The main reason Mimosa pudica shows movement even though it lacks a nervous system is due to rapid changes in turgor pressure in specialized motor cells at the pulvinus (the joint-like thickening at the base of the leaf stalk). When stimulated by touch or other external factors, electrical signals (action potentials) trigger ion redistribution in the pulvinus cells, leading to osmotic water movement. This causes a sudden loss of turgor pressure in certain cells, making the leaf fold or droop as a defensive response. These movements are mediated by calcium and electrical signals, not by a nervous system like in animals. In summary, Mimosa pudica's leaf movements arise from electrical and calcium signaling that induce changes in water pressure inside motor cells, causing mechanical bending at the pulvinus.