Microstructure of tracheids of nymphaea

Hand sections of root and stem xylem of diverse species of Nymphaea (including Ondinea) were studied with SEM in order to explore the diversity of wall structure within the genus. Lateral walls of root tracheary elements are untextured at the magnifications used, but end wall structure suggests that lysis of pit membranes leaves a cellulosic network with a large reticulum. Stem tracheids have lateral walls that are untextured or have a few prominent fibrils, but end walls have a dense accretion of coarse fibrils. These fibrils form a spongiform or compressed network overlying the pit membranes. In addition, on the lumen side of pit membranes, coarse fibrils cross the pits in an axial direction. These coarse fibrils are connected to pit borders, and they may extend onto wall surfaces of the tracheary elements. These fibrillar accretions are considered here to fulfill criteria as secondary-wall components. Variation within the genus with respect to the fibrillar accretions may occur, but there may be more extensive diversity within any given sample than among species. Root tracheary elements could be considered to be vessel elements because of the extensive removal of wall material from the end-wall pit membranes. Stem tracheary elements qualify as tracheids because, rather than showing lysis of pit-membrane portions, end-wall pit membranes bear fibrillar accretions that are only sparsely porose. The absence of coarse fibrillar accretions on lateral walls of stem tracheids (on tracheid-to-parenchyma interfaces) provides difficulty for any hypothesis that would relate wall thickenings to turgor pressure. The patterns of fibrillar accretion observed in Nymphaea include all patterns thus far reported from stem tracheids in the other genera of the family, and they must be considered to be a characteristic of Nymphaeaceae.