双角草 shuang jiao cao

Herbs, perennial, prostrate to weakly ascending, to 60 cm tall; stems 4-angled, retrorse pilose or -pilosulous along angles. Leaves petiolate; petiole ca. 3 mm; blade drying papery, elliptic-lanceolate to oblanceolate, 20-32 × 4-8 mm, both surfaces glabrescent, base cuneate and sometimes oblique, apex acute; secondary veins 4 or 5 pairs; stipule sheaths truncate to rounded, 2-3 mm, glabrous, with 3-5 setae 1-4 mm. Flowers 1 or sometimes 2 or 3 per axil, 2 or sometimes 4-6 per node; bracts reduced. Calyx villous to pilose; ovary portion obconic, ca. 1 mm; lobes narrowly triangular-lanceolate, [2-]5-7[-10] mm, often unequal on an individual flower. Corolla white, glabrous inside and outside; tube 5.5-6 mm; lobes triangular, 4-5[-6] mm, sometimes pubescent adaxially. Fruit corky, ellipsoid, 6-9 × 4-6 mm, distinctly 8-ridged, pilose or villous to glabrescent, usually not separating into mericarps; seeds 5-6 × 2-3 mm, reticulate. Fl. and fr. Aug-Sep.

Bamboo forest sides. Naturalized in Taiwan [native to C and E North America; perhaps naturalized in Mexico and Central America; adventive in Japan].

This species is rather widely but infrequently and locally naturalized in tropical and subtropical areas, especially near seaports (Taylor, Monogr. Syst. Bot. Missouri Bot. Gard. 85(3): 2206-2284. 2001), and probably should be expected elsewhere in Asia. The plants are usually found in microsites with moving water, and apparently the fruit are frequently water-dispersed.

Hsieh and Chaw (Bot. Bull. Acad. Sin. 28: 44-45. 1987) gave the number of calyx lobes as 2; this is the first report of this condition for Diodia virginiana, and their illustration of the flowers suggest that there may at least sometimes be 4 lobes. They also gave some unusual measurements for the corolla of their plant, with the tube said to be 15 mm and the limb to 18 mm wide; both of these are exceptionally large measurements that are unknown in other plants of this species.