Stephen Hales, (born Sept. 7/17, 1677, Bekesbourne, Kent, Eng.—died Jan. 4, 1761, Teddington, near London), English botanist, physiologist, and clergyman who pioneered quantitative experimentation in plant and animal physiology. Hales introduced new techniques of measurement to the study of plant physiology. One of the best-known of these concerned the measurement of water vapour emitted by plants. He measured this emission, known as transpiration, finding that it was the leaves that transpired and that this process encouraged a continuous upward flow of water and dissolved nutrients from the roots. He determined the direction in which sap flows in plants (it flows upward), and he measured the sap’s pressure. He also measured the rates of growth of shoots and leaves and the pressure roots exert on sap, and he investigated plant respiration.