Ryberg, Patricia E. , Taylor, Edith L. , Taylor, Thomas N. .
Phloem anatomy in Cycadeoidea (Lower Cretaceous): anatomical oddity, functional reality.
MESOZOIC bennettitaleans have been extensively studied for their reproductive anatomy, including cone development and pollen ultrastructure. Although equally well preserved, vegetative anatomy is relatively poorly known. Petrified Cycadeoidea stems from the Lower Cretaceous Lakota Formation, Black Hills, South Dakota, contain well preserved vascular tissues which allow us to detail secondary phloem anatomy in this genus. Phloem development is extensive and can equal that of the xylem, an unusual situation in seed plants. Secondary xylem consists of tracheids that measure 39-72 μm in diameter with scalariform secondary wall thickenings and vascular rays that vary from uni- to multiseriate. The phloem consists of alternating bands of fibers and sieve cells interrupted by vascular rays every 1-3 rows. Dark colored fibers are 8-30 μm in diameter with very thick walls and slit-like apertures in the simple pits. Sieve cells appear lighter in color due to thinner walls and measure 16-25 μm in diameter; they are identified by the presence of numerous oval-elliptical sieve areas on their radial walls. Sieve areas measure up to 10 μm in diameter, and contain multiple sieve pores that appear as darker spots within each sieve area. Axial parenchyma is rare, but when present, cells are rectangular and approximately 33 μm wide by 99 μm long. Secondary phloem anatomy of Cycadeoidea includes several unusual characters when compared with other gymnosperms, both extant and fossil. These include the alternating bands of sieve cells and fibers, large number of phloem fibers, and extensive development of the secondary phloem. These features will be discussed in light of our current understanding of the phylogenetic position of the Bennettitales among the seed plants.
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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 31st, 2006
Time: 1:30 PM