This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Amaranthus retroflexus L. Sp. Pl. 991. 1753.
Roughish-puberulent, rather light green, stem stout, erect or ascending, commonly branched, 1°-10° tall. Leaves ovate, rhombic-ovate or the upper lanceolate, slender-petioled, acute or acuminate at the apex, narrowed or acuminate at the base, the larger 3'-6' long, their margins undulate or entire; flowers green, polygamous, densely aggregated in terminal and axillary spikes, which are sessile, stout, obtuse or subacute, ovoid-cylindric, erect or ascending, 1/2'-2 1/2' long, 4"-7" thick; bracts subulate, twice as long as the 5 scarious narrowly oblong or slightly spatulate mucronate-tipped obtuse or often emar-ginate sepals; stamens 5; utricle slightly wrinkled, thin, circumscissile, rather shorter than the sepals.
Amaranthus hybridus L. Sp. Pl. 990. 1753. Amaranthus hypochondriacus L. Sp. Pl. 991. 1753. A. chlorostachys Willd. Amaranth. 34. pl. 10. t. 19. 1790. A. paniculatus L. Sp. Pl. Ed. 2, 1406. 1763.
Similar to the preceding species but darker green, or purple, pubescent or nearly glabrous; stem usually slender, erect, usually branched, 2°-8° tall. Leaves bright green on both sides or paler beneath, usually smaller, slender-petioled; spikes linear-cylindric, axillary and forming dense terminal panicles, ascending, somewhat spreading or drooping; bracts awned or awn-tipped, twice as long as the 5 oblong acute or cuspidate sepals; stamens 5; utricle scarcely wrinkled, circumscissile.
A weed, in waste grounds, range nearly of the preceding species, its races differing in color, pubescence and length of the awns of the bracts. Naturalized from tropical America. Slender pigweed. Red amaranth or cockscomb. Prince's-feather. Flower-gentle. Careless. Floramor. Aug.-Oct.
Somewhat resembling the two preceding species, stem erect, slender, branched, 2°-3° tall, usually pubescent above. Leaves ovate, rhombic-ovate or the upper lanceolate, blunt at the apex, narrowed at the base, prominently veined, slender-petioled, the lower petioles often longer than the blades; flowers polygamous or dioecious, borne in elongated erect or drooping spikes often 1° long or more, and some of them commonly in small clusters in the upper axils; bracts subulate, spiny-awned, spreading, twice as long as the sepals; sepals 5, spatulate, clawed; utricle dry, circumscissile.