Amaranthus
Sp. Pl. 2: 989. 1753.
Gen. Pl., ed. 5, 427. 1754.
Herbs, usually annual, rarely perennial, monoecious (subg. Amaranthus and Albersia) or dioecious (subg. Acnida), glabrous or pubescent. Stems erect, ascending, decumbent, or prostrate, usually branched, occasionally simple or nearly so; without nodal spines (except in A. spinosus). Leaves alternate, petiolate; blade rhombic-ovate, ovate, obovate, spatulate, lanceolate, oblanceolate, or orbiculate to linear, base rounded to narrowly cuneate, margins usually entire, usually plane, slightly undulate, or crispate, rarely undulate-erose, apex acute, obtuse, or emarginate, usually mucronulate. Inflorescences terminal and/or axillary or exclusively terminal, compound dichasia arranged in spikes, thyrses, panicles, or glomerules; components of terminal inflorescences often subtended by reduced leaves (pseudobracts), each dichasium unit subtended by persistent bracts. Bracts ovate, lanceolate, linear, subulate, deltate, or broadly triangular (in A. acanthochiton), or proximal bracts modified into spines (in A. spinosus); bracts of pistillate flowers not keeled (keeled in A. scleropoides and A. crassipes); bracteoles absent or 1–2. Flowers unisexual. Pistillate flowers: tepals absent or (1–) 3–5, distinct (connate in proximal 1/3 in A. polygonoides), equal or outer tepals larger than inner ones, usually membranaceous, sometimes scarious at maturity; stamens absent [rudimentary]; pistil 1; ovule 1; style 0.1–1 mm, or absent; stigmas 2–3 (–5), slender. Staminate flowers: tepals 3–5, equal or subequal; stamens 3–5, filaments distinct, anthers 4-locular, pseudostaminodes absent; pistils absent or rudimentary. Utricles loosely enclosed by inner tepals, occasionally conspicuously 3 (–5) -veined, usually globose, ovoid, or elongate-ovoid, thin walled, membranaceous, rugose or tuberculate, glabrous, dehiscence regularly circumscissile, irregularly dehiscent, or indehiscent. Seeds 1, subglobose or lenticular, usually smooth, shiny, sometimes indistinctly puncticulate or reticulate; embryo annular. x = 16, 17.
Distribution
Mostly tropical, subtropical, and warm-temperate zones, some species in temperate zones, some taxa are at present almost worldwide as and naturalized weeds
Discussion
Species ca. 70 (38 in the flora, including cultivated species).
Some segregate genera of Amaranthus, in the broad sense, have been proposed and sometimes recognized (see synonymy). In the present treatment, Amaranthus is accepted in its broad sense. Three subgenera are currently recognized (S. L. Mosyakin and K. R. Robertson 1996): subg. Acnida, subg. Amaranthus, and subg. Albersia.
Morphologic terminology in Amaranthus, as used in different floristic and taxonomic treatments, is rather confusing, especially regarding the terms applied to inflorescences and flowers. In the present treatment, we follow the traditional inflorescence terminology only for brevity and convenience; see T. A. Fedorova (1997) for a more complex scheme. A flower is subtended by a bract, often termed a “bracteole,” and 0–2 lateral bracts, the true bracteoles. Structures that are clearly reduced green leaves subtending portions of the inflorescence are sometimes incorrectly called bracts.
Specimens of Amaranthus are often difficult to identify by someone not familiar with the group. When using the key, look closely at the tips of pistillate inflorescence branches for staminate flowers to determine whether the plant is monoecious or dioecious; this is especially important for some monoecious species that produce few staminate flowers. Also, pistillate plants of dioecious species are usually required for positive identification. Descriptions and measurements of floral parts are given in more detail for pistillate flowers, unless noted otherwise.
