Mecranium

Description

Description Author and Date: James D. Skean, Jr., January 2011, based on Skean, J. D., Jr. 1993. Monograph of Mecranium (Melastomataceae-Miconieae). Syst. Bot. Monogr. 39: 1-116.

Type species: Lectotype: Mecranium virgatum (Swartz) Triana, listed as Cremanium virgatum (Swartz) Grisebach in Britton & Wilson, New York Acad. Sci., Sci. Surv. Porto Rico Virgin Islands 6: 13. 1925-30.

Description: Evergreen shrubs or small trees with pleonanthic shoots (Rauh's Model) and gray-brown smooth or longitudinally furrowed bark. Twigs slightly to moderately 4-angled (strongly 4-angled and ridged in M. haemanthum and M. tuberculatum), green or red-purple, nodose, smooth or minutely tuberculate-roughened, essentially glabrous to densely pubescent, with a minute ridge (to 0.2 mm thick) encircling each node (or, in M. crassinerve, each node with a flange-like outgrowth) and 2 longitudinal grooves (to 0.5 mm wide) on opposite faces of internode positioned 90 from adjacent upper node. Indumentum of minute, obconical to cylindrical, thin-walled, multicellular glandular hairs, to ca 0.1 mm long, and unbranched to irregularly branched and matted, multicellular hairs, to ca 1 mm long (nearly dendritic-stellate in M. plicatum), in most species restricted to youngest buds, leaf axils, and nodal ridges, less commonly in internodal grooves or evenly distributed on young twigs. Buds narrowly ovoid, sylleptic, essentially glabrous to densely pubescent. Leaves opposite, decussate, petiolate, estipulate, chartaceous or coriaceous, plane, slightly to strongly curved abaxially, or plicate, those on young shoots often tinged with red-purple on veins and margins if growing in areas of high sunlight (entirely red-purple in some leaves of M. haitiense); adaxial surface often essentially glabrous or with some minute glandular hairs or larger hairs similar to those on stem persisting in vein impressions or at petiole junction; abaxial surface sparsely to moderately pubescent with minute glandular hairs and sometimes also sparsely to moderately pubescent with larger hairs similar to those on stem, these often caducous or persisting in axillary tufts at junction of midvein and largest secondary veins; marsupiform domatia often located in axils of junction of midvein and 2 largest secondary veins; margin usually serrate in distal ca 3/4 of lamina to nearly entire, plane to strongly revolute, often more so near base where basal "pseudodomatia" may be formed from the folded leaf margin, the teeth typically with early caducous, acicular, multicellular, apically-curved projections to 0.5 mm long (to 1.5 mm long in M. crassinerve); venation acrodromous, basal or suprabasal, with a prominent midvein, 1-3 pairs of secondary veins (1 of these usually intramarginal and inconspicuous), numerous percurrent tertiary veins more or less perpendicular to midvein and convex, and reticulate quaternary and higher order veinlets; petiole canaliculate, green or red-purple, glabrous to densely pubescent with hairs similar to those on stem.

Description (cont.): Inflorescences axillary, paniculate cymes borne in the leaf axils and/or on leafless nodes below the leaves, usually borne singly, but in robust shoots sometimes branched at base and appearing to have 1-3 per axil, 1-2 (4) branched, bracteate, with opposite, narrowly triangular bracts (the 2 lowest protecting inflorescence bud and early caducous). Flowers perfect, 4-merous, sessile (in M. virgatum appearing pedicellate due to prolonged hypanthium base), lacking fragrance. Hypanthium green to tinged heavily with red-purple, subglobose, cylindrical, or slightly obconical, essentially glabrous to moderately pubescent with hairs similar to those on stem. External calyx teeth, 4, 0.2-0.5 mm long at anthesis. Internal calyx lobes free and imbricate (sect. Sagraeoides) or fused in a conspicuous, dome-like (conical in M. racemosum) apiculate, calyptra (sect. Mecranium), which is pushed aside or ruptures irregularly as style elongates at anthesis. Petals 4, usually white/cream-colored, sometimes pink-tinged, uniformly pink, or red-purple, usually obovate, with rounded, retuse apices (sect. Mecranium), or broadly obovate to ovate or narrowly triangular (sect. Sagraeoides), glabrous, many-veined (except 1-veined in M. acuminatum), spreading or, less commonly, reflexed. Stamens 8, isomorphic, white, or red-purple with white anther sacs, glabrous, inserted above an erose-ciliate toral fringe located just internal to the point of filament attachment, i.e., on the torus of the hypanthium, geniculate, folded adaxially in bud; proximal segment (filament) narrowly ovate or narrowly obovate, flattened dorsally, strongly curved; distal segment (anther) narrowly obovate or narrowly ovate, the anther sacs often slightly inrolled adaxially, the connective often prolonged beyond bases of anther sacs, the 2 anther sacs dehiscing confluently by means of a single small apical pore, or opening by means of a single gaping, dorso-apical, 2-chambered foramen formed from 2 lateral slits, with the septum torn, or opening separately by 2 apical pore-like slits that extend down the sides and often to the adaxial surface of the anther, with the septum between the anther sacs intact; pollen tricolporate, with 3 additional pseudocolpi, ellipsoid, obscurely rugulate. Ovary 1/2 to fully inferior, 4-locular, with axile placentation, ovules numerous, anatropous, the apex prolonged into a collar-like, 8-ribbed appendage, which encases the folded distal stamen segments in bud and encircles the style. Style white, or rarely light red-purple, cylindrical or very slightly obconical, straight or very strongly curved; stigma papillose, capitate, or less commonly not expanded. Berries deep purple-black, essentially glabrous or with minute glandular hairs. Seeds narrowly ovoid, papillose, tan. Testa 1 cell layer thick, each cell sinuous-margined, flattened, or more commonly, comprising a single papilla. Embryo occupying nearly entire seed, with 2 fleshy cotyledons, endosperm absent. Germination epigeal.

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