URTICACEAE (Nettle Family)

Plants annual or perennial herbs (shrubs elsewhere), usually monoecious or dioecious, sometimes incompletely so, sometimes armed with stinging hairs, not producing milky sap. Leaves alternate or opposite, petiolate. Stipules absent or present, when present usually small and herbaceous to scalelike, sometimes (in species with opposite leaves) fused between the adjacent leaf bases, often shed early. Leaf blades simple, pinnately veined or with 3 main veins, the margins entire, scalloped, or toothed, the surfaces usually appearing minutely dotted, the dots due to small structures (known as cystoliths) containing calcium carbonate crystals present in enlarged epidermal cells. Inflorescences axillary or terminal, small clusters, spikes, or spikelike racemes, these sometimes arranged into panicles, the flowers sometimes subtended by short bractlets. Flowers minute, actinomorphic (except sometimes in Pilea), hypogynous, mostly imperfect. Staminate flowers usually short-stalked, the calyces of 4 or 5, small, free sepals (usually absent in Parietaria). Pistillate flowers sessile or short-stalked, the calyces of (2)3–5, small, free or fused sepals, persistent at fruiting. Corollas absent. Stamens 4 or 5 (absent in pistillate flowers, except in Pilea, where reduced to 3 strongly inward-curved, scalelike staminodes), free, opposite the sepals, the filaments bent inward in the bud, reflexing suddenly as the bud opens and ejecting the pollen explosively, the anthers attached basally, yellow, dehiscing by longitudinal slits. Pistil 1 per flower (reduced to a small peglike structure in staminate flowers), of 1 carpel, the ovary superior, 1-locular, with 1 ovule, the placentation basal. Style absent or 1, the stigma 1, linear or capitate. Fruits achenes, not fused into groups. About 55 genera, about 1,650 species, nearly worldwide.

Members of the Urticaceae mostly are of relatively little economic importance (N. G. Miller, 1971). The principal crop plant is Boehmeria nivea (L.) Gaudich. (ramie), which originated in eastern Asia. After chemical treatment to remove pectins and other impurities, the inner bark of this herbaceous perennial yields a strong fiber that for many centuries has been woven into fine fabrics and yarns. Some species of Urtica were used similarly historically until ramie production became more widespread. Otherwise, members of a few genera, such as Pilea, are cultivated as ornamentals, outdoors in warmer climates and as house plants farther north. Some of the species produce sufficient windborne pollen to be of significance as allergens, particularly in portions of Europe. The species with stinging hairs have sometimes been used for culinary purposes and medicinally.

The stinging hairs of some genera act as syringes. They consist of a bulbous reservoir at the base and a slender shaft, which breaks off along a zone of weakness below a slightly bulbous tip, leaving a sharp beveled tip to pierce the skin. The hairs contain a cocktail of alkaloids and other toxic substances, including some similar in action to acetylcholine, and cause a strong histamine reaction that involves contact dermatitis and a lingering burning sensation (Burrows and Tyrl, 2001). The hairs of some Urtica species have been touted as a treatment for various ailments, including rheumatism. However, some members of the Australian genus of stinging trees, Dendrocnide Miq., have been documented to cause fatalities in humans and other mammals.