Origins and nature of vessels in monocotyledons. 11. Primary xylem microstructure, with examples from zingiberales

Using scanning electron microscopy of hand sections of alcohol-fixed material, microstructure of primary xylem of roots, stems, and rhizomes of Canna was studied. Comparable material of selected other species of Zingiberales, representing five families other than Cannaceae was examined. The appearances present in Canna proved to be shared by other Zingiberaceae. In protoxylem of inflorescence axes, primary walls that experience appreciable elongation bear longitudinally oriented strands facing the lumen. These can be exposed by sectioning and can also be seen in face view on inner surfaces of tracheary elements. Exterior to these strands, primary walls contain reticulate meshworks of cellulosic strands; longitudinal orientation of the strands is present and corresponds to the degree of elongation of the tracheary elements. These meshworks are mostly embedded in amorphous wall materials (primary middle lamella) and are often faintly seen on the inner primary wall surfaces of tracheary elements but may be variously exposed by sectioning. Metaxylem tracheary elements have much the same microstructure but with meshworks composed of randomly oriented fibrils. The meshwork layers can often be sectioned away from the amorphous wall material wholly or in part. Cellulosic strands, mostly longitudinal in orientation and not embedded in amorphous material, were observed in end walls of protoxylem tracheary elements of Canna inflorescence axes; these should be considered intermediate between tracheids and vessel elements. Perforation plates retaining porose pit membrane remnants were observed in stems of Strelitzia. Use of ethanol-fixed material avoids the tendency of macerating fluids to dissolve primary walls and offers many opportunities for further understanding of microstructure of primary xylem, particular in monocotyledons.