The primary body of Rellimia thomsonii: Integrated perspective based on organically connected specimens

The Blenheim-Gilboa locality in New York contains Rellimia thomsonii specimens preserved as both compressions and permineralizations that offer an unusually complete perspective on the reproductive structures, primary body, and secondary anatomy of this Middle Devonian plant. Here we describe the vegetative appendages, axis and branch size relationships, and trace departure as well as primary anatomy, including phloem sieve cells of interconnected axis orders of vegetative and fertile specimens. At least five orders are present, and each axis order is approximately half the diameter of the parent axis. Three-ribbed protosteles are evident in all axes. Protoxylem in transverse section is a continuous three-ribbed sheet one or two tracheids wide, enveloped by several cell layers of metaxylem in turn surrounded by primary phloem. Sieve cells have vertically arranged sieve areas. Trace departure to axes is similar in vegetative and fertile axes. Traces are apparent proximally as radial elongations of the tip of the parent primary xylem rib. In distal sections, the rib tip is tangentially elongated; ultimately, the trace becomes three-ribbed. Traces to vegetative appendages and fertile organs divide dichotomously. The new features described here allow us to expand and complement previous work and to discuss our whole-plant concept of Rellimia.