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Amaranthus

See list of 18 species in this genus

Determining the type of flowers (i.e., staminate or carpellate) on a plant or branch is very important and requires careful study because some species may show very few staminate flowers among the carpellate ones. Carpellate plants are sometimes necessary for identification of dioecious species. Hybridization has been documented in various parts 
of the world within Amaranthus, with A. hybridus, A. retroflexus, and A. tuberculatus most frequently involved in cross-species mating. References: Costea et al. (2001a, 2001b), Mosyakin and Robertson (2003).

  • 1a. Plants bearing 2 rigid, sharp-pointed spines (2–) 5–11 (–25) mm long at most nodes; terminal spike with staminate flowers in apical portion and carpellate flowers near the base
  • 1b. Plants without nodal spines; terminal spikes either entirely unisexual or with scattered flowers of each sex
    • 2a. Plants dioecious; carpellate flowers lacking sepals or with 1 or 2 vestigial ones 
(or with 5 sepals in A. palmeri); fruit an indehiscent utricle or an irregularly rupturing 
utricle (or a pyxis in A. palmeri)
      • 3a. Carpellate flowers with 5 spatulate sepals 2–4 mm long and often only 2 style branches; bracteoles of staminate flowers (2.5–) 3–4 mm long; fruit circumscissily dehiscent
      • 3b. Carpellate flowers lacking sepals or with 1 or 2 vestigial ones shorter than 1 mm and 3 style branches; bracteoles of staminate flowers 0.6–1.5 mm long; fruit indehiscent or irregularly dehiscent (dehiscent in rare forms of A. tuberculatus)
        • 4a. Utricle 2.2–3.5 (–4) mm long, fleshy, not dehiscent; seeds 1.9–3 mm long; bracteoles of staminate flowers 0.6–1 (–1.3) mm long, the midrib weakly excurrent; leaf blades linear to lanceolate; plants primarily of saline and brackish wetlands 
and shores
        • 4b. Utricle 1.5–2 mm long, membranaceous, sometimes dehiscent, though irregularly in most forms; seeds 0.8–1 mm long; bracteoles of staminate flowers 1–1.5 mm 
long, the midrib evidently excurrent; leaf blades lanceolate to ovate [Fig. 321]; plants of fresh water wetlands and shores, gardens, dumps, and open rights-of-way
    • 2b. Plants monoecious; flowers with a calyx of 3–5 sepals (rarely only 2), the carpellate sepals persistent in fruit; fruit a pyxis or less commonly indehiscent
      • 5a. Inflorescence composed entirely of small, axillary glomerules 2–11 mm in diameter [Fig. 316]; leaf blades 0.5–4.2 (–7) cm long
        • 6a. Fruit warty-rugose, evidently exceeding the sepals
        • 6b. Fruit neither warty nor rugose, not or only somewhat exceeding the sepals
          • 7a. Carpellate sepals 2.5–3 (–4) mm long; fruit indehiscent, fleshy, 2.5–4 (–5) mm long; seed oval in outline, 1.7–2.2 (–2.5) mm long; leaf blades fleshy, obovate to orbicular [Fig. 319]
          • 7b. Carpellate sepals 0.8–2.7 mm long; fruit circumscissilely dehiscent (indehiscent in A. blitum), dry, 0.9–2.2 (–2.5) mm long; seeds orbicular in outline, 0.7–1.6 (–1.8) mm long; leaf blades herbaceous, elliptic or oblong to obovate, rhombic-ovate, or spatulate [Fig. 316]
            • 8a. Leaf blades shallowly to deeply retuse at the apex [Fig. 316]; fruit indehiscent
            • 8b. Leaf blades rounded to obtuse at the apex (rarely with an inconspicuous apical notch); fruit circumscissilely dehiscent
              • 9a. Bracteoles of carpellate flowers 1–2.5 mm long, subequal in length with the sepals; carpellate flowers with 4 or 5 sepals 1.2–2.7 mm long; seeds 1.3–1.6 (–1.8) mm long; plants prostrate
