A Reevaluation of the Discontinuity Plate in Psoraleeae and Amorpheae (Leguminosae)

Abstract

The discontinuity plate, once thought to be a unique feature of the Leguminous tribes Psoraleeae and Amorpheae, has been used as evidence for a close relationship of the two tribes. It is here further studied for taxonomic utility. It is shown that the discontinuity plate comprises four phenomena, distributed in four tribes. Tribe Amorpheae has a discontinuity between the pedicellar and gynoecial traces with a profusion of tracheids at the junction of the gynoecial vascular traces, forming a plate-like structure. This is present and developed at very early stages of the flower and corresponds to the discontinuity plate as originally described. Segregate genera of Psoralea as proposed by Stirton (1981) and Rydberg (1919) also have distinctive vascular features, though these are not the same as the discontinuity plate found in Amorpheae. Psoralea, Otholobium, and all New World genera of Psoraleeae have simple delayed maturation of xylem through the receptacle. Some species of Tephrosia (Tephrosieae) were found to have a similar development. Cullen and Bituminaria (Psoraleeae) have delayed maturation of xylem with an inverted cone-like structure developing in early fruit. A similar structure was found in species of Desmodium (Desmodieae). Because the discontinuity plate is shown to comprise more than one phenomenon, and since these vascular irregularities are now known in four tribes, the discontinuity plate does not necessarily indicate a close relationship between the tribes Psoraleeae and Amorpheae. Furthermore, the phenomena cannot be assumed to be homologous. More complete developmental studies and a larger survey of the Papilionoideae is needed before the vascular features can be confidently used in systematic hypotheses.