When you touch a mimosa plant (Mimosa pudica), its leaves rapidly fold inward and the stem droops, then reopen a few minutes later. This is a quick response called thigmonasty, a type of nastic movement triggered by touch or other stimuli like heat or shaking
. The mechanism behind this involves changes in turgor pressure within specialized cells at the leaf base called pulvini. When touched, potassium and chloride ions move out of certain cells, causing water to flow out as well. This loss of water decreases turgor pressure, making the cells flaccid and causing the leaflets to fold and the stem to droop
. This rapid movement is thought to serve as a defense mechanism, making the plant appear smaller or wilted to discourage herbivores from eating it. It may also help dislodge insects from the leaves
. In summary, touching a mimosa plant causes its leaves to fold and droop quickly due to ion and water movement in cells that reduce turgor pressure, protecting the plant from potential harm
