Dunn, Michael T .
The vegetative structure of Tetrastichia bupatides Gordon.
THE stem morphotaxon Tetrastichia bupatides was first described by Gordon (1938) and ranges from the late Famennian of Ireland to the late Tournaisian of Oxroad Bay, Scotland. Fifty-five additional stems have recently been analyzed from material collected from the Oxroad Bay locality by R. M. Bateman and G. W. Rothwell and suggest that many characters originally used to describe this taxon exhibit a wider range of variation than has been reported. For example, these new data suggest that secondary xylem was produced by all stems regardless of size as well as some larger rachides, and rachis trace morphology can range from “butterfly” shaped, to U-shaped, to broadly U-shaped, to nearly T-shaped. Lobes of the protostele most commonly are four in number, but can range from three to five, phyllotaxy ranges from opposite decussate to 2/5, and rachis forking can be equal or unequal dichotomous. Pinnules are vascularized by a single trace that diverges from the outermost protoxylem pole of a pinna trace, and based on angle of trace divergence, are inserted at a low angle to the pinna axis. Pinnules are usually thick and fleshy, and broadly attached but arrangement ranges from opposite to sub-opposite to alternate. The range of variation of the above mentioned characters makes meaningful diagnosis difficult, however these specimens can by placed in a single clade due to the consistant mode of rachis trace production. Cauline protoxylem poles are located approximately midway between the center of the stele and the apex of each arm, and rachis trace formation is initiated by the division of a protoxylem pole, producing 2 radially oriented poles. The distal strand then divides producing 2 tangentially oriented poles. The distal protoxylem strands divide a number of times and the trace then diverges from the stele.
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1 - Cameron University, Department of Biological Sciences, Science Complex, Lawton, Oklahoma, 73505, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 31st, 2006
Time: 10:45 AM