Modular construction of the protoderm and peripheral root cap in the "open" root apical meristem of Trifolium repens cv. Ladino

Roots with open apical organization are defined by not having specific tier s of initial cells in the root apical meristem: those with closed apical or ganization have specific initial tiers to which all cell files can be trace d. An example of the clear organization of closed roots is the development protocol of the root cap and protoderm. The key event in differentiating th ese tissues is the T-division, a periclinal division of the root cap/protod erm (RCP) initial that establishes a module. Each module comprises two pack ets, the protoderm. and peripheral root cap. Consecutive T-divisions of the same RCP initial produce up to five modules on average in a lineage of cel ls in white clover (Trifolium repens cv. Ladino), with all lineages around the circumference of the root dividing in "waves" to form one module prior to the next. On average, clover has approximately 32 axial protoderm and pe ripheral root cap cells in each module, and 32 RCP lineages. The occurrence of RCP T-divisions in white clover, a root with open apical organization, and the subsequent modular construction of the root cap and protoderm, prov ides a link between open and closed roots and suggests a common development al feature that most roots of seed plants may share independent of their ro ot meristem organization type. The open apical organization of the white cl over root varies from roots with closed apical organization in that the RCP initials occur in staggered positions instead of connected to discrete tie rs, and the peripheral root cap and columella daughter cells form additiona l layers of cells. White clover also forms root hairs on all protoderm cell s irrespective of their position relative to the underlying cortical cells.