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Vessels in Brasenia New perspectives on vessel origin in primary xylem of angiosperms


, : Vessels in Brasenia New perspectives on vessel origin in primary xylem of angiosperms. SIDA Contributions to Botany 17(2): 437-443

Studies by means of scanning electron microscope (SEM) of primary xylem of Nymphaeaceae have revealed diversity in vessels and rudimentary expressions of pore formation on end walls of vessel elements. A similar situation is true in Cabombaceae. In Brasenia schreberi, vessels occur not only in roots but also in rhizomes. Vessels in roots have lignified helical or annular bands only at the ends of the vessel elements; central portions of the elements have diminished versions of the thickenings, lacking borders and lignification sbd a configuration hitherto unreported for vascular plants. This may be the first demonstrated instance of a midpoint in reduction of tracheary tissue in relation to the aquatic habitat. Perforation plates in roots consist of a series of pores of various sizes in primary wall areas between the lignified bands; pores are fewer in the perforation plates of tracheary elements of rhizomes. Vessels in Brasenia likely illustrate early stages in vessel origin. Because the patterns of vessel structure now available by means of SEM are so different from those hitherto figured for primary xylem of dicotyledons currently placed in basal portions of cladograms, we are extending our studies to primary xylem of other vascular plants likely to show incipient stages of vessel evolution. Evidence now available forces us to envision vessel origin in angiosperms in terms of formation of numerous pores on end walls, rather than elimination of entire pit membranes at the outset, and this poses new questions about the nature of conduction and embolism spread in species with porose end walls.

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