Wilson, Jonathan P. , Holbrook, Noel , Knoll, Andrew H. .
A Physiological Interpretation of the Paleozoic Seed Plant Medullosa.
The Paleozoic stem seed plant Medullosa has been an enigma for over one hundred years due to its unusual anatomy and morphology, with anomalous secondary development modifying a eustele into a series of vascular strands that anastomose throughout the plant. Medullosan fronds have large surface areas relative to the dimensions of the stems that support them; furthermore, medullosan tracheids are among the widest and longest found in the terrestrial plant record. Modeling fluid flow through tracheids shows that the wide, long, and pitted tracheids of Medullosa could conduct water at volumes on par with angiosperm vessels, rates far greater than those of the Palezoic coniferophyte Cordaites and the extant conifer Pinus, even at comparable xylem cell widths. Tracheid structure, thickness-to-span ratio, and distribution within the stem suggest that medullosan xylem risked lethal implosion at modest pressure gradients; thus, xylem could not have been a source of structural support for the plant. We interpret many of Medullosaís unusual anatomical features as adaptations to meet the evapotranspirative demands imposed by a large leaf area, from the close association of xylem cells with living parenchyma cells to the complex vascularization of each petiole from discrete segments in the stem. Medullosa represents the independent evolution of highly efficient transport tissue early in the seed plant clade, and this analysis provides further support for the hypothesis that some medullosan species were vines or relied on extraxylary tissues for support.
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1 - Harvard University, Earth and Planetary Sciences, 26 Oxford St., Cambridge, Massachusetts, 02138, USA
2 - Harvard University, 16 Divinity Avenue, Cambridge, Massachusetts, 02138, USA
3 - Harvard University, Organismic and Evolutionary Biology, 26 Oxford St., Cambridge, Massachusetts, 02138, USA
Presentation Type: Oral Paper:Papers for Sections
Location: Williford A/Hilton
Date: Monday, July 9th, 2007
Time: 8:15 AM