The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety

The plexus of vascular bundles in the nodes of grasses is notoriously complex, where long axial bundles pass through a network of transverse bundles. The xylem pathways for water in maize stems have been investigated anatomically and with dye and particulate tracers, revealing some of the details of this complexity. Only approx. 3% of axial vessels pass through nodes without being interrupted by end walls. Axial bundles at nodes differ from those in internodes in having the metaxylem and protoxylem vessels connected by small tracheary elements. So it is only at nodes that exchange of sap occurs between the large vessels within a bundle. End walls, acting as filters for particles and gas bubbles, always separate axial vessels from vessels in transverse bundles. The high redundancy of bundle connections in the nodal plexus is interpreted as providing alternative water pathways to bypass embolisms and damaged or diseased sections of the xylem. The pores in the filters at the base of nodes and between axial and transverse vessels within nodes are <20 nm in diameter. Where axial vessels connect to transverse vessels, a variety of unusual shapes of vessel elements mediate two- and three-way connections within the plexus.