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Key to Amaranthus, Key A: Amaranthus, subgenus Acnida

Amaranthaceae

Amaranthus

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1 Plants pistillate.
  2 Tepals lacking, or rudimentary (often only 1-2 present, these < 1 (-2) mm long and lacking a visible midvein); subgenus Acnida, section Acnida].
    3 Seeds 2-3 mm long; utricle 2.5-4 mm long
    3 Seeds 0.7-1.2 mm long; utricle 1-2.5 mm long.
      4 Utricle with conspicuous and regular longitudinal ridges; bract > 1.5 mm long, with a stout midrib not far excurrent beyond the bract blade
      4 Utricle smooth or irregularly tuberculate; bract < 1.5 mm long, with a slender excurrent midrib
        5 Leaf blades narrow, all or nearly all < 1 cm wide
        5 Leaf blades broader, well-developed leaves 1-3 cm wide
  2 Tepals present and well-developed (usually 5 present, at least the outer tepals > 2 mm long and with a visible midvein).
          6 Bracts foliaceous (leaflike), these deltate to rhombic-deltate in shape with erose to crenate or denticulate margins; leaves linear to linear-lanceolate with erose to crispate or undulate margins; [subgenus Acnida, section Acanthochiton]
          6 Bracts not foliaceous, these ovate to narrowly lanceolate in shape with entire margins, leaf shapes various with entire to undulate margins.
             7 Tepals 1 or 2, lanceolate to linear; [subgenus Acnida, section Acnida]
             7 Tepals 5, at least the inner spatulate; [subgenus Acnida, section Saueranthus].
               8 Outermost tepal acute or acuminate (dissimilar to the inner obtuse tepals), the midvein excurrent into a rigid point
               8 Outermost tepal obtuse or notched (similar to the others), the midvein excurrent slightly or not at all.
                 9 Leaf blades membranous; style branches erect; plants erect; inflorescences erect to nodding; [widespread]
                 9 Leaf blades coriaceous; style branches spreading; plants erect to ascending; inflorescences erect; [LA to TX and se. Mexico]
1 Plants staminate (some identifications following this lead may not be reliable).
                     11 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.
                          13 Leaves membranous; plants erect; inflorescences erect to nodding; [widespread]
                     11 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).
                                16 Leaf blades broader, well-developed leaves 1-3 cm wide