Amaranthus spinosus
Amaranthus polygonoides
spiny amaranth, thorny amaranth
smartweed amaranth, tropical amaranth
erect or sometimes ascending proximally, much-branched and bushy, rarely nearly simple, 0.3–1(–2) m; each node with paired, divergent spines (modified bracts) to 1.5(–2.5) cm.
erect-ascending to prostrate, branched mostly at base and in proximal 1/2, 0.1–0.5 m. Leaves: petiole ± equaling blade;
blade ovate, obovate-rhombic to narrowly ovate, sometimes lanceolate, 1.5–3(–4) × 0.5–1.5(–2) cm, base cuneate, margins entire to undulate-erose, apex rounded, obtuse, or emarginate, mucronate.
petiole ± equaling or longer than blade;
blade rhombic-ovate, ovate, or ovate-lanceolate, 3–10(–15) × 1.5–6 cm, base broadly cuneate, margins entire, plane or slightly undulate, apex acute or subobtuse to indistinctly emarginate, mucronulate.
of pistillate flowers lanceolate to ovate-lanceolate, shorter than tepals, apex attenuate.
of pistillate flowers lanceolate or linear, 1–1.5 mm, 1/2 as long as tepals.
simple or compound terminal staminate spikes and axillary subglobose mostly pistillate clusters, erect or with reflexed or nodding tips, usually green to silvery green.
axillary, congested clusters.
often terminal or in proximal glomerules;
tepals 5, equal or subequal, 1.7–2.5 mm;
stamens 5.
intermixed with pistillate;
tepals (4–)5;
stamens 2–3.
tepals 5, obovate-lanceolate or spatulate-lanceolate, equal or subequal, 1.2–2 mm, apex mucronate or short-aristate;
styles erect or spreading;
stigmas 3.
tepals 5, connate in proximal 1/3 (entirely distinct in all other species), with 3 prominent veins abaxially, spatulate or somewhat clawed, equal or subequal, 2–3 mm, apex rounded or retuse, mucronate;
style branches somewhat spreading;
stigmas 3.
black, lenticular or subglobose-lenticular, 0.7–1 mm diam., smooth, shiny.
dark reddish brown to black, lenticular, 0.8–1 mm diam., shiny.
ovoid to subglobose, 1.5–2.5 mm, membranaceous proximally, wrinkled and spongy or inflated distally, irregularly dehiscent or indehiscent.
cylindric or narrowly turbinate, 2–2.5 mm, ± equaling tepals, smooth proximally or roughened toward tips, indehiscent or tardily dehiscent.
Amaranthus spinosus
Amaranthus polygonoides
Amaranthus spinosus is native to lowlands in tropical America; at present it is a pantropical weed that also occurs in some warm-temperate regions.
Amaranthus spinosus, or its ancestral taxon, probably gave rise to the allopolyploid A. dubius by hybridization with some species of the A. hybridus aggregate (see above). Section Centrusa probably occupies a basal position, at least for the clade of subg. Amaranthus sect. Amaranthus, and probably for some representatives of subg. Acnida as currently outlined. Recent results of sequencing the ITS region (including ITS-1, 5.8S rDNA, and ITS-2) of nuclear ribosomal DNA from 15 species of Amaranthus occurring in China also suggest the basal position of A. spinosus among the studied species (Song B. H. et al. 2000). These results also confirm a profound divergence between subgenera Amaranthus and Albersia; the latter is called “sect. Paucestamen by the above authors. Data on the electrophoretic variation of seed proteins (R. H. Sammour et al. 1993) are also in accord with the segregation of these two subgenera; in the cited article, these groups are called sect. Amaranthus and sect. Blitopsis.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Amaranthus berlandieri often has been recognized as a separate species related to A. polygonoides. J. Henrickson (1999) clarified the confusion that existed in earlier descriptions of these two taxa and showed that the main characters used for their separation (dehiscent versus indehiscent utricles, leaf shape, etc.) are inconsistent and cannot be applied for segregation of two independent species. The subspecies rank may be more appropriate for A. berlandieri, as was suggested by A. Thellung (1914–1919). The relationships between these taxa of the A. polygoniodes aggregate require additional study; in the present treatment we follow the solution proposed by Henrickson.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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