2. TETRAZYGIA Richard ex de Candolle in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 3: 172. 1828.
Clover ash [Greek tessares, four, and zygon or zygos, yoke or crossbar, alluding to 4-merous flowers]
Shrubs or trees. Stems erect, terete, lepidote, scales dense, radiate, white or light brown and glabrescent; bark smooth to scaly or shallowly fissured on largest stems. Leaves petiolate, subsessile, or sessile; blade with 3 primary veins and prominent cross veins, margins entire, revolute, surfaces lepidote abaxially, glabrous adaxially. Inflorescences terminal, paniculate cymes; bracts usually persistent, triangular. Flowers pedicellate; hypanthium campanulate [urceolate]; calyx (0 or)5-lobed, lobes rounded to triangular; petals [4 or] 5 [or 6], spreading or slightly deflexed, symmetric, white or pink, often drying yellowish; stamens [8 or] 10 [or 12], subequal, in 1 whorl, connective bases unappendaged, relatively thick, oblong, shorter than thecae; anthers slightly downcurved, linear, 2-locular, apically poricidal; ovary inferior, adnate to floral tube except apically, 3-locular; style curved, linear, exserted; stigma minute, densely papillose. Fruits berries. Seeds cuneate. x = 17.
Species 20–25 (1 in the flora): Florida, West Indies; introduced in Pacific Islands.
Species of Tetrazygia with conspicuous, elongated external calyx lobes were transferred by Borhidi to the new genus Tetrazygiopsis. In generic realignments of terminal-flowered taxa of Miconieae, W. S. Judd and J. D. Skean (1991) considered Tetrazygiopsis as a synonym of Tetrazygia and were followed in this assessment by Hno. Alain (1985–1997, vol. 4, Puerto Rico; 1982+, vol. 9, Hispaniola). Judd and Skean suggested that Tetrazygia eventually might be broadened to include some species now placed in Miconia, redefined by having a distinctive glabrous style, strongly curved apically and with a minute stigma, irrespective of the presence or absence of a constriction between calyx and hypanthium.
Tetrazygia is an Antillean genus of ca. 20 (W. S. Judd and J. D. Skean 1991; Hno. Alain 1982+, vol. 9) or 25 species (R. A. Howard 1974–1989, vol. 5). The genus has been defined conventionally by a vesture of radiate scales, a terminal, paniculate inflorescence, the presence of a strong constriction in the hypanthium proximal to the calyx, and calyx lobes reduced or absent.
Hno. Alain (1957) considered Tetrazygia an artificial genus, perhaps better treated as a section of Miconia Ruiz & Pavon. Molecular-based phylogenetic analyses (F. A. Michelangeli et al. 2004, 2008) indicate that species of Tetrazygia are cladistically intermixed with members of other genera of Miconieae, especially Pachyanthus A. Richard and Calycogonium de Candolle, both predominantly Antillean. Species with 4-merous flowers may not be closely related to those with 5-merous or 6-merous flowers and conspicuously constricted hypanthia, such as T. barbata Borhidi, T. bicolor, T. Coriacea Urban, and T. lanceolata Urban, which appear to be cladistically coherent. Other data (Michelangeli et al. 2004) suggest that Charianthus D. Don, and possibly Calycogonium de Candolle (in part), are derived from within Tetrazygia.