We have investigated the basal Angiosperms and Dicotyledons to correlate the patterns of cellular organization in the root apical meristem (RAM) and cortex with the phylogeny of major orders and families, as recently proposed by the Angiosperm Phylogeny Group. Our investigation of primary roots and a few adventitious or lateral roots from 100 families and over 500 species by standard light microscopic methods reveals not only that open RAMs are most common in basal Angiosperms and major groups of the Dicotyledons from Eudicots through Euasterids II, but the open RAMs have important and distinct variations, usually consistent with phylogenetic grouping. These variations involve the degree to which the inner, middle, and/or outer cortices are derived from different parts of open RAMs by periclinal divisions/lineages and the positions of the columella bases relative to the tip of the cortical initials; in turn, these cortices exhibit various irregular patterns of cell organization, depending on the organizations of the open RAMs. Closed RAMs are usually the derived patterns in these Angiosperms, and a closed RAM produces a cortex with radial cell file alignments from the innermost layer, the endodermis, throughout the cortex.

Key words: Angiosperms, root cortex, root meristem