The mode of dry matter partitioning among organs of higher plants is as yet poorly understood. Two kinds of partitioning models have been proposed: a) A 'hierarchical' model, assuming absolute organ priorities. b) A 'proportional' model, in which every organ gets a fixed portion of the photosynthate supply. Pot-grown, fruiting Calamondin (C. reticulata cv. austera x Fortunella spp.) trees were used to examine the agreement of fruit/root partitioning data with the two models. Half of the trees were 50% defoliated to reduce the supply of photoassimilates (source manipulation). Half of the plants were transferred to larger pots to induce and enhance growth of new roots (sink manipulation). Thus, four treatments were obtained: small pots with full (S-100), or half (S-50) foliage, and large pots with full (L-100), or half foliage (L-50). In defoliated trees total growth (g dry weight) was reduced by ca. 50%. Transfer to larger pots invariably increased the portion of dry matter allocated to new roots. In trees with young fruitlets, 7 DAFS (days after fruit set), defoliation strongly interfered with fruit growth (55% reduction), but growth of new roots was not affected. In trees with older fruit (30 DAFS), the relative portions allocated to fruit and roots were not altered by defoliation. Large developing fruits dominated dry matter distribution, yet, significant emergence and growth of new roots occurred following transfer to large pots. In addition, the 50% defoliation changed the fruit size distribution, causing about 50% of the fruit to remain small. A computer simulation model was developed to test the hierarchical against the proportional approach. The data, as well as the simulation model, suggest that both kinds of partitioning models may be realistic. It is concluded, however, that the existence of a hierarchical mechanism of dry matter partitioning is essential in order to allow for the emergence and development of new plant organs.