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Published In: Species Plantarum 2: 989. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

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1. Amaranthus L. (pigweed, amaranth)

Plants annual, monoecious or dioecious (rarely a few perfect flowers present in monoecious species). Stems erect to spreading, glabrous or variously pubescent, green to yellowish white or reddish purple, often with pink to purple longitudinal lines or ridges. Leaves alternate, short- to long-petiolate. Leaf blades 0.8–20.0 cm long, variously shaped, usually pubescent with short, curved hairs when young, usually becoming nearly glabrous (except often along the margins) at maturity. Inflorescences axillary and/or terminal, dense spikes, spikelike racemes, panicles (often appearing as spicate branches arranged racemosely along a central spicate axis), or axillary clusters. Flowers imperfect. Sepals 1–5 or sometimes absent in pistillate flowers, free, more or less similar or differentiated into inner and outer series, often with an awnlike extension, this sometimes becoming somewhat hardened and spinelike at maturity, herbaceous to hardened or leathery, green, at least centrally, but often with thin, papery margins. Staminate flowers with the stamens 3–5, the filaments free. Pistillate flowers with the ovary ovoid. Ovule 1. Style absent or very short, the 2 or 3 stigmas tapered and slender, persistent. Fruits mostly with papery walls, ovoid or ellipsoid, with 2 or 3 small beaks, indehiscent or more commonly with irregular or circumscissile dehiscence, 1-seeded. Seeds often somewhat flattened, circular or nearly so in outline, rounded or angled along the rim, the surface smooth (sometimes somewhat roughened in A. blitum and A. viridis), shiny. Sixty to 70 species, nearly worldwide.

The genus Amaranthus has great economic importance. Several of the Missouri species (the weedy amaranths) are serious weeds in crop fields. All of the pigweeds produce copious quantities of potentially allergenic, wind-dispersed pollen that are major causes of hay fever during some times of the year. On the plus side, however, certain species have a long history of cultivation for food. Three species, A. caudatus, A. cruentus, and A. hypochondriacus, have histories of cultivation in Latin America as pseudo–grain crops that stretch back to long before European colonization of the Americas, and these cereal amaranths also subsequently became cultivated in parts of Europe and Asia (Sauer, 1967; Robertson, 1981; Costea et al., 2001 a, b). Red-pigmented forms of these species and others, principally A. hybridus, also have been cultivated as garden ornamentals, and in Latin America they are used to produce a red dye for religious ceremonies (Robertson, 1981; Heiser, 1985). Yet other species, such as A. tricolor, have been used widely for food, fresh as greens and boiled as potherbs, and also as a green supplement to some livestock feeds. That these uses persist today is evidenced by the fact that all of the species mentioned have been recorded outside of cultivation in Missouri. In fact, grain amaranths are available to consumers in many grocery and health food stores, the seeds dried and ground into flour, popped for use in baked goods and cereals, and powdered as a component of health food beverages (Robertson, 1981). They are quite nutritious, a good source of protein, and contain high levels of lysine. Wild food enthusiasts are cautioned, however, that amaranths growing in the wild occasionally can absorb and accumulate in their foliage toxic substances present in the soil.

The species of Amaranthus considered native in Missouri and the central portion of the United States occur in a variety of habitats with moist, bare soil. The natural limits of the distributional ranges of these species are not well understood, as they have been spread in recent times by agriculture and other human activities as well as by natural means. These plants are primary colonizers of both habitats prone to frequent natural disturbance, especially in the floodplains of major rivers and streams, and also anthropogenically altered sites, like crop fields and railroad embankments. Distributional limits of these disturbophiles had already expanded both westward across the Great Plains and eastward to the Atlantic seaboard as a result of European colonization by the time that botanists began paying attention to the biogeography of such species (Sauer, 1957, 1967, 1972).

The weedy amaranths frequently grow in mixed populations, and a number of hybrids have been reported. These hybrids are sterile, producing inflorescences with many bracts but with the flowers lacking or vestigial, or in some cases with reduced flowers having shrunken, misshapen, or absent fruits, so detecting such plants is not difficult. However, evaluating the presumed parentage of such sterile individuals with confidence is often not possible. Carl Sauer, a specialist on the genus, determined one or a few specimens as representing each of the following hybrid combinations in Missouri: A. hybridus × A. palmeri, A. hybridus × A. retroflexus, A. hybridus × A. rudis, A. hybridus × A. tuberculatus, and A. retroflexus × A. rudis (note, however that A. rudis is reduced to synonymy under A. tuberculatus in the present treatment). Other combinations are to be expected where two or more species grow together, but apparently they are rare, even in large mixed populations.

