Revision of the Genus Sterigmapetalum (Rhizophoraceae)

Abstract

The genus Sterigmapetalum consists of 7 species, with 4 newly described species (S. exappendiculatum, S. heterodoxum, S. resinosum, and S. tachirense), and 1 new subspecies (S. guianense subsp. ichunense). On the basis of the presence or absence of a resinous exudate, development or reduction of lateral flanges of the petals, and opposite or verticillate leaves, two subgenera are recognized: Sterigmapetalum and Balsamocaulon. On the basis of their hypogynous flowers and superior ovaries, Cassipourea and Sterigmapetalum are placed in a new tribe, Hypogyneae, within the Rhizophoraceae. A key to the species is provided, followed by a description of the species and their geographical distribution. Newly discovered anatomical evidence, which reveals the presence of a P-type sieve element in the Rhizophoraceae, indicates that the family should be excluded from the Myrtales and is additional evidence supporting the view provided by the gross morphological floral details that the family is atypical in the Myrtales, and should be removed from that order. Sterigmapetalum was established in 1925 by Kuhlmann with a single species, S. obovatum, collected from Amazonian Brazil. It was differentiated from what was considered a related genus, Blepharistemma Wall. ex Benth., by the possession of dioecious flowers, of calyx segments and stamens, absence of a disc, and a greater number of cells of the ovary. Since that time, two additional species, S. colombianum Monachino (1944) from Colombia, and S. guianense Steyermark (1952) from the Venezuelan Guayana, have been published. During recent explorations in Venezuela near the Colombian border, a fourth species has been discovered. In an attempt to bring our present knowledge of the genus up-to-date, material has been borrowed from the herbaria of F, NY, and US. Unfortunately, the dioecious character of the genus, together with the paucity of specimens available in herbaria, have added to the difficulties of revision. However, as a result of this study, additional taxa have become manifest and are described below. We wish to thank the curators of the aforementioned herbaria for the loan