The temperatures of the roots, the apical meristem, and the shoots of Zea mays plants were varied independently of each other and the rates of leaf extension were measured. When the temperature of the apical meristem and region of cell expansion at the base of the leaf was kept at 25 °C, changes of leaf extension in response to changes of root and shoot temperatures were less pronounced. When the temperature of the meristematic region was changed by increments of 5 or 10 °C from 0 to 40 °C, and the root and shoot temperatures were kept at 25 °C, rapid changes in leaf extension occurred. It was concluded that the rates of leaf extension were controlled at root-zone temperatures of 5 to 35 °C by heating or cooling of the meristematic region. Changes in rates of leaf extension in response to changes in air temperature were attributed to direct effects on the temperature of the meristematic region and on the physiology of the leaf.