Occurrence and specialization of vessels in Xyridales

Abstract

The occurrence and variation among vessels in available parts of 41 species in 16 genera of Rapateaceae and of 20 species in the four genera of Xyridaceae were determined. The vessels in Xyridaceae are more specialized in all organs of the plant than they are in Rapateaceae. Simple and scalariform perforation plates occur in the inflorescence axes and leaves of nearly all species of Xyridaceae but only scalariform plates occur in these organs of Rapateaceae – infrequently vessels are lacking in stems and leaves, at least in early metaxylem. Vessels in roots and stems of Mono–tremeae are most specialized (simple and scalariform plates) among tribes of Rapateaceae, with those in Rapateeae intermediate, and those in Schoenocephalieae and Saxofridericieae most primitive (only scalariform perforation plates). Brief comments are made about vessels as possible indicators of relationships with other families.