              • 9b. Bracteoles of carpellate flowers 2–2.6 mm long, mostly 2–3 times as long as the sepals; carpellate flowers with 3 sepals 0.8–1.2 mm long; seeds 0.7–1 (–1.2) mm long; plants upright, bushy branched
      • 5b. Inflorescence composed of a terminal spike or panicle 20–300 mm tall, as well as often axillary spikes or glomerules [Figs. 317,320]; leaf blades 1–30 cm long, the larger usually longer than 4 cm (shorter in A. deflexus)
        • 10a. Carpellate flowers with 2 or 3 sepals; fruit indehiscent (i.e., a utricle)
          • 11a. Fruits rugose when dry; terminal panicle mostly 10–20 cm tall; stems erect, 2–10 dm tall; fruit 1–1.6 mm long; seed orbicular in outline
          • 11b. Fruits smooth to slightly wrinkled; terminal panicle mostly 2–5 cm tall; stems prostrate to erect, 1–4 (–5) dm tall; fruit 2.3–2.6 mm long; seed obovate in outline
        • 10b. Carpellate flowers with (3–) 5 sepals; fruit circumscissilely dehiscent (i.e., a pyxis)
          • 12a. Seeds dark brown to black; bracteoles exceeding the stigma branches in intact infructescences (except in A. dubius); inflorescence not showy, dull green (sometimes tinged with red in A. hybridus)
            • 13a. Bracteoles shorter than 2 mm; styles spreading; seeds 0.8–1 mm long
            • 13b. Bracteoles 2.5–6 (–8) mm long; styles erect to ascending; seeds 
1–1.4 mm long
              • 14a. Carpellate sepals rounded to truncate to emarginate at the apex, (2–) 2.5–3.5 (–4) mm long, outwardly curved; terminal inflorescence usually with many short, crowded, thick branches [Fig. 320]
              • 14b. Carpellate sepals acute to subacute at the apex, 1.2–3 mm long, 
erect; terminal inflorescence unbranched or with a few, widely spaced, long branches or with many, crowded, slender branches [Fig. 317]
                • 15a. Bracteoles 4.5–6 (–8) mm long, 2–3 (–4) times as long as the 
sepals; sepals numbering 3–5, with yellow midveins, evidently unequal, 
1 sepal 2.2–3 mm long, the others 1.2–1.6 mm long; apex of fruit gradually narrowed; inflorescence with few, erect, rigid, widely spaced branches
                • 15b. Bracteoles 2.5–4 (–6) mm long, 1.2–2 times as long as the sepals; sepals numbering 5, with green midveins, subequal or 1 longer, 
1.5–2.5 (–3) mm long; fruit abruptly narrowed to apex; inflorescence with few to many, slender, lax or flexuous branches [Fig. 317]
          • 12b. Seeds white-yellow to red-brown (often dark brown in ornamental forms); bracteoles equaling or exceeded by stigma branches in intact infructescences; inflorescence often large and showy, usually red to purple (less frequently yellow or white)
            • 16a. Terminal inflorescence with stiff, thick branches 9–14 mm wide; bracteoles subequal in length to the stigma branches in intact infructescences [Fig. 318]; stigma branches thick at the base; fruit ± truncate at apex
            • 16b. Terminal inflorescence with lax, narrower branches 4–10 (–12) mm wide; bracteoles shorter than the stigma branches in intact infructescences; stigma branches slender at the base; fruit gradually or abruptly tapering to apex
              • 17a. Sepals narrow-oblong, acute at the apex, not overlapping with one another; stigma branches erect from the base; bracteoles 2–3 mm long, equal to or slightly longer than the sepals
              • 17b. Sepals obovate to spatulate, obtuse to rounded at the apex, overlapping with one another; stigma branches spreading from the base; bracteoles 3–4 mm long, 1.2–1.5 times as long as the sepals

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