Amaranthus muricatus
Amaranthus tuberculatus
African amaranth, muricate amaranth
common waterhemp, rough-fruit amaranth, rough-fruit water-hemp, tall water-hemp
ascending or prostrate, much-branched from stout rootstock, 0.1–0.4 m.
erect to sometimes ascending or rarely prostrate, branched, rarely simple, usually (0.5–)1–2(–3) m.
petiole to 1/2 as long as blade;
blade linear to narrowly lanceolate, 1.5–8 × 0.2–0.5(–1) mm, base tapering, margins entire, plane to undulate, apex obtuse and often emarginate.
petiole 1/4–1/2 length of blade;
blade ovate or obovate proximally, oblong or elliptic to narrowly lanceolate distally, 1.5–15 × 0.5–3 cm, base cuneate, margins entire, plane, apex obtuse or rounded to acute.
of pistillate flowers linear, 0.7–1.2 mm, 1/2–2/3 as long as tepals.
of pistillate flowers 1–2 mm; of staminate flowers with inconspicuous to prominent midrib, 1–2 mm, apex acuminate to short-subulate.
terminal, compact pyramidal panicles and axillary glomerules, erect or reflexed, green, leafless at least distally.
terminal, linear spikes to panicles, occasionally interrupted-moniliform, remote, globose glomerules.
intermixed with pistillate or at tips of inflorescences;
tepals 5;
stamens 5.
tepals 5, inner tepals with prominent midribs excurrent as rigid spines or not, equal to unequal, 2–3 mm, apex obtuse to acute or acuminate or indistinctly mucronulate;
stamens 5.
tepals 5, narrowly oblanceolate, not clawed, equal, 1.5–2 mm, apex obtuse or subacute;
style branches erect;
stigmas 3.
tepals absent or 1–2, often rudimentary, 1–3 mm;
style branches ± erect;
stigmas 3.
black, lenticular, 1–1.2 mm diam., semiglossy.
dark reddish brown to dark brown, 0.7–1 mm diam., shiny.
compressed, subglobose, 1.7–2 mm, ± equaling or slightly exceeding tepals, muricate, indehiscent.
dark brown to reddish brown, not ribbed, obovoid to subglobose, 1.5–2 mm, thin, almost smooth or irregularly rugose, indehiscent, irregularly dehiscent, or dehiscence regularly circumscissile.
Amaranthus muricatus
Amaranthus tuberculatus
The vernacular name “African amaranth” is sometimes used for this species; it is a misnomer; the species is native to South America and naturalized in Africa.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
J. D. Sauer (1967b, 1972b) separated Amaranthus rudis (A. tamariscinus) as distinct from A. tuberculatus, based primarily on absence of tepals in the pistillate flowers and indehiscent fruits. Recent work by D. B. Pratt and L. G. Clark (2001) showed that those characteristics are not constant and they recognized only one polymorphic species, A. tuberculatus. Long-term observations by K. R. Robertson strongly support the inclusion of A. rudis within A. tuberculatus. Amaranthus rudis probably was originally native to the Great Plains west of the Mississippi, from Texas to Iowa. Amaranthus tuberculatus likely had a more northern range, north of Missouri and Tennessee to the Great Lakes. The emerging evolutionary differentiation between the two related taxa was erased by agriculture and human-induced introduction and invasion. Amaranthus tuberculatus has become a major weed of agricultural fields and other disturbed habitats and is now introduced in parts of North America far outside its original range.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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