Anatomy of the Late Devonian sphenopsid Rotafolia songziensis, with a discussion of stelar architecture of the Sphenophyllales

A previous study of the Late Devonian (Famennian) sphenopsid Rotafolia songziensis Wang, Hao, and Wang provided detailed descriptions of the morphology and a sketchy illustration of a three-ribbed primary xylem. The present anatomical data show that the protostele of this plant also has four-ribbed primary xylem of exarch maturation. Located at the tip of each xylem rib, the tracheids of the protoxylem strand bear helical wall thickenings. Tracheids of metaxylem and secondary xylem possess scalariform pits and/or bordered pits. Ray cells seldom occur in secondary xylem. In contrast to fertile organs demonstrating great diversity in bract shape, structure, and sporangiophore number, the stelar architecture of the Sphenophyllales is consistent in ribbed primary xylem of fundamentally exarch maturation, although secondary xylem indicates moderate structural variations. Stelar architecture of the Sphenophyllales is compared with that of basal euphyllophytes and lycophytes. The position of peripheral protoxylem strands in the Sphenophyllales and Iridopteridales corresponds to the presence of leaves or leaf precursors. The origination of these strands differs from that of radiate protoxylem strands in the Aneurophytales.