Developmental and Structural Section
Frederick, Lafayette , Richardson, FC .
Comparative Studies on Nodal Vasculature in Species of Ulmus.
A detailed study of nodal vasculature has been made of several species of Ulmus (9). These studies were undertaken to determine whether elm species differ in nodal structure and whether there might be any correlation between xylem dysfunction at this anatomical site and leaf wilt in Dutch elm disease(DED). Current season twigs have been collected from trees in different localities over several years. Transverse and longitudinal sections from the uppermost through four lower nodes have been examined. Typical of each species is a trilacunar node. A median trace is inserted in the stele directly opposite and on the same side of the stem as the leaf that it supplies. Lateral traces originate in the stele on opposite sides of the stem at positions 60 to 90 degrees from the median trace. Emergent lateral traces follow an undulating course through the cortex to the petiole base where they fuse to form the petiolar trace. Median traces for upper nodes originate from a fusion of stelar vascular bundles located between the median and lateral trace gaps of the immediate lower node. New lateral traces originate from vascular bundles bordering each lateral trace gap on the side away from each departing median trace. Species vary in the level below a node where median and lateral traces emerge from the stele; the number, size and alignment of tracheary elements in leaf traces; wall sculpturing of protoxylem and metaxylem vessels in leaf and petiolar traces; and level of anastomosis of stipular traces with leaf traces. In leaf and petiolar traces the number of water conducting elements is much less than the number in the stem axis. We postulate that xylem dysfunction at these anatomical sites could substantially impede water flow to leaves and might represent the primary cause of wilting in DED.
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1 - Howard University, Department of Biology, 415 College Street Northwest, Washington, DC, 20059, USA
2 - Indiana University Southeast, Office of Chancellor, 4201 Grant Line Rd, New Albany, IN, 47150
Dutch elm disease.
Presentation Type: Poster:Posters for Sections
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM