Touch me nots have always been a wonder. The scientific name of touch me not is ‘Mimosa Pudica’. True to its name, the leaves of these plants fold themselves inward when touched. They close during darkness and open in light. They are also referred to as sensitive or humble plant.
Inside the cells of the plant, there are specialized sub units that perform specific functions. One such sub unit is the cell vacuoles. These are compartments filled with water containing organic and inorganic molecules. All the cells are surrounded by a membrane which is permeable to water but restricts the dissolved solute.
The pressure caused by the flow of water from a region of low solute concentration outside of the cell into the cell's vacuole, which has a higher solute concentration is called turgidity. Plants rely on turgidity to maintain rigidity.
Now as a reaction to external stimuli (touching, blowing etc), specific parts on the stems are stimulated to release chemicals like potassium ions. These force water out of the cell vacuoles. As water diffuses out of the cells, it creates a difference in pressure within the regions of the cell. This differential turgidity causes the cell to collapse and thus causes closure of the leaflets.