Flora of North America species comparison

Species 8 (3, including 1 hybrid, in the flora).

Circaea occurs throughout the temperate and boreal northern hemisphere, but is most diverse in eastern Asia, where all but one species occur. Reproductive features include: self-compatible; flowers diurnal, outcrossing, and pollinated by syrphid flies and small bees, or, sometimes, autogamous. It is found in rich, moist soils in deciduous forests and thickets, forest margins, and in moss or soil in mixed, coniferous-broadleaved deciduous, boreal forests. Circaea alpina subsp. alpina and C. canadensis subsp. canadensis often grow in close proximity and hybridize in eastern North America to produce C. ×sterilis. The unilocular C. alpina, with petals less than 2 mm, is self-pollinating under adverse weather conditions, but outcrosses on warm, sunny days. Because of its shorter style and much smaller pollen grains, it is probably the pollen recipient during hybridization events. Artificial hybridization experiments in England using C. alpina as the pollen donor and C. lutetiana as the pollen recipient failed to result in offspring, although hybrids were easily produced in the other direction (P. M. Benoit 1966). Recent molecular phylogenetic analysis supported the separation of the C. canadensis complex into two species; C. alpina subsp. pacifica was found to be sister to the remainder of the genus rather than being nested with other members of C. alpina (Xie L. et al. 2009). Thus, despite the strong morphological similarities of taxa within the C. canadensis and C. alpina complexes, these North American taxa may be better treated as separate species. Further detailed molecular studies are underway to examine this in more detail (Xie et al., unpubl.).

(Discussion copyrighted by Flora of North America; reprinted with permission.)