A non-woody plant is held upright by the pressure of water in its cells. These cells are tightly expanded like balloons. If too little water reaches the cells, as happens in a drought or if the stems are damaged by disease, the cells soften and the plant sags or wilts. Usually it can stiffen up again when water is available, provided the plant has not dried out too much. Woody plants can also wilt at the tips of their shoots.
Picture credit: google
