Groundwork for this manuscript was laid during a sabbatical visit by Dr. Powell to Wageningen University in the Netherlands, funded in part by NSF grant INT-9813421 and by the Wageningen University Research School. Dr. van der Werf gratefully acknowledges the Dutch Technology Foundation STW for providing visiting scientist grant WBI.4958 which allowed him to make critical visits to Utah State University. Further support was provided by Strategic Expertise Development Project 620-33001-76 of Plant Research International, Wageningen. We all acknowledge helpful conversations with Dr. Geert Kessel, who read an earlier version of this manuscript and provided much helpful biological commentary. Early ideas on a precursor of the model presented here were developed in lively discussions among Wopke van der Werf, Walter Rossing, Geert Kessel and Hans Heesterbeek in the framework of the aforementioned Strategic Expertise Development Activity.