Developmental and Structural Section
Schneider, Edward , Carlquist, Sherwin .
Xylem of early angiosperms: Microstructure in root and stem tracheids of Nymphaeales.
Pit membranes of tracheid end walls of Nymphaeaceae and Cabombaceae were studied with scanning electron microscopy (SEM). Root tracheid end wall pit membranes are highly porous and qualify the elements as vessels. Pit membranes of stem tracheids are composed of two thick layers. The outer (distal) layer, which comprises the continuous primary wall, is spongiform, perforated by porosities of relatively uniform size, and confined to or most prominent on end walls. The second layer consists of thick widely-spaced fibrils that are oriented axially and laid down proximally (facing the cell lumen) to the first layer, although continuous with it. These axial fibrils are attached at their ends to the pit cavities. We were unable to observe any similar microstructure in the stem of species of Trithuria (Hydatellaceae) studied, but this may be correlated to the predominance of protoxylem. In Nymphaeaceae and Cabombaceae the fibrils are typically represented in scalariformly pitted metaxylem tracheids and the dense spongiform reticula of coarse microfibrils are probably composed of secondary wall material, in accord with the non-extendable nature of metaxylem.
The tracheid microstructure reported is different from pit structures observed in other groups of vascular plants. The longitudinally-oriented threads and strands in perforation plates of secondary xylem of wood and stems of a variety of primitive woody angiosperms (e.g., Illicium) are not homologous to the pit membrane structure observed in stem tracheids of Nymphaeaceae and Cabombaceae, which have only primary xylem and no perforation plates. The structures seen in woody genera represent remnants of primary walls after lysis within secondary xylem, whereas the pit microstructures reported herein are accretion of secondary wall material in primary xylem. In addition the pit microstructure in Nymphaeales are three-dimensional, consisting of a two-layered coarse fibril system, contrasted to the thin, laminar, one layered architecture of woody angiosperms.
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1 - Santa Barbara Botanic Garden, 1212 Mission Canyon Road, Santa Barbara, California, 93105-2199, USA
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Ballroom 3/Cliff Lodge - Level B
Date: Monday, July 27th, 2009
Time: 3:00 PM