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Key to Amaranthus

Amaranthaceae

Amaranthus

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1 Plants monoecious (the pistillate and staminate flowers intermingled, or in separate inflorescences on the same plant); [subgenera Albersia and Amaranthus]
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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.
                   10 Bracts ca. 4 mm long, equaling or exceeding 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).
                       12 Leaf blades broader, well-developed leaves 1-3 cm wide
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Key to Amaranthus, Key B: Amaranthus, subgenera Albersia and Amaranthus

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1 Inflorescences axillary clusters of glomerules (sometimes leafy terminal spikes also present); [subgenus Albersia].
  2 Pistillate flowers usually with 3 tepals; utricles usually regularly dehiscent (indehiscent in A. blitum).
    3 Utricles indehiscent; leaf blades usually deeply notched at the tip.
      4 Plants prostrate; leaves (0.75-) 1-2 (-3.5) cm long; inflorescences axillary cymes (terminal inflorescences reduced or absent)
      4 Plants ascending; leaf blades 2-4 (-6) cm long; inflorescences primarily terminal, thin and flexuous
    3 Utricles dehiscent; leaf blades obtuse, acuminate, or very shallowly notched at the tip.
        5 Tepals of pistillate flowers acute to short-acuminate at the tip, not reflexed; seeds 0.6-1.0 mm in diameter
        5 Tepals of the pistillate flowers long-aristate at the tip, usually reflexed outward; seeds 1.0-1.4 mm in diameter
  2 Pistillate flowers usually with (4-) 5 tepals; utricles usually indehiscent or tardily dehiscent (regularly dehiscent in A. blitoides).
          6 Inflorescence axes not thickened, not indurate at maturity.
                 9 Leaves ovate, obovate-rhombic, to narrowly ovate or lanceolate; plants not fleshy; [exotic of disturbed situations]
                 9 Leaves orbicular or obovate; plants fleshy; [native of sea-beaches]
1 Inflorescences terminal spikes or panicles, leafless or nearly so at least in the distal portions (axillary spikes or clusters usually also present).
                   10 Utricles indehiscent; tepals of pistillate flowers usually 2-3 (5 in A. spinosus); inflorescence bracts shorter than the tepals.
                     11 Stems with paired nodal spines; tepals of pistillate flowers 5; [subgenus Amaranthus]
                     11 Stems lacking spines; tepals of pistillate flowers 2-3; [subgenus Albersia].
                       12 Utricles distinctly rugose, equaling or slightly exceeding the tepals; terminal inflorescences usually thin and interrupted
                       12 Utricles smooth to faintly rugose (occasionally wrinkled or rugose in dried material), distinctly exceeding the tepals; terminal inflorescences usually thick and dense (or thin and interrupted in some forms of A. blitum).
                          13 Utricles subglobose to obovate, compressed; seeds filling the fruit almost completely; leaf blades usually deeply notched at the tip; annual.
                          13 Utricles ellipsoid, slightly to distinctly inflated; seeds filling only the proximal portions of the fruit; leaf blades shallowly notched at the tip; short-lived perennials, or annuals
                   10 Utricles dehiscent; tepals of pistillate flowers usually 5 (3-5 in A. powellii); inflorescence bracts exceeding the tepals (shorter than the tepals in some cultivated forms); [subgenus Amaranthus].
                              15 Fully developed inflorescences large and robust, usually brightly colored (red, purple, occasionally white or yellow, rarely green); bracts usually not exceeding style branches at maturity (occasionally longer than the style branches in A. hypochondriacus); seeds white, ivory, red, brown, or black; [cultivated, only weakly naturalized].
                                  17 Tepals of pistillate flowers (at least the inner tepals of the pistillate flowers) obovate or spatulate, the tip obtuse to slightly notched; style branches spreading or reflexed
                              15 Fully developed inflorescences moderately large, usually green (rarely with some whitish or reddish coloration); bracts exceeding the style branches and tepals; seeds brown or black; [wild and weedy].
                                    18 Tepals of pistillate flowers acute, acuminate, or aristate at the tip; plants slightly pubescent when young, becoming glabrous or nearly so.