Why does the touch-me-not shrink when touched?

The touch-me-not shrinks due to loss of turgidity (the pressure of content against the cell wall) by cells within the pulvini-specialised motor organs at leaf joints. Upon stimulation, like touching, the leaf cells lose potassium ions, triggering water to leave the cells through osmosis. It takes about 10 minutes for the cells to regain turgidity and the leaves to open out.