Herbs, perennial, rhizomatous, often forming large colonies. Leaves petiolate, opposite, becoming alternate and bractlike in inflorescence. Inflorescences simple or branched racemes, terminal on main stem and at apices of short axillary branches. Flowers 2-merous, with a floral tube. Sepals and petals alternate. Petals obcordate or obtrullate, notched at apex, white or pink. Stamens opposite sepals; nectary wholly within floral tube or elongated and projecting above opening of floral tube as a fleshy cylindric or ringlike disk. Ovary locules 1 or 2; ovules 1 per locule; style equaling or longer than stamens, stigma 2-lobed. Fruit an indehiscent capsule, with stiff uncinate hairs, with or without conspicuous rows of corky tissue. Seeds smooth, fusiform or broadly clavoid to narrowly ovoid, adhering ± firmly to inner ovary. 2n = 22.
Eight species: temperate and boreal forests of the N hemisphere, from near sea level to 5000 m and from 10°-70° N; seven species (one endemic) and five natural hybrids (two endemic) in China.
Hybrids are common and often abundant in naturally disturbed habitats in Europe, Japan, and North America, but few gatherings of hybrids are known from China.
Ascherson and Magnus (Bot. Zeitung (Berlin) 28: 47-49, 745-787. 1870) divided Circaea into two groups, which they called "divisions," based on the number of locules in the ovary. Included in their "Uniloculares" are C. alpina and C. repens, with all of the other species placed in "Biloculares." These groups were later given sectional status by Steinberg (in Schischkin & Bobrov, Fl. URSS 15: 634. 1949). The single line of specialization, leading from the 2-loculed, outcrossing species to the 1-loculed, self-pollinating C. alpina, through the intermediate C. repens, represents a continuum that makes formal recognition of two infrageneric groups unwarranted.
Flowers, mature fruits, and carefully collected rhizomes are highly desirable to facilitate identification. The nature of the nectary is most easily determined in living plants.
