1. Cycloloma atriplicifolium (Spreng.) J.M. Coult. (winged pigweed, tumble ringwing)
Pl. 357 j, k;
Map 1546
Plants annual,
the taproot not tuberous-thickened. Stems 10–80 cm long, erect to loosely
ascending, not succulent, not appearing jointed, much-branched, moderately to
densely pubescent with woolly hairs when young, becoming nearly glabrous at
maturity. Leaves alternate, well developed, not succulent, sessile to
short-petiolate, shed early. Leaf blades 1–8 cm long, lanceolate to oblong or
ovate, flattened in cross-section, not clasping the stem, narrowed to a sharply
pointed tip, tapered at the base, the margins coarsely and irregularly wavy
and/or toothed, the surfaces hairy when young, becoming glabrous at maturity.
Inflorescences terminal, consisting of loose, interrupted spikes usually
appearing as irregular panicles, the flowers solitary at the nodes, not sunken
into the axis. Flowers perfect or pistillate, green, red, or purple. Bract 1, 3–10
mm long, linear to narrowly elliptic, the margins entire or sparsely toothed.
Calyx 5-lobed to about the midpoint, persistent at fruiting, enclosing the
fruit, longitudinally angled or ridged, at fruiting the entire calyx developing
a prominent, continuous, papery, transverse wing with 5 shallow lobes and an
otherwise irregular margin, the lobes 0.4–0.6 mm long, triangular. Stamens 5
(sometimes absent). Ovary superior. Style absent or 1 and very short, the
stigmas (2)3, linear. Fruits 0.5–1.0 mm long, 2–4 mm in diameter (including the
wing), circular to somewhat 5-angled in cross-section, depressed-elliptic in
outline, flattened vertically, indehiscent, the wall thin and papery, glabrous.
Seed adhering loosely to the fruit wall, positioned horizontally, 1.3–1.7 mm in
diameter, nearly circular in outline, strongly flattened, the surface smooth,
black, shiny, the embryo appearing more or less ring-shaped. 2n=36. June–October.
Scattered, mostly
in counties with large rivers, nearly absent from the Ozark Division (western
U.S. east to Indiana, Arkansas, and Texas; Canada, Mexico, introduced eastward
to South Carolina and Massachusetts). Banks of rivers and less commonly
streams, and sand prairies; also fallow fields, railroads, and open, disturbed
areas, in sandy soil.
The unique
equatorial wing in this species, which develops as the fruit matures, makes it
one of the easiest members of the family to recognize. The plants often form
dense, irregularly spherical masses that break off with age and act as
tumbleweeds, dispersing fruits as the wind blows them across the substrate. The
wing of the fruit presumably also aids in dispersal by wind, although it also
helps the fruit to float during floods.