Why will a plant wilt if it is not watered?

When a plant is watered, water moves into the plant by osmosis (the diffusion of water), this means that water moves into the cells causing them to become turgid. When the plant is not being watered, water moves out of the cells by osmosis, as the concentration of water will be higher in the cell than outside, and water always moves down its concentration gradient. Thich causes the cells to lose their turgidity and become plasmolysed, leaving the cells flaccid and causing the plant to wilt.