Developmental and Structural Section
Schneider, Edward , Carlquist, Sherwin .
What ultrastructure shows about monocot tracheids: SEM studies of orchid xylem.
XYLEM of the orchids studied provided unusually favorable material for demonstration of how conductive tissue evolves in monocotyledons. In the end walls of tracheary elements of many Orchidaceae, remnants of pit membranes can be demonstrated with SEM. Illustration of the full range from tracheids to vessel elements, featuring many intermediate stages, was possible. Hand sections of ethanol-fixed material of roots, stems, and inflorescence axes of 13 species representing four subfamilies of Orchidaceae were studied. Pit membranes in end walls of tracheary elements are porose to reticulate in roots of all species studied, but nonporose in stems of Cypripedioideae and Vanilloideae and porose to reticulate in stems of Orchidoideae and Epidendroideae. The pattern of distribution of pit membranes and pit membrane remnants in end walls of tracheary elements of orchids parallels the findings of Cheadle. The position of Cypripedioideae and Vanilloideae as outgroups to Orchidoideae and Epidendroideae, claimed by earlier authors, is supported by clades based on molecular studies and by our studies. Very little hydrolysis of pit membranes in tracheary element end walls was observed in pseudobulbs or inflorescence axes of epidendroids. The pervasive presence of network-like pit membranes of various extents and patterns in end walls of tracheary elements in Orchidaceae calls into question the traditional definitions of tracheids and vessel elements, not merely in orchids, but in angiosperms at large. These two concepts, based on light microscope studies, are blurred when ultrastructural studies are taken into account. More importantly, the intermediate expressions of pit membrane presence in tracheary element end walls of Orchidaceae and some other families of angiosperms are, in fact, important as indicators of steps in evolution of conduction with respect to organs (more rapid flow in roots than in succulent storage structures) and habitat (less obstruction to flow correlated with shift from terrestrial to epiphytic).
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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 Sections
Location: 312/Bell Memorial Union
Date: Monday, July 31st, 2006
Time: 1:45 PM