During plant embryogenesis an embryo with cotyledons, a shoot apical meristem, a hypocotyl and a root apical meristem, is formed. The primary root and shoot meristems initiate post-embryonic growth generating all plant organs. The root meristem forms the primary root, and the shoot meristem forms the aerial portion of the plant including secondary meristems. Histological and fate map data have shown that there is no precise correlation between the shoot meristem cells and their descendants (Steeves and Sussex, 1989). This indicates that cell fate is flexible. In contrast, in the root a more strict relationship between differentiated cells and their meristematic ancestors is seen. Little is known about the mechanisms specifying cell fate in meristems.