Exacum tetragonum Roxb.

Robust herb with rather thick, fibrous roots. Stem erect, sparsely branching in lower part, to 150 cm high. Basal leaves withered at flowering time. Stem leaves lanceolate to sub-linear, 3.5–15 cm long, 0.5–3 cm wide, acute, prominently 3-veined, usually shortly petiolate. Inflorescence sub-corymbose, usually 5–7-flowered; pedicels 0.5–1.5 cm long. Flowers 4-merous, 2–3 cm diam. Calyx lobes ovate to lanceolate, 3–5 mm long, ±acuminate. Corolla purple to violet-blue, usually with white or yellow centre, rarely entirely white; tube 4–5 mm long; lobes 10–15 mm long. Stamens 4; anthers 3–5.5 mm long, deep yellow. Ovary 2.5–3 mm long with numerous ovules; style 6–11 mm long; stigmas ±reniform, c. 0.5 mm wide. Capsule 6–8 mm long. Seeds c. 0.3 mm diam., finely rugose, reddish brown.