The water flow through the 'Soil-Plant-Atmosphere Continuum' is a dynamic process: transpiration losses are not always equal to water uptake from the soil. A non-steady state mathematical model will be designed based on measurements of sap flow, water potential and stem fluctuations of young model trees (Quercus robur L., Fagus sylvatica L. en Pinus sylvestris L.) in a controlled environmment. All model parameters will receive anatomical/morphological meaning. This mathematical model will be validated.