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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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