The specific locations of the evaporating surfaces in the leaf are not yet known. However, Maercker (1965) repeated successfully the early experiments of Stahl (1894) and concluded from her evidence that the cuticular component oftranspiration could be further divided into "cuticular" and "peristomatal" transpiration, where the latter term refers to transpiration from the surfaces of the guard cells and their subsidiary cells. Peristomatal transpiration has been the subject of considerable recent interest (e.g. Lange et al. 1971).