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Key to Amaranthus

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1 Plants dioecious; [subgenus Acnida]
1 Plants monoecious (the pistillate and staminate flowers intermingled, or in separate inflorescences on the same plant); [subgenera Albersia and Amaranthus]

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

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 thickened, becoming indurate at maturity
..........6 Inflorescence axes not thickened, not indurate at maturity.
............ 7 Utricles with regular, circumscissile dehiscence
............ 7 Utricles indehiscent (or tardily and irregularly dehiscent).
............ ..8 Leaves crisped-erose, conspicuously undulate (non planar)
............ ..8 Leaves entire or erose, plane or slightly undulate.
............ ....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.
............ ............ ..14 Plants ascending; leaf blades 2-4 (-6) cm long; inflorescences primarily terminal, thin and flexuous
............ ............ ..14 Plants prostrate; leaves (0.75-) 1-2 (-3.5) cm long; inflorescences axillary cymes (terminal inflorescences reduced or absent)
............ ............ 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].
............ ............ ......16 Inflorescences lax, erect to drooping.
............ ............ ........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
............ ............ ........17 Tepals of pistillate flowers oblong to lanceolate, the tip acute; style branches erect or slightly 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 obtuse, rounded, or slightly notched at the tip; plants rather densely pubescent
............ ............ ..........18 Tepals of pistillate flowers acute, acuminate, or aristate at the tip; plants slightly pubescent when young, becoming glabrous or nearly so.
............ ............ ............ 19 Bracts 2-4 mm long; inflorescences usually soft and lax, with spreading branches
............ ............ ............ 19 Bracts 4-7 mm long; inflorescences usually stiff, with erect branches
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