Phylogenetic Relationships of Didymocistus and Hymenocardia ( Euphorbiaceae )

The genus Hymenocardia has been placed either in Euphorbiaceae subfamily Phyllanthoideae or in its own family and then associated with the Urticales, particularly the Ulmaceae. However, the pollen exine wall of Hymenocardia lacks the microchanneled tectum, granular interstitium, and thin foot-layer of the Urticales. Hymenocardia instead has many palynological features found in the Phyllanthoideae and should be retained in the Euphorbiaceae in that subfamily. Didymocistus, which has scalelike foliar trichomes and exine sculpturing and ultrastructure similar to Hymenocardia, should be transferred from the Phyllanthoideae-Aporuseae to a position near Hymenocardia. The relationships of Hymenocardia Wallich ex Lindley have been controversial ever since Airy Shaw (1965) segregated the genus as its own family, Hymenocardiaceae. Previous authors had placed Hymenocardia in the Euphorbiaceae with genera now included in subfamily Phyllanthoideae, either without special attention (Baillon, 1874; Bentham, 1880; Pax & Hoffmann, 1922, 1931), in a distinct tribe but with other genera (Hutchinson, 1969), or in a tribe (or subtribe) by itself (Mueller, 1866; Webster, 1975). Radcliffe-Smith (1973, 1987a) and Leonard & Mosango (1985) have also accepted the Hymenocardiaceae, though Webster (1967, 1975, 1987, 1994) has not. Leonard & Mosango (1985) and Radcliffe-Smith (1987b) reviewed the history of this controversy more completely. Levin (1986a-c), studying leaf architecture and epidermal morphology, suggested that Didymocistus Kuhlm., a monotypic South American genus, was closely related to Hymenocardia. Webster (1975) had placed Didymocistus in the tribe Aporuseae of the Phyllanthoideae, following suggestions Kuhlmann (1940) made when he described the genus. (Note that Webster (1994) more recently treated the Aporuseae as subtribe Scepinae of the tribe Antidesmeae; for convenience we will continue to refer to this group as the Aporuseae.) Levin based his proposal on the absence in Didymocistus of marginal glands, enlarged tanniniferous epidermal cells, and anisocytic stomata, all synapomorphies of Aporuseae, and the presence in that genus and Hymenocardia of relatively organized leaf venation, which in turn linked these genera with some members of Phyllanthoideae tribe Phyl-