One of the most pervasive laws in biology is the maintenance of the relationship between organ sizes among individuals, populations and/or species. The existence of such allometric scaling, together with the fact that most of the organ growth regulators are pleiotropic, suggests that the same genes are controlling the size of different organs. Yet, in many instances during evolution, the morphological scaling relationship among organs has been modified and new relationships have evolved. How allometry is maintained and evolve at the molecular level is, however, poorly understood. It is equally unclear how identity factors influence growth processes in different organs. We hypothesise that organ identity factors quantitatively modulate the expression of general growth regulators through chromatin-based mechanisms that modify their sensitivity to hormonal signals. I will test this hypothesis by studying the molecular basis of one prominent example of allometric scaling evolution in plants, the decrease in flower size after the transition to selfing, using a combination of genomics, molecular genetics and cell biology approaches. This work will determine how organ growth is affected, spatially and temporally, by organ identity and highlight the molecular mechanisms quantitatively regulating the transcription of general growth regulators, determining the size of different organs.