Gensel, Patricia G. .
A Pertica-like euphyllophyte revisited.
A plant resembling the genus Pertica, described by Gensel (1984), represents a new taxon and along with some other taxa may possess features that allow for renewed evaluation of basal euphyllophyte relationships. Like Pertica, this plant consists of a dominant main axis, with helically arranged lateral branches. Fertile ultimate branches terminate in paired sporangia; the last few dichotomies occur at closely spaced intervals so the sporangia are tightly clustered. It differs from presently known perticas in that lateral branches may be single or paired, and both fertile and sterile lateral branches are pseudomonopodially arranged, bearing synchronously isodichotomous second order branches at 1- 3 cm intervals. While anatomy is unknown in most presently recognized species of Pertica, the new taxon exhibits a 3 or 4-armed actinostele comprised of primary xylem.
Protoxylem is present in the middle and near tips of each arm, and sometimes along arms during trace formation. Traces to first order lateral branches are ovoid or shallowly triangular (in larger ones), and those to second order branches are ovoid to round. Mode of trace departure is variable in both vegetative and fertile regions. Some features of this plant indicate a restudy of some Pertica specimens is needed to verify lateral branch organization.
This plant is very different from aneurophytealeans in that the anatomical change to a “leaf equivalent” occurs between the main axis and first order branch. In contrast, this change happens at the ultimate, or sometimes penultimate, order of branching in aneurophytes.
Iridopteridaleans and cladoxyls also exhibit a change in vascular strand shape between main axis and first order traces. Details about lateral trace formation and trace configuration, as well as protoxylem position and presence/absence of protoxylem parenchyma are variable in these groups and represent potential new characters for inclusion in analyses directed at delineating early/basal euphyllophyte lineages.
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1 - University of North Carolina, Department of Biology, Cb#3280, Chapel Hill, North Carolina, 27599-3280, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 31st, 2006
Time: 9:30 AM