a seed containing a simple mature embryo that displays an apical–basal axis of polarity and a radial pattern of concentric tissue layers perpendicular to the apical–basal axis. The axis of polarity has, at its top end, the primary shoot meristem, which is flanked by one or two cotyledons, and the primary root meristem at its bottom end. How this basic body organization is established during
embryogenesis is the focus of this review.