Fl. Bor.-Amer. 1: 10, plate 2. 1803.
Annuals [perennials], forming mats. Stems prostrate to weakly ascending, glabrous. Leaves cauline, opposite or whorled; petiole absent or present; blade not leathery, margins entire. Inflorescences axillary, solitary flowers; bracts absent. Flowers: sepals 4, connate proximally, calyx nearly radially symmetric, campanulate, not winged, lobes lanceolate or oblanceolate, equal or unequal; petals 4, corolla white, weakly to strongly bilaterally symmetric, weakly bilabiate and ± rotate or unilabiate, tube 0.3–0.8 mm, abaxial lobes 1 or 3, lateral 0 or 2, adaxial 0 or 1, adaxial lip sometimes prominent; stamens 2, filaments gibbous or bearing a process near base, glabrous, appendage present; staminode 0. Capsules: dehiscence irregular. Seeds 15–20, yellowish, ellipsoid or oblong to narrowly obconic.
s, e United States, West Indies, Central America, South America
Species 14 (3 in the flora).
Morphological data (corolla and stamen structures) can be interpreted as providing support for recognizing the genus Hemianthus (which would include Micranthemum glomeratum and M. micranthemoides), as was done by J. K. Small (1933) and F. W. Pennell (1935).
|1||Leaf blades orbiculate, ovate, or obovate; corollas shorter than or equal to calyces.||Micranthemum umbrosum|
|1||Leaf blades elliptic to oblanceolate; corollas longer than calyces.||> 2|
|2||Seeds ellipsoid; flowers chasmogamous.||Micranthemum glomeratum|
|2||Seeds oblong to narrowly obconic; flowers cleistogamous.||Micranthemum micranthemoides|