9. ROTALA Linnaeus, Mant. Pl. 2: 143, 175. 1771.
Toothcup [Latin rota, wheel, alluding to whorled leaves of R. verticillaris Linnaeus]
Shirley A. Graham
Herbs, annual or perennial, terrestrial, amphibious, or aquatic, often turning pink or red, 0.4–4 dm, glabrous throughout. Stems erect, decumbent, or creeping [floating], unbranched or branched, often rooting at nodes. Leaves usually opposite, rarely whorled [subalternate], dimorphic or uniform; sessile or subsessile; blade obovate, orbiculate, oblong-elliptic, oblanceolate, subspatulate, or linear [lanceolate, spatulate], base attenuate, obtuse, or rounded, margins membranous or cartilaginous. Inflorescences indeterminate, terminal or axillary racemes, axillary spikes, or solitary flowers (on main stem). Flowers sessile or subsessile, actinomorphic, monostylous [distylous]; floral tube perigynous, campanulate; epicalyx segments shorter to longer than sepals or absent; sepals [3 or] 4[–6], to 1/2 floral tube length; petals caducous or persistent, 0 or 4[–6], rose, pink, or white [purple]; nectariferous tissue variably present at base of inner wall of floral tube; stamens (1–)4[–6], in 1 whorl; ovary 2–4-locular; placenta globose, ultimately nearly free-central; style slender or stout; stigma capitate or thick-capitate. Fruits capsules, walls thin and dry, finely, transversely striate (10\×), 2–4-valved, dehiscence septicidal. Seeds 30–100+, obovoid-semiovoid, convex-concave, 0.3–0.5 mm; cotyledons ± complanate. x = 8.
Species ca. 50 (3 in the flora): North America, Mexico, West Indies, Central America, South America, Europe, s Asia, Africa.
The native Rotala ramosior and introduced R. indica and R. rotundifolia are weeds in North American rice fields, irrigation ditches and canals, and other disturbed wet areas. Rotala ramosior is increasingly found elsewhere in the world in rice-growing regions. The genus often grows with one or more species of the superficially similar Ammannia (S. A. Graham et al. 2011). Some species of Rotala are part of the international aquatic plant trade.
SELECTED REFERENCE Cook, C. D. K. 1979. A revision of the genus Rotala (Lythraceae). Boissiera 29: 1–156.