By Harold McGee
Cell walls are thus firm but flexible containers. The cells that they contain are mostly water. When water is abundant and a cell approaches its maximum storage capacity, the vacuole swells and presses the surrounding cytoplasm against the cell membrane, which in turn presses against the cell wall. The flexible wall bulges to accommodate the swollen cell. The pressure exerted against each other by many bulging cells— which can reach 50 times the pressure of the surrounding air—results in a full, firm, turgid fruit or vegetable. But if the cells are low on water, the mutually supporting pressure disappears, the flexible cell walls sag, and the tissue becomes limp and flaccid.