Figure 4.

R. carbonica Is Distinct from All Extant Root Meristems

Schematic diagrams show the cellular organization of a typical gymnosperm (A and C) and Radix carbonica meristem (B and D).

(A and B) Schematics are color coded for the major tissue types within the meristem (yellow, root cap; pink, promeristem [yellow lines in the R. carbonica promeristem indicate the positions of anticlinal cell divisions within the promeristem]; orange, ground tissue; blue, epidermis; and green, procambium).

(C and D) Same schematics as in (A) and (B) but color coded to mark the position of the Körper- (red) Kappe (blue) complexes. Note the difference in the Körper-Kappe boundary between R. carbonica (D) and the gymnosperm meristem (C).

(E) A simplified vascular plant cladogram [26], showing the two hypothesized origins of roots [27], and schematics of lycophyte, monilophyte, and gymnosperm root meristems. Lyc stands for the Lycopodiales that typically have multicellular promeristems consisting of either three or four tiers of initials. Sel stands for the Selaginellales that typically have a single initial cell (apical cell). Iso stands for the Isoetales that typically have multicellular promeristems consisting of either two or three tiers of initials. Monilophyte root meristems typically have a single initial cell (apical cell). Gymnosperm root meristems have multicellular promeristems consisting of a zone of common initials for all tissues, or common initials for all non-vascular tissues and a separate set for all vascular tissues.

For a detailed review of meristem types, see Supplemental Information.