Description from Flora of China
Gratiola tenuifolia Colsmann, Prodr. Descr. Gratiol. 8. 1793; Bonnaya tenuifolia (Colsmann) Sprengel; Ilysanthes tenuifolia (Colsmann) Urban; Vandellia tenuifolia (Colsmann) Haines.
Annuals, to 15 cm tall. Roots fibrous, fascicled. Stems erect or subprostrate and ascending, much branched, striate, glabrous, rarely rooting from 2nd or 3rd nodes. Leaves sessile, linear, 1-2.8 cm X ca. 2 mm, glabrous, base somewhat amplexicaul, margin obscurely short serrate to subentire; midrib conspicuous. Flowers few, opposite to leaves; bracts linear, 1.5-3.5 mm. Pedicel 5-10 mm, glabrous, reflexed in fruit. Calyx basally connate; lobes linear-lanceolate, ca. 2 mm, glabrous, apex obtuse, midrib conspicuous. Corolla purple-red; lower lip 3-lobed, middle lobe larger than other lobes and slightly longer than upper lip; upper lip reduced, obscurely 2-lobed. Fertile stamens 2, posterior. Staminodes 2, anterior. Capsule cylindric, to 1.5 cm, 2-3 X as long as persistent calyx, apex acuminate.
Beside water, trailsides; low elevations. Guangdong, Guangxi, Taiwan [Cambodia, India, Indonesia, Laos, W Malaysia, Myanmar, New Guinea, Philippines, Vietnam].