Figure 34.8 Stomata (A) Scanning electron micrograph of an open stoma formed by two sausage-shaped guard cells. (B) Potassium ion 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.