Why do the leaves of ‘Touch-me-not’ plant fold when touched?

            All plants respond to touch, but Mimosa pudica, or the “touch-me-not” plant is more sensitive than the others. Whenever touched, the leaves of the mimosa plant close and drop immediately, appearing like a dead plant. Possibly it is a defense mechanism by which the plant protects itself against its enemies.

   The opening and closing of the leaflets and the entire leaf are controlled by fluid-filled sac-like structures found at the base of not only the compound leaf but of each leaflet as well. When the leaf is touched, a chemical signal immediately moves to the base of each leaflet making the fluid drain away from the sac which makes the leaflet fold. When the signal reaches the base of the whole leaf, it makes the entire leaf drop. If left untouched for some time, the sacs fill up and the leaflets open again.