Common histogen -- dermatocalyptrogen for rhizodermis and rootcap

Special histogen -- calyptrogen for rootcap in monocots

In most dicots, cells of the rhizodermis have a common origin with the rootcap cells as shown in Figs. A, B & C), while in most monocots all cells of the root proper have independent origin from the rootcap, as shown in Figs. D, E & F).

In Figure A, showing a longitudinal section of the root apical meristem in the dicotyledon, hemp-nettle (Galeopsis speciosa), the protoderm (shown in brown) gives rise to the rhizodermis and comes from the same initials as the rootcap.

Figures B & C diagrammatically show the mode of root apical growth in dicotyledons. There are three different tiers of initials. The lower tier of cells (the dermatocalyptrogen, shown in brown) gives rise to both protoderm and the rootcap (in white). The middle tier (periblem) is a precursor of ground meristem (cortex) and the upper tier is the source of procambium (central cylinder).

Figure D is a longitudinal section of the root tip in Heleocharis palustris, a monocotyledon; and Figures E & F are diagrammatic representations of growth where rootcap and protoderm are produced by different histogens -- a calyptrogen (shown in red) and a dermatogen (shown in brown) respectively.

1 = plerome; 2 = periblem; 3 = dermatocalyptrogen; 4 = dermatogen; 5 = calyptrogen; 6 = rootcap; 7 = protoderm; 8 = ground meristem; 9 = procambium