A reevaluation of the discontinuity plate in Psoraleeae and Amorpheae (Leguminosae)

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. THE SUBTRIBE PSORALEEAE (Leguminosae: Papilionoideae) of the tribe Galegeae was proposed by Bentham in 1865. It included those leguminous plants characterized by one-seeded indehiscent fruits and glandular foliage. In 1928 Rydberg elevated the group to tribal level. Barneby (1977) proposed dividing the tribe into a reduced Psoraleeae (corresponding to Psoralea Bentham) and Amorpheae Borissova emend Barneby. Stirton (1981) more recently proposed dividing Psoralea Bentham into six genera: Cullen, Bituminaria, Otholobium, Hallia and Psoralea to include the Old World species, and Orbexilum for the New World ones. This treatment has been accepted in the most recent overview ofthe Leguminosae (Polhill and Raven, 198 1). I am currently revising the New World Psoraleeae, and along with Orbexilum, will be reinstating the genera Hoita, I Received for publication 1 February 1985; revision accepted 25 February 1986. I thank James D. Mauseth and R. Malcolm Brown for advice, technical assistance and use of facilities. I owe special thanks to Eric M. Roberts for technical assistance and Melissa Luckow for discussion. I also thank the curators of the following herbaria for permission to use herbarium material in this study: CAS, LL, MEXU, MO, NY, RSA, SD, TEX, and UB. Support of this project by a Grantin-Aid of Research from Sigma-Xi is gratefully acknowledged. Psoralidium, and Pediomelum, all first proposed by Rydberg (1919). A brief nomenclatural history of the group is shown in Table 1. Barneby (1977) further proposed, however, that Psoraleeae and Amorpheae might not be as. closely related as their taxonomic history implies. This proposal remains to be confirmed, and the question of relationships of the two tribes to other tribes in the subfamily Papilionoideae is unclear. Three features have been used to argue a close relationship of Amorpheae and Psoraleeae (Stirton, 1981): the presence of glands, a similar adnation of the stipules, and the discontinuity plate. This paper presents the results of a study of the discontinuity plate, an anatomical feature of the floral vasculature described and studied by Lersten, Wemple and colleagues (Brady, Wemple and Lersten, 1964; Lersten and Wemple, 1966; Wemple and Lersten, 1966). They found that the xylem of the gynoecium, and usually that ofthe stamens and petals, is discontinuous with that ofthe pedicel. At the base of the gynoecium where the gynoecial traces merge, xylem proliferates and forms "a mass of tracheary elements flaring out horizontally from the lower end of the merged gynoecial bundles" (Lersten and Wemple, 1966). These studies were done prior to Barneby's proposal to divide Psoraleeae into two