| 1 |
Stipules connate; inflorescences often branched. |
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(2) |
| + |
Stipules mostly free (sometimes partly connate with 2-lobed apex on upper parts of plant in U. laetevirens); inflorescences unbranched or branched. |
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(6) |
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| 2 (1) |
Leaf blade ovate to lanceolate with margin dentate to doubly denticulate or doubly serrulate. |
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(3) |
| + |
Leaf blade broadly ovate or subcordate, with margin lobed or rarely doubly denticulate. |
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(4) |
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| 3 (2) |
Leaf blade ovate to lanceolate, surface wrinkled when dried, margin sharply doubly serrulate; stem densely pubescent with many stinging hairs. |
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13 U. ardens |
| + |
Leaf blade lanceolate or rarely narrowly ovate, surface nearly smooth when dried, margin crenate or inconspicuously double-denticulate to serrulate; stems sparsely pubescent with a few stinging hairs at least when old. |
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14 U. parviflora |
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| 4 (2) |
Stipules partly connate, apex 2-lobed or -cleft. |
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10 U. thunbergiana |
| + |
Stipules wholly connate, apex obtuse. |
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(5) |
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| 5 (4) |
Leaf margin shallowly 5-7-lobed or palmately 3-lobed (lobes ± pinnately lobed again), lobe margin serrulate; inflorescences with a few short branches or almost unbranched. |
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11 U. fissa |
| + |
Leaf margin usually 10- or more lobed, lobe margin doubly serrulate; inflorescences with many long branches. |
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12 U. mairei |
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| 6 (1) |
Plants usually dioecious; inflorescences branched. |
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(7) |
| + |
Plants monoecious; inflorescences, at least female ones, unbranched or nearly so. |
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(8) |
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| 7 (6) |
Plants often with dense stinging and setose hairs; leaf blade ovate or lanceolate, base cordate; petiole 1/6-1/2 as long as leaf blade. |
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9 U. dioica |
| + |
Plants with sparse stinging and setose hairs; leaf blade lanceolate to linear, base rounded or notched; petiole 1/15-1/5 as long as leaf blade. |
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8 U. angustifolia |
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| 8 (6) |
Inflorescences bisexual, containing proximal female flowers and distal male flowers. |
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(9) |
| + |
Inflorescences often unisexual, containing either male or female flowers. |
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(11) |
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| 9 (8) |
Plant always annual; leaf blade broadly elliptic, apex obtuse-rounded; perianth lobes of female flowers connate at base, sparsely setulose along margin. |
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3 U. urens |
| + |
Plant perennial; leaf blade ovate to lanceolate, apex acute to shortly acuminate; perianth lobes of female flowers connate 1/2 of length, subglabrous. |
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(10) |
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| 10 (9) |
Stipules 4-7 mm; leaf margin dentate or doubly serrate; achene ca. 0.8 mm. |
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1 U. atrichocaulis |
| + |
Stipules ca. 2 mm; leaf margin incised-serrulate; achene ca. 1 mm. |
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2 U. taiwaniana |
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| 11 (8) |
Male inflorescences in distal axils; female perianth lobes without stinging hairs; cystoliths often botuliform. |
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7 U. laetevirens |
| + |
Male inflorescences usually in proximal axils; female perianth lobes with stinging hairs; cystoliths often punctiform. |
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(12) |
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| 12 (11) |
Female perianth lobes membranous, inner pair 2 times as long as achene; petiole 0.2-0.5(-1.6) cm. |
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6 U. hyperborea |
| + |
Female perianth lobes herbaceous, inner pair subequal to achene; petiole 1-8 cm. |
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(13) |
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| 13 (12) |
Achene smooth; female inflorescences in fruit drooping; outer 2 female perianth lobes 1/7-1/5 as long as inner 2. |
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9 U. dioica |
| + |
Achene verrucose; female inflorescences in fruit erect or spreading; outer 2 female perianth lobes 1/4-1/2 as long as inner 2. |
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(14) |
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| 14 (13) |
Leaf margin 3-5-palmatisect or -palmatipartite, with pinnatisect lobes; female perianth lobes connate for 1/3 of length. |
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5 U. cannabina |
| + |
Leaf margin coarsely dentate or sharply serrate; female perianth lobes connate at base only. |
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4 U. triangularis |
