1. Background State-of-the-art mechanistic models can simulate intra-daily stem growth (of both xylem and phloem) and internal stem hydraulics with an (sub-)hourly resolution by combining dendrometer and sapflow measurements. PhytoSim was designed to apply such models, consisting of a set of dynamic algebraic and/or first order (ordinary) differential equations - it is a dynamic Plant Modelling and Simulation software.