A simulation model describing the root growth, among other things depending on the water content of the soil has been developed by Huck (Auburn,U.S.A.) and Penning de Vries (Wageningen, The Netherlands). The main purpose of the model is to investigate root growth in soillayers with different initial watercontents. The distribution of reserves, produced by photosynthesis, over shoot and root is not studied in detail, but works according to the functional shoot-root balance concept. (Brouwer and De Wit;1968).