Herbs , annual or perennial, with stinging and nonstinging hairs on same plant. Stems simple or branched, erect, ascending, or sprawling. Leaves opposite; stipules present. Leaf blades elliptic, lanceolate, ovate, or orbiculate, margins dentate to serrate; cystoliths rounded or ± elongate. Inflorescences axillary, lax, of cymes arranged in racemes or panicles. Flowers unisexual, staminate and pistillate flowers in loose to tight clusters in separate inflorescences or intermixed in same inflorescence on same or different plants; bracts narrowly triangular to lanceolate, lacking hooked hairs. Staminate flowers: tepals 4, distinct, equal; stamens 4; pistillode cuplike. Pistillate flowers: tepals 4, distinct, inner 2 equal to achene, outer 2 smaller, without hooked hairs; staminodes absent; style absent; stigma tufted, persistent or deciduous. Achenes sessile, laterally compressed, ovoid or deltoid, loosely enclosed by inner tepals. x = 12, 13.
PLANT: Erect, ascending, or sprawling, monoecious or dioecious, annual or perennial herbs with stinging and non-stinging hairs. LEAVES: opposite, stipulate, the blades elliptic, lanceolate, ovate to broadly ovate, or circular, dentate to serrate, with punctiform or more or less elongate cystoliths. INFLORESCENCE: axillary, of cymes arranged in lax racemes or panicles, or in loose to tight clusters, staminate and pistillate flowers in separate inflorescences, or staminate and pistillate flowers alternately arranged in the same inflorescence. STAMINATE FLOWERS: with 4 equal tepals, 4 stamens, and a rudimentary, cuplike ovary. PISTILLATE FLOWERS: with 4 tepals (the 2 outer smaller, at least in fruit); style absent; stigmas tufted. FRUIT: flattened, ovoid or deltoid, loosely enclosed by the inner tepals, the stigmatic tuft deciduous or persistent on the mature fruit. NOTES: A genus of about 45 spp. indigenous to temperate areas of both hemispheres. Woodland, D. W. 1982. Syst. Bot. 7:269 281. Woodland, D. W. 1982. Syst. Bot. 7:282 290. Woodland, D. W., I. J. Bassett, & C. W. Crompton. 1976. Canad. J. Bot. 54:374 383. (Latin: based on urere = to burn). REFERENCES: Boufford, David E. 1992. Urticaceae. Ariz.-Nev. Acad. Sci. 26(1)2.
Dioecious or monoecious; male fls with 4 subequal sep, 4 stamens, and vestigial ovary; sep of the female fls unequal, the outer 2 small and inconspicuous; stigma capitate-tufted; fr a lenticular achene enclosed by the 2 inner sep; herbs with opposite lvs, ±beset with stinging bristles; fls minute, in panicles, spikes, or head-like clusters from the upper axils. 25, widespread.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.