Identification of Amaranthus species is difficult, especially when only staminate flowers are present, not only because of the presence of occasional hybrids but also because determination depends on observation of details of the small, morphologically similar flowers and fruits. Within an inflorescence, the numerous flowers are at various stages of development, and fruits often continue to mature in the plant press during drying. Taxonomists traditionally have divided the genus into two main subgroups, the dioecious subgenus Acnida (L.) Aellen ex K.R. Robertson and the monoecious subgenus Amaranthus. Recently, a few authors (Costea et al., 2001a) have segregated subgenus Albersia (Kunth) Gren. & Godr. from subgenus Amaranthus, based on its mostly axillary clusters of flowers (vs. mostly terminal spikes or panicles), indehiscent (vs. circumscissile) fruits, and minute differences in leaf and seed anatomy, but most botanists still treat this group as section Blitopsis Dumort. of subgenus Amaranthus. In spite of the instinctive appeal of these groupings, the validity of maintaining such infrageneric taxa is drawn into question by the existence of various hybrids between them.

 

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1 1. Stems with conspicuous paired spines at most nodes ... 12. A. SPINOSUS

Amaranthus spinosus
2 1. Stems unarmed (but bracts and sepals sometimes becoming spinelike at maturity)

3 2. Plants dioecious

4 3. Flowers pistillate

5 4. Sepals absent or irregularly 1 or 2, linear to lanceolate; stigmas 3 or 4 ... 14. A. TUBERCULATUS

Amaranthus tuberculatus
6 4. Sepals 5, at least the inner ones oblanceolate to spatulate; stigmas 2(3)

7 5. Sepals rounded or bluntly pointed at the tip, the midrib not or only slightly extending beyond the main body as a minute, sharp point; outer sepals 2.02.5 mm long; inner sepals only slightly shorter than the outer sepals and bracts ... 2. A. ARENICOLA

Amaranthus arenicola
8 5. At least the outer sepals sharply pointed at the tip, the midrib extending beyond the main body as a short awn, usually spinelike at maturity; outer sepals 34 mm long; inner sepals conspicuously shorter than the outer sepals and bracts ... 9. A. PALMERI

Amaranthus palmeri
9 3. Flowers staminate

10 6. Bracts 46 mm long, slightly to more commonly conspicuously longer than the sepals ... 9. A. PALMERI

Amaranthus palmeri
11 6. Bracts 1.52.5 mm long, shorter than to about as long as the sepals

12 7. Bracts with the midrib not or only slightly extending beyond the main body as a minute, sharp point ... 2. A. ARENICOLA

Amaranthus arenicola
13 7. Bracts with the midrib extending beyond the main body as a short awn, usually somewhat spinelike at maturity ... 14. A. TUBERCULATUS

Amaranthus tuberculatus
14 2. Plants monoecious (the staminate and pistillate flowers either on different parts of the plant or intermingled in each inflorescence)

15 8. Main inflorescences mostly small, axillary clusters (short, terminal spikes occasionally also present)

16 9. Stems spreading to less commonly loosely ascending, often forming dense mats; bracts of the pistillate flowers about as long as the 4 or 5 sepals ... 3. A. BLITOIDES

Amaranthus blitoides
17 9. Stems loosely to strongly ascending or erect, often forming dense, irregularly globose masses; bracts of the pistillate flowers longer than the 3 sepals

18 10. Stems loosely to strongly ascending, often forming dense, irregularly globose masses; leaf blades 0.54.0(8.0) cm long, elliptic to obovate, rounded or shallowly and minutely notched at the tip; bracts 2.02.6 mm long; sepals 0.82.3 mm long ... 1. A. ALBUS

Amaranthus albus
19 10. Stems erect or strongly ascending, not forming globose masses; leaf blades 315 cm long, ovate to broadly triangular-ovate, narrowed or tapered to a bluntly or sharply pointed tip (often minutely notched at the very tip); bracts 36 mm long; sepals 24 mm long ... 13. A. TRICOLOR

