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..2 Flower clusters subglobose; mature achenes ovate, 1-1.5 mm long, < 1 mm wide; leaf teeth generally blunt, the sides of the tooth convex; [American clade]
..2 Flower clusters elongate; mature achenes triangular, 1.5-2.5 mm long, 1-1.5 mm wide; leaf teeth generally sharp, the sides of the tooth straight; [Urtica urens clade]
....3 Plants mainly dioecious (male and female flowers on separate plants), but some plants in populations monoecious (with basal inflorescences male and apical inflorescences female, or with male and female flowers intermixed in the same inflorescence); stems usually weak, sprawling, branching; stems strongly hispid with stinging hairs; leaf blades strongly hispid with stinging hairs on both surfaces; leaf teeth commonly 5-6 mm long
....3 Plants mostly monoecious (with male and female flowers in separate inflorescences on the same plant, usually with basal and apical inflorescences female and median inflorescences male), rarely an entire plant male or female; stems upright, erect, less branched; stems glabrous to puberulent or strigose, lacking (or nearly so) stinging hairs; leaf blades glabrous or glabrescent above (lacking stinging hairs), glabrous to puberulent below (with some stinging hairs); leaf teeth commonly 2-3.5 mm long