Ask Mr. Smarty Plants

Every plant cell has a vacuole and a nucleus. The vacuole, although the name means "empty space", actually is an inner sac containing much of the cell's stored water and occupies a large part of the volume of most cells. The vacuole is enclosed in a membrane to hold the water in place.

The nucleus of a cell is its control center from which comes instructions for the cell's operation, maintenance, and reproduction. Between and around the nucleus and vacuole is the cytoplasm, a soft jelly-like material in which most of the cell's metabolism takes place.

Water entering the cell is stored in the vacuole, which expands and presses the cytoplasm against the rigid cell wall, so the cell does not expand or increase in size. When a vacuole becomes full, the cell wall squeezes water out, a safety valve to keep the cell from inflating to the bursting point. This pressure against the cell wall is what holds the shape of the plant, keeping the leaves flat or the stems standing up straight. If too much water is released and more water is not added, the plant will begin to wilt.