Description from Flora of China
Nama zeylanica Linnaeus, Sp. Pl. 1: 226. 1753; Beloanthera oppositifolia Hasskarl; Hydrolea arayatensis Blanco; H. inermis Loureiro; H. javanica Blume; H. zeylanica (Linnaeus) Vahl var. ciliata Choisy; Steris aquatica N. L. Burman; S. javanica Linnaeus.
Herbs, erect to prostrate, sometimes rooting from lower nodes, 10-60(-100) cm tall, often branched, glabrous or pubescent, unarmed. Leaf blade lanceolate to ovate, 2-10 X 0.5-2.5 cm, glabrous or pubescent, base acute, margin entire, apex acute. Flowers in terminal panicles or clusters at branch apices, rarely solitary. Pedicel 1-3 mm, elongating after anthesis. Calyx lobes lanceolate, 4.5-8 mm, pubescent or glabrous. Corolla purple-blue or deep purple-green, 3-5 mm; tube white. Anthers scarlet. Capsule ovoid, septicidal or irregularly dehiscent, included in persistent calyx. Seeds oblong-ovoid, 0.3-0.4 mm. 2n = 18, 24.
On an herbarium sheet at BM, G. T. Sampson commented that this species occurs sporadically and postulated that it was introduced as a seed contaminant of rice.
Paddies, pond margins, streamsides, open forests, swampy or inundated soil; 0-1000 m. Fujian, Guangdong, Guangxi, Hainan, Taiwan, S Yunnan [India, Indonesia, Malaysia, Nepal, Philippines, Sri Lanka; Africa, SW Asia, Australia]