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When stomates of Zea mays open K and Cl migrate from the subsidiary cells into the guard cells; when the stomates close both elements return to the subsidiary cells. Subsidiary cells function as reservoirs for K and Cl. Import of K and Cl into the guard cells and loss of both elements from the guard cells become observable 1 or 2 min after light is turned on or off, both when histochemical methods and the electron-probe microanalyzer are used for detection. Each stomatal complex of maize contains on the average 10±3×10(-13) gram equivalents (eq) of K and 4±1×10(-13) eq of Cl. Guard cells accumulate K in the light and CO2-free air at an average rate of 10×10(-15) eq K per minute, and Cl at approximately half that rate.