Determining the exact distribution of some species of Amaranthus in North America requires additional floristic and taxonomic studies. Because of the weedy life strategies of some Amaranthus species, they may occasionally occur as naturalized weeds or waifs very far from their original areas of distribution. Some of such isolated populations exist only as long as conditions are favorable and may eventually disappear or, vice versa, become expansive and invasive. These factors, together with frequent misidentifications in herbaria and the literature, obscure the distribution patterns of some Amaranthus species in North America. Weedy and introduced species of Amaranthus are often neglected or misidentified by collectors. Consequently, some taxa are known only from scattered localities in various regions of the flora, and their actual distribution may be much wider than present data indicate. Some species have been reported for the flora only as rare, casual, non-naturalized aliens, e.g., on ballast, or as grain immigrants or wool contaminants, and may not now be present in North America. Because of all these factors, the maps and distribution statements in the treatment show the generalized distribution and may not properly reflect the actual changing distribution patterns of some species, especially those that have expanded their ranges over the decades due to various anthropic factors. In addition to the taxa discussed below, some other South American or Old World species may be found in North America in the future as introduced weeds.
Species of Amaranthus occasionally form interspecific hybrids. Such hybridization seems to be especially important and widespread in cultivated grain-amaranths, in wild representatives of the A. hybridus aggregate, between species of sect. Amaranthus, and between A. tuberculatus and species of sect. Amaranthus. The degree and scope of hybridization in Amaranthus are often overestimated, especially by European authors, and some taxa described as putative hybrids are in fact nonhybrid infraspecific forms of morphologically variable species. Hybrids between more distantly related species, if they occur at all, are usually highly sterile, such as hybrids between taxa of the subgenera Amaranthus and Acnida, or at least show much decreased fertility. There are no verified records of hybrids between representatives of the subgenera Amaranthus and Albersia.
Some species of Amaranthus are cultivated as pseudocereal and leaf-vegetable crops, or as ornamental or fodder plants (J. D. Sauer 1967; D. M. Brenner 1990; J. T. Williams and D. M. Brenner 1995; S. Cheatham et al. 1995). The most commonly cultivated taxa are A. caudatus Linnaeus, A. hypochondriacus Linnaeus, and A. cruentus Linnaeus of American origin, and south Asian A. tricolor Linnaeus. The cultivated species may occur occasionally as escapes near places of cultivation; they cannot be regarded as truly naturalized.
Species of Amaranthus were widely used by prehistoric and modern Native Americans as food, forage for livestock, medicinal plants, and, occasionally, for some other uses, such as face and body paint, ceremonial items, and fuel (S. Cheatham et al. 1995; D. E. Moerman 1998).
Selected References
Key
| 1 | Stems with paired nodal spines, rarely some forms spineless or with short and weak spines; utricles irregularly dehiscent or indehiscent [3b. subg. Amaranthus] | Amaranthus spinosus |
| 1 | Stems without nodal spines; utricles dehiscent or indehiscent | > 2 |
| 2 | Plants dioecious; inflorescences terminal spikes, thyrses, or panicles [3a. subg. Acnida] | > 3 |
| 2 | Plants monoecious; inflorescences of terminal spikes and panicles or axillary glomerules or clusters | > 21 |
| 3 | Plants in hand with pistillate flowers | > 4 |
| 3 | Plants in hand staminate (pistillate flowers are usually required for positive identification) | > 12 |
