Article: The Devonian plant Reimannia, with a discussion of the class Progymnospermopsida
The type specimen of Reimannia aldenense, from the Middle Devonian of New York, was reinvestigated in order to more critically evaluate the anatomy of this poorly understood monotypic genus. Three successive axis orders were observed in organic connection. The first-order axis contains a three-ribbed primary xylem column showing mesarch order of maturation with protoxylem strands along median-radial planes and near the tips of primary xylem ribs. Traces appear to be produced in a helical arrangement. The primary xylem of the second-order axis is proximally elliptical or shallowly four-ribbed, as seen in transverse section, and gives off a sub-opposite pair of traces, supplying third-order axes, which may divide once through the course of their departure. Distally, the primary xylem of the second-order axis assumes an increasingly three-ribbed configuration and probably gives off a single abaxial trace. No evidence for a histologically distinctive 'peripheral loop' has been found.Although placed within the Iridopteridineae by Arnold (1940), these features show that Reimannia has little in common with the other genera of this group. Instead, Reimannia should be considered a form genus for pennineralized axes within the Aneurophytales (Progymnospermopsida). Assignment is made to this group even though Reimannia shows neither of the diagnostic characters originally used in combination to establish the class: a free-sporing habit and 'typically gymnospermous' secondary vascular tissues. It is pointed out that the 'definition of the Progymnospermopsida, based upon a combination of primitive and derived characters, is inextricably tied to a prior phylogenetic assessment as the direct ancestors of seed plants. Although such a definition is acceptable on methodological grounds, primitive and derived characters should not be treated equally in the analysis of phylogeny or in the characterization of groups.