The cell wall is what gives plant cells their shape. The cell wall, not to be confused with the cell membrane, is a rigid structure made up of long cellulose fibers, branching hemicellulose fibers, and pectin. The fluid pressure inside the cell also contributes somewhat to their shape.
Cell walls are tough and rigid to support the upward growth of stems using their own structural strength. Their rigidity forces the plant cell to conform to a certain shape. Despite this rigidity, the fibrous consistency of the wall lends itself to a certain amount of flexibility, which is dependent upon the fluid pressure inside the cell. When a plant cell is healthy and full of water, the fluid presses against the cell wall and keeps it stiff, like a fully blown balloon. When a plant cell does not have enough water, the cell wall sinks in on itself, and the plant wilts.