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Click the number at the start of a key lead to highlight both that lead and its corresponding lead. Click again to show only the two highlighted leads. Click a third time to return to the full key with the selected leads still highlighted.

Key to Ambrosia

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1 Leaves either undivided, with 2 lateral teeth, or palmately 3-5-lobed.
..2 Leaves sessile to clasping, 2-7 cm long, undivided or with teeth at base or rarely lobed.
....3 Annual; leaves with (1-) 2 (-4) teeth near base; [widely distributed]
....3 Perennial or subshrub; leaves with no teeth near base; [TX southwards]
..2 Leaves petiolate, 7-30 cm long, (1-) 3 (-5) lobed.
......4 Petioles of upper leaves wingless; ribs of fruits ending in blunt or obsolete tubercles; leaf undersurfaces strongly scabrous
......4 Petioles of upper leaves wing-margined; ribs of fruits ending in short spines; leaf undersurfaces glabrescent to scabrous
1 Leaves 1- to 3-pinnatifid.
........5 Stems creeping, mat-forming; leaves grooved on the upper surface along the veins and strikingly "3-D”; [peninsular FL]
........5 Stems erect; largest leaves 1-2-pinnatifid, planar; stems erect; [collectively widespread in our area].
..........6 Annual, with fibrous roots; fruiting involucre (bur) with short, sharp spines 0.1-0.5 mm long
............ 7 Achenes 2.9-3.6 mm long (including the tubercle) and 1.5-1.9 mm in diameter; plant height:width ratio > 0.8; annual from a definite taproot; terminal leaf lobe > 2.9 mm wide; fresh foliage with a pungent odor, not noticeably viscid, and with hairs to 1.8 mm long; [of many habitats, widespread]
............ 7 Achenes 3.5-4.8 mm long (including the tubercle) and 1.7-2.3 mm in diameter; plant height:width ratio < 0.8; annual from shallow, fibrous roots only; terminal leaf lobe 1.0-2.0 mm wide; fresh foliage abundantly viscid, with a floral, jasmine-like odor, and with hairs to 0.3 mm long; [plants restricted to granitic outcrops; endedmic to nw. SC]
..........6 Perennial, with deep-seated, creeping roots; fruiting involucre (bur) either with spines (0.1-) 0.4-1.0 mm long or with 'spines' reduced to mere bumps.
............ ..8 Leaves lacinately 2-4 pinnately lobed; fruiting involucres (burs) 1-2 mm long; bur spines uncinate (hooked at tip)
............ ..8 Leaves pinnately toothed or weakly 1-pinnately lobed; fruiting involucres (burs) 2-3 mm long; bur spines straight at tip.
............ ....9 Plant 1-3 dm tall; leaves sessile, not gland-dotted; bur spines 1-5; [TX southwards]
............ ....9 Plant 1-10 dm tall; leaves sessile or not, gland-dotted; bur spines 1-6 or merely reduced to bumps; [widely distributed]