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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.
............ 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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