Perennial rhizomatous herbs, often forming large colonies. Leaves petiolate, opposite, becoming alternate and bract-like in inflorescence. Inflorescence terminal on main stem and at tips of short axillary branches, a simple or branched raceme. Flowers white or pink, 2-merous, with a floral tube; ovary uni- or bilocular; ovules 1 per locule; sepals and petals alternate, stamens opposite sepals; petals notched at apex; nectary surrounding base of style, wholly within floral tube or elongated and projecting as a fleshy, cylindrical or ring-like disc; style equalling or longer than stamens, stigma 2-lobed. Fruit an indehiscent capsule, covered with stiff uncinate hairs; with or without conspicuous rows of corky tissue. Seeds smooth, fusiform or broadly clavoid to slenderly ovoid, adhering firmly to inner ovary wall. n=11.
A genus of 7 species and an additional 7 subspecies in temperate and boreal forests of the northern hemisphere from near sea level to 5,000 m, and from 10 to 70 N. Lat; 4 species are in Taiwan. Interspecific hybrids are common and often abundant in naturally disturbed habitats in North America, Europe and Japan, but are unknown from Taiwan.
Flowers, mature fruit, and carefully collected rhizomes are highly desirable to facilitate identification. The nature of the nectary is most easily determined in living plants.
Boufford, D. E. 1983 ("1982"). The systematics and evolution of Circaea (Onagraceae). Ann. Missouri Bot. Gard. 69: 804-994.