| 4 | Bracts deltate or rhombic-deltate, leaflike, margins crenate or denticulate, completely enfolding flower; leaf blades linear or narrowly linear-lanceolate, margins crispate or erose, or irregularly undulate [3a.3. sect. Acanthochiton] | Amaranthus acanthochiton |
| 4 | Bracts ovate to narrowly lanceolate, not leaflike, margins entire, not enfolding flower; leaf blades variable in shape, margins entire to slightly undulate | > 5 |
| 5 | Pistillate flowers with tepals absent or 1-2(-3), usually less than 2 mm; utricles usually indehiscent (dehiscent in A. tuberculatus) [3a.1. sect. Acnida] | > 6 |
| 5 | Pistillate flowers usually with 5 tepals, at least outer tepals longer than 2 mm; utricle dehiscence usually circumscissile (indehiscent in A. greggii) [3a.2 sect. Saueranthus] | > 9 |
| 6 | Utricles 2.5-4 mm; seeds 2-3 mm diam | Amaranthus cannabinus |
| 6 | Utricles 1-2.5 mm; seeds 0.7-1.2 mm diam | > 7 |
| 7 | Utricles with 3(-5) prominent longitudinal ridges, smooth or only slightly rugose, usually stramineous to brown | Amaranthus australis |
| 7 | Utricles with longitudinal ridges faint or absent, usually ± rugose, reddish or brown | > 8 |
| 8 | Leaf blades consistently linear to narrowly oblong, 1 cm or less wide | Amaranthus floridanus |
| 8 | Leaf blades variable, narrowly to broadly ovate, obovate, elliptic, usually more than 1 cm wide | Amaranthus tuberculatus |
| 9 | Bracts longer than tepals, 4-6 mm | > 10 |
| 9 | Bracts shorter than or equaling tepals, 1.5-3(-4) mm | > 11 |
| 10 | Outer tepals acuminate or acute-acuminate at apex | Amaranthus palmeri |
| 10 | Outer tepals obtuse and short-mucronulate at apex | Amaranthus watsonii |
| 11 | Utricles 3 mm, indehiscent; outer tepals acuminate at apex | Amaranthus greggii |
| 11 | Utricles 1.5-2 mm, dehiscent; outer tepals rounded at apex | Amaranthus arenicola |
| 12 | Leaf blades linear or linear-lanceolate, margins crispate or erose [3a.3 sect. Acanthochiton] | Amaranthus acanthochiton |
| 12 | Leaf blades variable in shape, margins entire, or sometimes slightly undulate | > 13 |
| 13 | Outer tepals without prominent midribs (sometimes moderately prominent), not appreciably longer than inner tepals; bracts 2 mm or shorter, midribs mostly not prominent (moderately heavy in floridanus and australis) [sect. Acnida] | > 14 |
| 13 | Outer tepals with prominent midribs, usually longer than inner tepals; bracts longer than 2 mm (1-2 mm in tuberculatus), mostly with prominent midribs | > 17 |
| 14 | Bracts less than 1 mm, midribs scarcely excurrent | Amaranthus cannabinus |
| 14 | Bracts 1 mm or longer, and/or midribs conspicuously excurrent | > 15 |
| 15 | Cauline leaf blades consistently linear or narrowly elliptic, 1 cm or less wide | Amaranthus floridanus |
| 15 | Cauline leaf blades variable, narrowly to broadly ovate, obovate, elliptic, or spatulate, mostly more than 1 cm wide | > 16 |
| 16 | Bracts with moderately prominent midribs; midribs of outer tepals excurrent | Amaranthus australis |
| 16 | Bracts with slender midribs; midribs of outer tepals not excur- rent | Amaranthus tuberculatus |
| 17 | Outer tepals with apex acuminate, midribs excurrent as rigid spines | > 18 |
| 17 | Outer tepals with apex acute or obtuse, apiculate, dark midribs not excurrent [3a.2 sect. Saueranthus] | > 19 |
| 18 | Bracts 2 mm, shorter than outer tepals, apex acuminate to short-subu- late [3a.1 sect. Acnida] | Amaranthus tuberculatus |
| 18 | Bracts 4 mm, equaling or exceeding outer tepals, apex usually long- subulate [3a.2 sect. Saueranthus] | Amaranthus palmeri |
| 19 | Bracts equaling outer tepals, apex long-acuminate or mucronulate | Amaranthus watsonii |
| 19 | Bracts shorter than outer tepals, apex acute | > 20 |
| 20 | Leaves usually thin, soft | Amaranthus arenicola |
| 20 | Leaves usually thick, coarse | Amaranthus greggii |
