The Embryology and Relationships of Rhynchocalyx Oliv. (Rhynchocalycaceae)

Abstract

This paper presents the first embryological study of Rhynchocalyx, a unique and problematical monotypic South African genus. The genus agrees well with other Myrtales in its basic embryological and other characteristics, including an ephemeral anther endothecium, a micropyle formed by the inner integument alone, and by other subsidiary features of the anther, nucellus, endosperm, and seed, as well as its megasporogenesis. Rhynchocalyx differs absolutely from Axinandra in many embryological characteristics, a fact that is not in agreement with a broad definition of Crypteroniaceae to include both genera. Also, Rhynchocalyx, despite its shared distinctive multicelled archesporium, differs from Lythraceae in many attributes. Thus, evidence from embryology, combined with that from other sources, supports the conclusion that Rhynchocalyx is not directly related to Lythraceae, and is, therefore, best assigned to a family of its own, Rhynchocalycaceae. This paper reports the embryology of the rare monotypic South African genus Rhynchocalyx, and is the second concerning the unique genera of the order Myrtales. The first concerned Axinandra (Tobe & Raven, 1983b). The proper taxonomic assignment and relationships of Rhynchocalyx, like those of Axinandra, have often been disputed. Oliver (1895), who first described Rhynchocalyx lawsonioides, classified the genus under Lythraceae. Later, Engler (1900) treated Rhynchocalyx as one of the Gattungen der Lythraceen von unsicherer Stellung, and Koehne (1903) excluded it from Lythraceae. Sprague and Metcalfe (1937), however, returned it to Lythraceae, stating that Koehne's grounds for its exclusion from that family were not tenable. Recently, van Beusekom-Osinga and van Beusekom (1975) proposed a broad definition of Crypteroniiaceae that included Rhynchocalyx, together with Crypteronia, Dactylocladus, Axinandra, and Alzatea. Pollen morphology might or might not be taken to support a relationship of Rhynchocalyx with the other genera of Crypteroniaceae sensu lato (Muller, 1975): thus, Rhynchocalyx agrees with Dactylocladus and Axinandra in having heterocolpate pollen grains but differs from both Crypteronia (with bisyncolporate pollen grains) and Alzatea (with tricolporate pollen grains). Rhynchocalyx and Alzatea differ from the other three genera of Crypteroniaceae sensu lato in their wood anatomy, and share many characteristics with Lythraceae and Melastomataceae (van Vliet, 1975). On the other hand, leaf, twig, and nodal anatomy suggests that Rhynchocalyx is closer to some Lythraceae, Oliniaceae, and Melastomataceae than to the other members of Crypteroniaceae sensu lato (van Vliet & Baas, 1975). Nevertheless, differences in floral structure and the presence of foliar sclereids in the petioles of Rhynchocalyx (Rao & Das, 1979) indicate that it is not directly related to Lythraceae. In addition, Rhynchocalyx may be distinguishable from Lythraceae in not having nectaries in its flowers, although this feature needs to be reviewed in