Taylor, Wilson A. , Wellman, Charles H. .
Tackling the tubes: studies in nematoclast ultrastructure.
Fossil tubes (a.k.a., nematoclasts) form one third of the triumvirate of enigmatic early land plant phytodebris: tubes, cuticles, and cryptospores. The dispersed tubes are widely considered to have been derived at least in part from the nematophytes, a group of land dwelling organisms of uncertain taxonomic position that many feel independently colonized the land along side the ancestors of modern land plants in the lower Paleozoic. TEM analysis of dispersed tubes from the Upper Silurian of Pennsylvania and the Lower Devonian of Scotland reveals a surprising degree of variability among the dispersed tubes with respect to the shapes of the thickenings and the way they are attached to the outer wall. In cross section (longitudinal section of the tubes) the thickenings range from slightly thicker areas of the wall, through more prominent inward protrusion, through bulbous internal protrusion, to hollow cleft shaped internal protrusions. TEM comparisons to both fossil tracheids from the Lower Devonian of Canada and modern tracheids led to the conclusion that the dispersed tubes did probably have a separate evolutionary origin. Several key differences that support this contention are: 1) in most cases, the internal thickenings of dispersed tubes are an integral part of the wall and not deposited on the wall as appears to be the case for modern and fossil tracheids; 2) there is no convincing evidence of end walls in any of the dispersed fossil tubes; 3) there is no evidence of pores through the walls between the internal thickenings. All of these features are more consistent with a function of anchorage and/or transport of absorbed solute/water from outside the organism, rather than transport of water within the organism.
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1 - University of Wisconsin-Eau Claire, Department of Biology, 105 Garfield Ave., P.O. Box 4004, Eau Claire, Wisconsin, 54702, USA
2 - University of Sheffield, Animal and Plant Sciences, Alfred Denny Building, Western Bank, Sheffield, S10 2TN, UK
early land plants
Presentation Type: Oral Paper:Papers for Sections
Location: Williford A/Hilton
Date: Monday, July 9th, 2007
Time: 1:45 PM