In the closed
type of RAM, the cells that form the RC and root body
are typically produced by different tiers of cell initials; in
the open type they orginate from a common group of initials.
The RCJ arises as the consequence of a high rate of cell
division in the cap initial cells (resulting in a columellar
structure within the cap) and a low rate in the adjacent
tier of cells in the QC. The QC, first discovered by
Clowes (1956) with use of radiolabelled DNA autoradiography,
is a distinct central region of the RAM, which consists
of slowly dividing cells (Jiang and Feldman, 2005).