The ‘sensitive plant’, Mimosa pudica, also known as ‘humble plant’, or ‘touch-me-not’, is a source of fascination to adults and children alike. When you gently touch the narrow fern-like leaflets they almost instantaneously fold together and the leaf stalk droops. This sometimes sets off a chain reaction, with several leaf stalks falling on top of one another, causing the collapse of a whole section of foliage, or perhaps the whole plant. When left to its own devices, the plant gradually returns to normal, this taking up to about half an hour. This touch-induced movement of leaves is known scientifically as thigmonasty, and is thought to be a defensive mechanism against grazers.