Primary plant meristems are the shoot and root meristems that are initiated at opposite poles of the plant embryo. They contain stem cells, which remain undifferentiated, and supply new cells for growth and the formation of tissues. The maintenance of a long-lasting stem cell population in meristems is achieved by signal exchange between organizing regions and the stem cells, and also by feedback signals emanating from differentiating cells. Related peptide signals that make use of different receptor classes were found to control the stem cell populations in both meristem types by regulating evolutionarily conserved homeodomain transcription factors. The precise interplay of auxin and cytokinin signaling pathways is central to keep cells in the meristem, or direct them toward differentiation.