The root apical meristem (RAM) is the source of the cells making up the root cap and body. Initial cells in the RAM divide, producing derivative cells that elongate, mature and differentiate into specific cell types. In closed RAM organization these initials are arranged in specific tiers (histogen layers); in open organization cell files terminate in a zone of initials lacking apparent organization. Despite this obvious difference, both open and closed roots form protoderm and peripheral root cap through a periclinal T-division of a root cap protoderm initial. It has been commonly assumed that RAM organization remains constant over the developmental life of the root. The primary roots of Arabidopsis (a closed species) became determinate in 3-4 weeks, this process was correlated with a loss of closed RAM organization. In addition, the RAM of Pisum, an open species, changed in shape and size during the lifetime of the root. Studies of other dicot species reinforced these findings. T-divisions appeared to be a constant feature among dicots because it was present regardless of root age and organization. We will propose that RAM organization in dicot roots undergoes certain changes through ontogeny.  1. All the radicles of early embryos have closed organization.  2. The organization changes by the time germination occurs in open species, but remains the same in closed species.  3. By the time a root has reached its determinate stage, all RAMs have open organization. The RAM of dicots is a dynamic structure; therefore, it is important to consider the developmental state of the root when classifying apical organization types.

Key words: closed organization, determinacy, ontogeny, open organization, root apical meristem, t-division