can see it in botanical gardens around the country.

What you do

Touch the leaves down the centre and watch them fold up. If you touch a number of them a whole section of the planet may start to droop.

What's going on?

Mimosa Pudica is sensitive to touch, shaking and heat, and the leaves also close and droop at night. Looking at the leaves of the plant, each whole leaf structure is broken into something that looks like fingers, and these fingers themselves are formed of many tiny leaflets. Where each part joins the next there is a small section of cells, called pulvini, which control the movement of this plant. The pulvini have cells on opposite sides of the leaf, when the cells on one side of the leaf swell the other side shrink and this creates the folding motion. Research is still ongoing into this movement but it is thought that electric signals play a role in changing how much water and ions can pass through the cell membranes.

Although there is no firm way of telling why the plant evolved to behave this way, it could be because herbivores are less likely to eat the folded leaves. Folding the leaves can also prevent excessive water loss, something which might be crucial in the this plant's natural tropical habitat.