Trees, shrubs, or herbs, rarely climbers. Leaves alt. or opp., stipulate; sap milky or water; flowers regular, unisexual; perianth of sepals only, or lacking; 2-5, usually 4-merous; free or connate; stamens opposite the sepals, inflexed in bud, dehiscing explosively; ovary superior or inferior, 1-celled, ovule 1, basal, erect; fruit an achene, nut, or drupe, sometimes embedded in the perianth or fleshy receptacle. 42 genera, mostly tropical, a few (e.g. Urtica) in temperate regions; about 600 species. The hairs of some spp. are stinging; siliceous and brittle, containing a highly irritating fluid; as in Nettles.