Leaves are vascular plant organs, optimized for photosynthesis. Auxin plays a critical role during leaf development, controlling its patterning as well as tuning cellular growth and differentiation. Auxin is also important for the development of phyllids, the bryophyte leaf-like organs. However, its precise role in this process remains elusive. Here we use a combination of live imaging, genetics, chemical treatments, and modeling to understand how auxin controls phyllid development in the model moss species, Physcomitrium patens. We tracked basal and upper phyllid morphogenesis from a single initial cell until full maturity to uncover the cellular growth dynamics underlying this process. We explored the role of auxin by recording auxin synthesis and transport patterns throughout phyllid development and integrated our experimental data into a computational model of a growing phyllid. Our results elucidate the cellular basis of auxin function in the regulation of developmental decisions allowing moss leaf-like organs to reach their final shape and structure.