The movement observed in the Mimosa pudica plant when touched is called a thigmonastic movement (also known as seismonastic movement). This is a type of nastic movement, which means it is a non-directional response to stimulus such as touch or mechanical disturbance. When touched, the leaves of Mimosa pudica fold or close rapidly. This movement results from a sudden loss of turgor pressure in the motor cells (pulvini) of the leaf, which causes the leaflets to fold inward and droop. The change in turgor pressure is due to water moving out of the cells triggered by an electrical signal or action potential that travels through the plant, causing the motor cells to collapse and the leaves to close as a defensive mechanism against herbivores or environmental stress.
