Origins and nature of vessels in monocotyledons. 12. Pit membrane microstructure diversity in tracheary elements of Astelia

Xylem of stems and roots of three species of Astelia, a monocot with relatively unspecialized xylem, was examined with scanning electron microscopy (SEM) to better understand structural conditions intermediate between tracheids and vessel elements. Both macerations and hand-sectioned material were studied. Tracheary elements of roots of Asteliaceae can be characterized as tracheids, with some degrees of transition to vessel elements. Pit membrane remnants, which take the form of pores, reticula, or threads, are present commonly in end walls of tracheary elements of roots of Astelia. Stems of Astelia have tracheids with less-conspicuous porosities in the pit membranes of end walls than those of roots. Sectioned materials show that the porose (reticulate) cellulosic layers of the primary wall, which is embedded in a matrix of amorphous material, can be exposed to various degrees by the sectioning process; the cellulosic network faces the lumen, and the amorphous material is the compound middle lamella. Astelia shows stages of transition between vessel elements and tracheids. These character expressions relate to occupancy of moist habitats (Astelia) with steady availability of moisture during the year. There appears to be little difference between a terrestrial species (A. chathamica) and the scandent/epiphytic species A. argyrocoma and A. menziesiana in terms of tracheary element microstructure, suggesting that habitat is more important than habit as a determinant of tracheary element microstructure and the degree to which lysis of pit membranes occurs. Freehand sectioning of ethanol-fixed materials, as in earlier studies in this series, provides a reliable way of observing pit membrane/ perforation structure when viewed with SEM. Astelia is one of several monocots that demonstrate the difficulty of discriminating between tracheids and vessel elements.