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Amaranthus Linn., Sp. Pl. ed. 1: 989. 1753. Gen. P1. ed. 5: 427. 1754; Boiss., Fl. Or. 4:988. 1879; Hook. f., Fl. Brit. Ind. 4: 718. 1885; Schinz in E. and P., Pflanzenfam. ed. 2, 16c: 35. 1934.
Annual or more rarely perennial herbs, glabrous or furnished with short and gland-like or multicellular hairs. Leaves alternate, long-petiolate, simple and entire or sinuate. Inflorescence basically cymose, bracteate, consisting entirely of dense to lax axillary cymose clusters or the upper clusters leafless and ± approximate to form a lax or dense “spike” or panicle. Flowers unisexual, dioecious or monoecious, bibracteolate; perianth segments (2-) 3-5, free or connate at the base, membranous, those of the female flowers slightly accrescent in fruit. Stamens free, usually similar in number to the perianth segments, with no alternating pseudostaminodes. Stigmas 2-3. Ovule solitary, erect. Fruit a dry capsule, indehiscent, irregularly rupturing or commonly dehiscing by a circumscissile lid. Seeds usually black and shining, testa thin; embryo annular, endosperm present.
Approximately 50 species, chiefly in the warmer regions of the world, about a dozen species being ± cosmopolitan weeds in such areas. Amaranthus is a genus of considerable difficulty to the taxonomist, especially the “grain amaranths”, which have long been cultivated. Apart from those used for grain, the foliage of several species is eaten in various regions as a kind of spinach.
Type species: Amaranthus caudatus L.
Although the species concept employed therein is not adopted in the present account, the interesting and important account of the grain amaranths by Sauer [Ann. Mo. bot. Gdn. 54: 95-137 (1967] should be consulted. It contains much of interest concerning these species-viz. Amaranthus caudatus and Amaranthus hybridus (incl. Amaranthus hypochondriacus and Amaranthus cruentus, treated by Sauer as full species)-and their past and present cultivation, including within our own region and adjacent territories. The figures of tepals, ovary, style-bases etc. given in Aellen’s account of the genus in the second edition of Hegi’s “Illustrierte Flora von Mitteleuropa” (Bd. 3/2, Lf. 1: 465-516) are most useful for identification, and should be referred to in the absence of reliably determined material for comparison.
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| 1 |
Leaf axils with paired spines |
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Amaranthus spinosus |
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Leaf axils without paired spines |
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(2) |
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| 2 (1) |
Inflorescence consisting entirely of axillary, cymose
clusters, no terminal leafless spike present |
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(3) |
| + |
Inflorescence with a terminal leafless spike, or a panicle whose apical part is spike-like |
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(5) |
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| 3 (2) |
Capsule hyaline and transparent, with 4-5 prominent
green longitudinal ribs; perianth segments 2 |
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Amaranthus tenuifolius |
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Capsule neither hyaline and transparent nor longitudinally ribbed; perianth segments 3 |
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(4) |
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| 4 (3) |
Perianth segments (without the arista) longer than the constantly circumscissile capsule |
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Amaranthus tricolor |
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Perianth segments (without the arista) shorter than the circumscissile or irregularly rupturing capsule |
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Amaranthus graecizans |
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| 5 (2) |
Terminal spike of inflorescence usually much elongated and pendulous; females perianth segments broadly obovate to broadly spathulate, distinctly imbricate. |
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Amaranthus caudatus |
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Terminal spike of inflorescence rarely elongate and pendulous; female perianth segments acute, or if spathulate then not distinctly imbricate |
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(6) |
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| 6 (5) |
Capsule indehiscent, irregularly rupturing |
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(7) |
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Capsule circumscissile |
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(9) |
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| 7 (6) |
Capsule scarcely exceeding the female perianth, strongly muricate, ± globose; seeds with shallow, scurfy verrucae on the reticulate pattern of the testa |
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Amaranthus viridis |
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Capsule distinctly exceeding the female perianth; seeds without shallow verrucae |
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(8) |
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| 8 (7) |
Fruit ellipsoid, scarcely compressed, seed ellipsoid; plant perennial. leaves rarely (and then shallowly) retuse |
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Amaranthus deflexus |
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Fruit compressed, rotund to very shortly pyriform;
seed round and compressed plant annual, leaves broadly and conspicuously emarginate, rarely broadly
truncate |
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Amaranthus lividus |
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| 9 (6) |
Female perianth segments spathulate to narrowly
oblong-spathulate, obtuse or emarginate, longer than
the fruit |
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Amaranthus retroflexus |
| + |
Female perianth segments lanceolate to ellipticoblong
or oblong. usually acute, if occasionally blunt then shorter than the fruit |
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(10) |
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| 10 (9) |
Terminal inflorescence alone usually spiciform, the axillary inflorescences ± rounded clusters-or if axillary spikes present, these with globose clusters at junction with stem; bracteoles terminating in awns which are fine, flexuose and hair-like above; perianth segments 3 |
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Amaranthus tricolor |
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Terminal and at least some (generally most) of the upper axillary inflorescences spiciform, without dense globose clusters at the junction of peduncle and stem; bracteoles terminating in stout, rigid awns; perianth segments usually 5 |
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Amaranthus hybridus |
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List of lower taxa
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