Mature plants are the result of two formally separable developmental processes. Embryogenesis provides a mature embryo with axis, cotyledons, and the root and shoot apical meristem. The embryo has a structure of limited complexity which is relatively uniform in flowering plants. Upon seed germination post-embryonic development starts. The primary root and shoot meristems, together with newly formed secondary meristems, elaborate the architecture of the mature plant. We are interested in finding out how meristematic cells in the embryo are programmed to form organs with appropriately spaced tissues and cell types.