Premise of research. Two permineralized stems of Medullosa leuckartii Goppert & Stenzel provide evidence for the stelar architecture and mode of leaf trace production and for the growth form of a complex Early Permian species of medullosan seed ferns. Methodology. Anatomical features were characterized from the surfaces of stem cross sections and from ground thin sections mounted on microscope slides. Vascular architecture and the mode of leaf trace production were reconstructed from the analysis of protoxylem strands. Pivotal results. Protoxylem consists of a ring of independent cauline bundles that divide repeatedly to produce numerous leaf traces to each frond. Some leaf traces diverge as small collateral bundles. Others separate from the stele as a large body of primary xylem surrounded by secondary vascular tissues and divide distally into small collateral bundles. As is common for gymnosperms with helical phyllotaxis, the number of cauline bundles appears to conform to numbers of the Fibonacci series. Some tracheids near the periphery of the stelar region extend horizontally around the circumference of the stem, as do the girdling leaf traces that characterize stems of the Cycadales. Conclusions. Despite complex anastomosing vascular segments that each consist of primary xylem surrounded by secondary vascular tissues, protoxylem architecture reveals that M. leuckartii has an incompletely dissected eustele that is basically similar to the steles in more typical gymnosperms. Secondary growth specializations are characteristic of lianas, and the absence of cauline adventitious roots suggests that the plant grew as a canopy-supported vine. While variations over the length of stems and the nature of horizontally oriented tracheids that are reminiscent of girdling leaf traces remain to be clarified, this study demonstrates that stems of M. leuckartii differ from more typical eustelic seed plants primarily by having specializations for vine architecture.