Lateral organs in plants arise from the meristem in a stereotypical pattern known as phyllotaxy. Spiral patterns result from initiation of successive organs at a fixed angle of divergence but variable patterns of physical contact. Such patterns ultimately give rise to individual leaves and flowers at positions related to each other by consecutive terms in the mathematical series first described by Leonardo Fibonacci. We demonstrate that a BELL1 related homeodomain protein in Arabidopsis, BELLRINGER, maintains the spiral phyllotactic pattern. In the absence of BELLRINGER, the regular pattern of organ initiation is disturbed and lateral organs are initiated more frequently. BELLRINGER is also required for maintenance of stem cell fate in the absence of the regulatory genes SHOOT MERISTEMLESS and ASYMMETRIC LEAVES1. We propose a model whereby BELLRINGER coordinates the maintenance of stem cells with differentiation of daughter cells in stem cell lineages.