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Keyed in multiple places:

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Key to Amaranthus, Key A: Amaranthus, subgenus Acnida

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1 Plants pistillate.
..2 Tepals present and well-developed (usually 5 present, at least the outer tepals >2 mm long and with a visible midvein).
....3 Tepals 1 or 2, lanceolate to linear; [subgenus Acnida, section Acnida]
....3 Tepals 5, at least the inner spatulate; [subgenus Acnida, section Saueranthus].
......4 Outermost tepal obtuse or notched (similar to the others), the midvein excurrent slightly or not at all
......4 Outermost tepal acute or acuminate (dissimilar to the inner obtuse tepals), the midvein excurrent into a rigid point
..2 Tepals lacking, or rudimentary (often only 1-2 present, these <1 (2) mm long and lacking a visible midvein); subgenus Acnida, section Acnida].
........5 Seeds 2-3 mm long; utricle 2.5-4 mm long
........5 Seeds 0.7-1.2 mm long; utricle 1-2.5 mm long.
..........6 Utricle with conspicuous and regular longitudinal ridges; bract > 1.5 mm long, with a stout midrib not far excurrent beyond the bract blade
..........6 Utricle smooth or irregularly tuberculate; bract < 1.5 mm long, with a slender excurrent midrib
............ 7 Leaf blades narrow, all or nearly all < 1 cm wide
............ 7 Leaf blades broader, well-developed leaves 1-3 cm wide
1 Plants staminate (some identifications following this lead may not be reliable).
............ ..8 Outer tepals with prominent midribs, usually longer than the inner tepals; bracts >2 mm long (or 1-2 mm long in A. tuberculatus), mostly with prominent midribs.
............ ....9 Outer tepals with apex acute or obtuse; dark midribs not excurrent
............ ....9 Outer tepals with apex acuminate; midribs excurrent as rigid spines.
............ ......10 Bracts ca. 4 mm long, equaling or exceeding the outer tepals
............ ......10 Bracts ca. 2 mm long, shorter than the outer tepals
............ ..8 Outer tepals without prominent midribs, not appreciably longer than the inner tepals; bracts <2 mm long, the midribs usually not prominent (except sometimes in A. australis).
............ ........11 Bracts < 1 mm long; midribs scarcely excurrent
............ ........11 Bracts > 1mm long; midribs often conspicuously excurrent.
............ ..........12 Leaf blades narrow, all or nearly all < 1 cm wide
............ ..........12 Leaf blades broader, well-developed leaves 1-3 cm wide
............ ............ 13 Bracts with moderately prominent midribs; midribs of outer tepals excurrent
............ ............ 13 Bracts with slender midribs; midribs of outer sepals not excurrent
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