Histochemical tests for K+ were carried out on the epidermis of aerial organs from plants which varied in evolutionary development (e.g., clubmosses, ferns, angiosperms) and general morphology (e.g., monocotyledons, succulent dicotyledons, woody dicotyledons). These tests made on epidermal tissue with open or closed stomata suggested that K+ transport is implicated in stomatal movements regardless of the evolutionary level and the stomatal location in the plants investigated. In all species that displayed substantial stomatal opening, K+ was detectable in the guard cells. With small stomatal apertures, K+ was located in guard and subsidiary cells of Commelina communis L. leaves and Glycine max. L. cotyledons. When stomata were closed, K+ was detectable in the subsidiary cells of all the grass species, Kalanchoë pinnata Pers., C. communis, and, occasionally, in the epidermal cells surrounding the stomata of some ferns. A condition was also observed when virtually no K+ was detectable in the guard or subsidiary cells of C. communis leaves or G. max cotyledons. Commonly, when stomata were closed, K+ was not detectable in any cells of the epidermis. Exceptions were the "K+ storage cells," trichomes and epidermal cells of Arachis hypogaea L., and in the more primitive plants from and including the level of the ferns.