| 21 | Inflorescences axillary clusters or glomerules, distal nodes sometimes condensed into leafy spikes | > 22 |
| 21 | Inflorescences terminal spikes and/or panicles, leafless or almost leafless at least in the distal part, axillary spikes or clusters usually also present | > 36 |
| 22 | Pistillate flowers usually with only 1 well-developed tepal, sometimes with 1-3 distinctly unequal tepals [3c. subg. Albersia] | Amaranthus californicus |
| 22 | Pistillate flowers with 3-5 equal or subequal tepals, at least 2 tepals well developed | > 23 |
| 23 | Tepals of pistillate flowers fan-shaped, margins fimbriate or denticulate; utricles dehiscent [3b. subg. Amaranthus] | Amaranthus fimbriatus |
| 23 | Tepals of pistillate flowers spatulate or narrowly ovate to oblanceolate, lanceolate to linear, margins entire to minutely erose; utricles indehiscent or dehiscent | > 24 |
| 24 | Pistillate flowers usually with 3 tepals; fruits usually regularly dehiscent (indehiscent in A. blitum) [3c. subg. Albersia] | > 25 |
| 24 | Pistillate flowers usually with (4-)5 tepals; fruits usually indehiscent or tardily dehiscent (regularly dehiscent in A. blitoides, A. scleropoides, A. tamaulipensis, and A. torreyi) | > 29 |
| 25 | Utricles indehiscent; leaf blades usually deeply and broadly emarginate at apex | Amaranthus blitum |
| 25 | Utricles dehiscent; leaf blades obtuse or acuminate to short-mucronate or shallowly emarginate at apex | > 26 |
| 26 | Leaf blades brightly colored, showy; plants cultivated | Amaranthus tricolor |
| 26 | Leaf blades green; plants wild, usually weedy | > 27 |
| 27 | Tepals of pistillate flowers long-aristate apically, usually re- flexed outward; seeds 1-1.4 mm diam | Amaranthus thunbergii |
| 27 | Tepals of pistillate flowers acute to short-acuminate apically, not reflexed; seeds 0.6-1.6 mm diam | > 28 |
| 28 | Bracts shorter to slightly longer than tepals of pistillate flow- ers, subspinescent; seeds 1-1.6 mm diam | Amaranthus graecizans |
| 28 | Bracts 2 times as long as tepals of pistillate flowers, not spinescent; seeds 0.6-1 mm diam | Amaranthus albus |
| 29 | Inflorescence axes thickened, becoming indurate at maturity [3c. subg. Albersia] | > 30 |
| 29 | Inflorescence axes not thickened, not indurate at maturity | > 31 |
| 30 | Utricles indehiscent | Amaranthus crassipes |
| 30 | Utricles with dehiscence regularly circumscissile | Amaranthus scleropoides |
| 31 | Utricles indehiscent or tardily dehiscent | > 32 |
| 31 | Utricles with dehiscence regularly circumscissile | > 34 |
| 32 | Leaf blade margins crisped-erose, conspicuously undulate | Amaranthus crispus |
| 32 | Leaf blade margins entire or erose, plane or slightly undulate | > 33 |
| 33 | Leaf blades ovate, obovate-rhombic, to narrowly ovate, sometimes lanceolate; plants not fleshy | Amaranthus polygonoides |
| 33 | Leaf blades orbiculate, broadly ovate, or obovate; plants fleshy | Amaranthus pumilus |
| 34 | Tepals narrowly ovate to broadly linear; leaf blades usually obovate to elliptic-spatulate [3c. subg. Albersia] | Amaranthus blitoides |
| 34 | Tepals spatulate; leaf blades various in shape [3b. subg. Amaranthus] | > 35 |
| 35 | Leaf blades lanceolate, oblanceolate, or ovate-lanceolate | Amaranthus torreyi |
| 35 | Leaf blades ovate or rhombic-ovate | Amaranthus tamaulipensis |
| 36 | Tepals of pistillate flowers fan-shaped to spatulate, base contracted into claw; terminal spikes unbranched or nearly so, usually interrupted, narrow and slender; leaf blades linear to ovate-lanceolate [3b. subg. Amaranthus] | > 37 |
| 36 | Tepals of pistillate flowers spatulate-obovate, oblanceolate, ovate-elliptic or elliptic to lanceolate-linear, base never contracted into claw; terminal inflorescences variable, usually branched and ± dense; leaf blades usually rhombic-ovate to elliptic (linear to narrowly lanceolate in A. muricatus) | > 39 |