Amaranthus tricolor
20 8. Main inflorescences elongate terminal spikes, these often grouped into panicles (additional axillary clusters, spikes, and/or panicles also often present)

21 11. Sepals 2 or 3

22 12. Leaf blades truncate to noticeably notched at the tip (the midvein sometimes extending as a short, sharp point) ... 4. A. BLITUM

Amaranthus blitum subsp. emarginatus
23 12. Leaf blades narrowed to tapered to a bluntly or sharply pointed tip (sometimes minutely notched at the very tip)

24 13. Fruits 1.41.7 mm long, indehiscent; seeds angled along the rim; sepals 0.91.2 mm long, shorter than the fruit, oblanceolate to oblong-oblanceolate; bracts 0.60.9 mm long, shorter than the sepals and fruit; axillary inflorescences elongate spikes ... 15. A. VIRIDIS

Amaranthus viridis
25 13. Fruits 1.82.5 mm long, with circumscissile dehiscence; seeds rounded along the rim; sepals 24 mm long, as long as or longer than the fruit, lanceolate to linear-lanceolate or ovate to elliptic-ovate; bracts 3.07.5 mm long, longer than the sepals and fruit; axillary inflorescences mostly more or less globose clusters

26 14. Bracts and sepals lanceolate to narrowly oblong-lanceolate or linear-lanceolate, the thin, papery margins only slightly broader than the strongly thickened green midrib ... 10. A. POWELLII

Amaranthus powellii
27 14. Bracts and sepals ovate to elliptic-ovate, with thin, papery margins much broader than the slightly thickened green midrib ... 13. A. TRICOLOR

Amaranthus tricolor
28 11. Sepals (4)5

29 15. Pistillate flowers with the main body of the bracts (excluding the awnlike or spinelike extension of the midrib) conspicuously longer than the fruits; inflorescence usually stiffly erect at the tip

30 16. Pistillate flowers with the sepals narrowed or tapered to a sharply pointed, stiff, erect tip, often tapered to a short, sharp extension of the midrib; staminate flowers usually with 3 stamens; terminal inflorescence a solitary spike or panicle of few long spikes (these branching from near the panicle base) ... 10. A. POWELLII

Amaranthus powellii
31 16. Pistillate flowers with the sepals rounded, truncate or shallowly notched at the soft, outward-curved tip, sometimes with an abrupt, minute, sharp extension of the midrib; staminate flowers with (4)5 stamens; terminal inflorescence usually a panicle with numerous clusters of short, dense spikes (these branching along most of the panicle axis) ... 11. A. RETROFLEXUS

Amaranthus retroflexus
32 15. Pistillate flowers with the main body of the bracts (excluding the awnlike or spinelike extension of the midrib) shorter than to slightly longer than the fruits; inflorescences stiffly erect or nodding or drooping at the tip

33 17. Pistillate flowers with the bracts (including the awnlike or spinelike extension of the midrib) conspicuously longer than the fruits; sepals sharply pointed at the tip; inflorescences dull green, occasionally dull reddish-tinged ... 7. A. HYBRIDUS

Amaranthus hybridus
34 17. Pistillate flowers with the bracts (including the short, awnlike extension of the midrib, if present) shorter than to about as long as the fruits; sepals of at least the pistillate flowers rounded to bluntly pointed at the tip; inflorescences bright green, yellow, or red

35 18. Main inflorescence with the tip stiffly straight, erect or nearly so ... 8. A. HYPOCHONDRIACUS

Amaranthus hypochondriacus
36 18. Main inflorescence with the tip nodding or drooping

37 19. Sepals of the pistillate flowers 1.52.0 mm long, obovate to spatulate, noticeably overlapping, at least the outer ones curved outward at the tip; stigmas spreading ... 5. A. CAUDATUS

Amaranthus caudatus
38 19. Sepals of the pistillate flowers 1.01.6 mm long, narrowly oblong to narrowly oblong-elliptic, slightly and inconspicuously overlapping, erect at the tip; stigmas erect or nearly so ... 6. A. CRUENTUS Amaranthus cruentus
 
 
 
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