The numbers of open and closed leaf stomata were investigated both in Plantago major and in Epilobium anguslifolium in relation to, respectively, the relative humidity of the surrounding air, the transpiration and the evaporation, by studying transparent replicas of the leaves obtained by using the replica method, described by Sampson (1961). Statistical operations with the data obtained showed in both cases a significantly lower number of open stomata when the relative humidity is lower and transpiration is higher.