Up to 90 percent of volume in the mature plant cell may be taken up by a single vacuole.
Vacuoles, or membrane-bound vesicles, have so-called sap inside, which is really water with dissolved salts and sugars. These nutrients are "pushed" inside the vacuole by special molecular pumps that use energy to actively transport chemicals across the membrane. As salts and sugars are inside, they are "locked" the membrane of the vacuole does not allow them to freely diffuse into the protoplasm. Water, on the other hand, freely moves across the membrane through the special pores about the size of a water molecule. As long as the concentration of salts and sugar inside the vacuole is higher than the concentration outside, more water will enter than leave the vacuole via osmosis, thus creating a strong osmotic pressure that expands the vacuole. Pressure from the expanding vacuole in turn creates a "hydrostatic" pressure (hydro water) of the protoplasm on the cell walls, called turgor. Turgor is the cause of rigidity in living plant tissue. It causes the stiffening cell walls similar to the inflating of a pneumatic tire when you pump the air.
Cells that are in cut leaves or flowers do not have enough energy to transport salts and sugars inside the vacuole, as they are now longer receiving nutrients that provides fuel for the pumps. The resulting loss of turgor pressure causes the flowers and leaves to wilt.