Rotala rotundifolia

Description

Rotala rotundifolia (Buch.-Ham. ex Roxb.) Koehne is a small amphibious herb of the Lythraceae with a wide Asian distribution and an extensive secondary range. GBIF, with 2,686 occurrences, records it natively across China, Taiwan, India, Nepal, Bhutan, Bangladesh, Myanmar, Thailand, Laos, Vietnam and Japan, and as an introduction in the USA, Australia, Brazil and elsewhere, partly through the aquarium trade. The species inhabits wet paddy margins, ditches, marshes, riverbanks and shallow standing water from the lowlands to around 1,500 m. The illustrated material from the Mewa-Khola in eastern Nepal at 1,195 m, in April, is typical of its subtropical wetland habitat.

The plant is a creeping to ascending annual or short-lived perennial, often rooting at the lower nodes and producing erect flowering shoots. Its stems are slender and four-angled, and the leaves are opposite, sessile, orbicular to ovate (as the epithet indicates) on submerged or basal parts but narrower on emergent shoots, a marked heterophylly. The flowers are minute and sessile, crowded into dense, spike-like terminal racemes; each has a four-lobed campanulate calyx, four small pink to rose-purple petals, and few stamens. The fruit is a small septicidal capsule with a finely striate surface containing numerous tiny seeds.

Rotala rotundifolia is taxonomically variable, with submerged and emergent forms differing strikingly in leaf shape and colour, a plasticity that has made it a popular ornamental aquatic. This same adaptability allows it to behave as an aggressive invasive in introduced regions, where it can clog drainage and displace native wetland flora. In its native Himalayan and Asian range it is an abundant and ecologically unremarkable component of seasonally inundated habitats.

Author: (Buch.-Ham. ex Roxb.) Koehne · Sources: POWO (Kew), GBIF, Flora of China Vol. 13, Flora of Nepal, Flora of British India Vol. 2, Cook: Aquatic and Wetland Plants of India