TURGOR PRESSURE

The tissues of plants, bacteria, fungi, and some protists are kept erect by water pressure within their cells, called turgor pressure. 

The cell plasma membranes of plants are differentially permeable, so that while water molecules can enter and exit, the larger salt molecules inside cannot escape: since turgor pressure is maintained by osmosis (the diffusion of solute molecules from an area of higher concentration to an area of lower concentration) the solute concentration within the cell affects water pressure to build up in the cell membrane, giving the plant a rigid structure.

This is why plants wilt when they have insufficient water: their cellular membranes are essentially deflated, which normally provide support to the cell walls.

Reference:
http://assoc.garden.org/courseweb/course1/week2/page10.htm
h/t biodiverseed