A stoma opens when the surrounding guard cell pair increases in turgidity. The increase results from active accumulation of potassium in the guard cell vacuole. The intracellular compartmentation of potassium evokes compensatory accumulation of a yet-unidentified solute in the guard cell cytoplasm. The source of potassium is other epidermal cells; this indicates that stomatal movements in situ are under control of these cells also. Presumably, guard cell potassium uptake, which is from the apoplast, is mediated by a proton -extruding ATPase on the guard cell plasmalemma. The energy source is. oxidative phosphorylation and, to a lesser extent, photosynthetic electron transport. Except for high flux capacity and different responses to applied chemicals, potassium uptake by guard cells is similar to potassium uptake by other plant cells.