Root Apical Meristem - Theories of Organisation

The root apex faces the opposite challenge from the shoot apex - it has to keep pushing forward through abrasive soil while still producing new tissue - and it has its own family of organisational theories. Nageli's Apical Cell theory again proposes a single working unit, but here it is a tetrahedral apical initial: one face of the tetrahedron produces the root cap (a soil-facing sacrificial shield), while the other three faces between them produce the epidermis, cortex and vascular tissue behind it. Hanstein's Histogen theory, applied to the root, adds a fourth histogen beyond the shoot's three: Dermatogen (outer, gives root epidermis), Periblem (middle, gives cortex), Plerome (inner, gives stele) and Calyptrogen (a layer dedicated purely to renewing the root cap). Schuepp's Korper-Kappe theory is the root's direct equivalent of Tunica-Corpus: a Korper (body) zone building the root proper, distinguished from a Kappe (cap) zone by the direction of a distinctive T-shaped ('Y') cell division - inverted-T in the Korper, straight-T in the Kappe. Clowes' Quiescent Centre concept (1961) is different in kind from the other three: rather than describing layers, it identifies a small group of cells, sitting between the root cap and the activ …