Decombeix, Anne-Laure , Taylor, Edith L. , Taylor, Thomas N. .
Secondary vascular anatomy of Vertebraria (Glossopteridales) roots from the Permian of Antarctica.
Glossopterid seed plants are the main component of Permian floras in Gondwana. At the locality of Skaar Ridge, Central Transantarctic Mountains, Antarctica, permineralized peat contains abundant glossopterid leaves, stems, and reproductive organs. In this communication, we investigate the associated root systems, represented by the morphogenus Vertebraria Royle.
A well-known distinctive feature of Vertebraria is the occurrence in the secondary xylem of air spaces, developed opposite the protoxylem strands, and interpreted as an adaptation to growth in waterlogged soils. The anatomically preserved specimens from Skaar Ridge range from less than a millimeter to several centimeters in diameter. The central part consists of an exarch actinostele with 2 to 7 protoxylem strands. During secondary growth, the vascular cambium produces radial spokes of secondary xylem alternating with these protoxylem strands. Secondary xylem tracheids have 1-5 elliptical to circular bordered pits on their radial walls. These pits have an opposite arrangement and form distinct horizontal rows. Rays are typically uniseriate and 1-5 cells high. In some specimens a small amount of secondary phloem is preserved at the extremity of secondary xylem spokes. The air spaces are initially filled with parenchyma cells that enlarge and break down during development. In the largest roots, air spaces are restricted to an inner part of the axis, beyond this zone a complete cylinder of wood is produced.
We present new details of secondary xylem anatomy and the first description of Vertebraria secondary phloem. A comparison with corresponding tissues in Glossopteris stems is provided. We also discuss the possible patterns of cambial activity resulting in air spaces formation and Schopf’s 1965 hypothesis that their enclosing is the result of the differentiation of peripheral parenchyma cells into new xylem initials.
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1 - University of Kansas, Department of Ecology & Evolutionary Biology, Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Avenue, Lawrence, Kansas, 66045-7534, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 28th, 2008
Time: 9:30 AM