Annual herb 10–60 m. tall, minutely papillose at the nodes and on the inflorescences when young, glabrous with age. Stem slender, simple or branched above, 4-ridged. Leaves petiolate in 4–7 distant pairs; the lower pair smaller; petiole 2–5 mm. long; lamina 10–50 x 3–10 mm., the upper ones linear-lanceolate, narrowly elliptic to elliptic, acute at the apex, cuneate at the base, the lower ones smaller and elliptic, rounded at the apex, 3–5 nerved. Flowers blue, pale violet or rarely white, turning yellow when dry in terminal and axillary dichotomous cymes; pedicel 4–15 mm. long; bracts 2–10 mm. long, linear, acute. Calyx tube 0.5–1 mm. long; lobes 2–4.5 x 1–1.5 mm., ovate-elliptic, membranous, hyaline, very broadly winged on the keel; wings reticulately veined, minutely papillose along the margin, accrescent in fruit, up to 2.5 mm. broad. Corolla tube 2.5–4 mm. long, broadly cylindrical, the lower portion enlarged in fruit; lobes erect, 2.25–3 x 1.5–2.5 mm., subcircular to ovate or obovate. Stamens inserted 0.5–1 mm. below the corolla sinuses; filaments 0.75–1.25 mm. long; anthers 0.75–1.75 mm. long. Ovary 2–3 x 1–1.5 mm., globose or ovoid-globose; style 1–2 mm. long; stigma capitate, often rather small. Capsule 2.5–4 mm. in diam., globose or obovoid-globose. Seeds 0.25–0.35 mm. in diam., subglobose-polyedric, minutely faveolate, brownish.