There are two basic types of root apical meristem (RAM) organization in dicotyledonous angiosperms: open and closed. In taxa with closed organization, longitudinal cell files terminate apically into distinct tiers of initials sometimes called histogens. In RAMs with open organization, the cell files terminate in a zone of initials with uncertain cell lineage. An intermediate type of open organization has specific initials for the epidermis and peripheral root cap cell lineages, but the other histogens do not appear to be clonally distinct. In a survey of one-week-old seedlings grown in sand, about half of the dicot families represented had closed organization. One-third of the families had the intermediate type of open organization, and about 10% of the families were mixed. Only Cucurbitaceae and Fabaceae had the true open type. Mapping RAM organization on recent phylogenies based on DNA sequences of several genes indicated that the ancestral state in angiosperms was the intermediate type of open organization, and the closed type originated in the eudicots, above Ranunculales. Several clades in the eudicots showed reversals to the more primitive, intermediate open type, resulting in some taxa containing a mix of closed and intermediate open members. What particular ecological adaptations these RAM organizations represent remains to be investigated.

Key words: angiosperms, eudicots, phylogeny, plant anatomy, root apical meristem