Figure 25.13: Stomata Regulate Gas Exchange and Transpiration (A) Scanning electron micrograph of an open stoma formed by two sausage-shaped guard cells. (B) Potassium ion (K+) concentrations affect the water potential of the guard cells, controlling the opening and closing of stomata. Negatively charged ions (e.g., Cl−) that accompany K+ maintain electrical balance and contribute to the changes in water potential that open and close the stomata.