| 37 | Utricles indehiscent | Amaranthus obcordatus |
| 37 | Utricles with dehiscence regularly circumscissile | > 38 |
| 38 | Tepals of pistillate flowers fan-shaped, margins fimbriate or denticulate | Amaranthus fimbriatus |
| 38 | Tepals of pistillate flowers spatulate, margins entire, rarely minutely erose | Amaranthus torreyi |
| 39 | Utricles indehiscent; tepals of pistillate flowers usually 2-3 (5 in A. muricatus); inflorescence bracts shorter than tepals [3c. subg. Albersia] | > 40 |
| 39 | Utricles dehiscent; tepals of pistillate flowers usually 5 (or 3-5 on the same plant in A. powellii); inflorescence bracts exceeding tepals (shorter than tepals in some cultivated species) [3b. subg. Amaranthus] | > 43 |
| 40 | Tepals of pistillate flowers 5; leaf blades linear to narrowly lanceolate | Amaranthus muricatus |
| 40 | Tepals of pistillate flowers usually 2-3; leaf blades ovate, rhombic, or elliptic | > 41 |
| 41 | Utricles distinctly rugose, equaling or slightly exceeding tepals; terminal inflorescences usually thin and interrupted | Amaranthus viridis |
| 41 | Utricles smooth to faintly rugose, occasionally wrinkled or rugose in dry plants, distinctly exceeding tepals; terminal inflorescences usually thick and dense (occasionally thin and interrupted in some forms of A. blitum) | > 42 |
| 42 | Utricles subglobose to obovate, compressed; seeds filling fruit almost completely; leaf blades usually deeply emarginate at apex; plants an- nual | Amaranthus blitum |
| 42 | Utricles ellipsoid, slightly to distinctly inflated; seeds filling only proximal portions of fruit; leaf blades retuse or shallowly emarginate at apex; plants short-lived perennial or annual | Amaranthus deflexus |
| 43 | Fully developed inflorescences large and robust, usually brightly colored, red, purple, deep beet-red, occasionally white or yellowish, rarely green in some forms; bracts usually not exceeding style branches at maturity, occasionally longer than style branches in A. hypochondriacus; seeds white, ivory, reddish, brown, or black; plants cultivated and rarely escaped | > 44 |
| 43 | Inflorescences moderately large, usually green, occasionally silvery green, sometimes with reddish tint; bracts in most species exceeding style branches and tepals, distinctly shorter than tepals in A. dubius, almost equal to tepals in some rare forms of A. retroflexus; seeds brown to black; plants wild, often weedy | > 46 |
| 44 | Inflorescences stiff, erect | Amaranthus hypochondriacus |
| 44 | Inflorescences lax, erect to drooping | > 45 |
| 45 | Tepals of pistillate flowers oblong to lanceolate, apex acute; style branches erect or slightly reflexed | Amaranthus cruentus |
| 45 | At least inner tepals of pistillate flowers spathulate-obovate or lanceolate-obovate, apex obtuse to emarginate; style branches spreading or reflexed | Amaranthus caudatus |
| 46 | Plants densely viscid-pubescent; inflorescences usually unbranched | Amaranthus viscidulus |
| 46 | Plants not viscid (occasionally slightly viscid in some forms of A. retroflexus); inflorescences branched | > 47 |
| 47 | Tepals of pistillate flowers obtuse, rounded, or emarginate at apex | > 48 |
| 47 | Tepals of pistillate flowers acute or acuminate to aristate at apex | > 49 |
| 48 | Plants glabrous or nearly so; tepals of pistillate flowers 1.5-2 mm | Amaranthus wrightii |
| 48 | Plants densely to moderately pubescent; tepals of pistillate flowers (2-)2.5- 3.5(-4) mm | Amaranthus retroflexus |
| 49 | Bracts shorter than 2 mm, shorter than tepals; style branches strongly spread- ing | Amaranthus dubius |
| 49 | Bracts 2-7 mm, longer than or equaling tepals; style branches erect or slightly reflexed | > 50 |
| 50 | Bracts 2-4 mm; inflorescences variable, usually soft and lax, with spread- ing branches | Amaranthus hybridus |
| 50 | Bracts 4-7 mm; inflorescences usually stiff, with erect branches | Amaranthus powellii |