Fig. 4. Measuring water potential with a pressure chamber. A cut branch (or leaf or compound leaf) is placed inside the chamber, with the cut end protruding from the seal. Once the chamber has been sealed (hermetically closed), pressure is gradually applied from the gas cylinder. When the pressure in the chamber equals the xylem pressure, a drop of water appears at the cut surface. Assuming that the xylem osmotic potential is very low, the balance pressure represents the equilibrium water potential of the plant material in the chamber.