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.)

Circaea ×sterilis is a sterile hybrid intermediate in morphology between C. alpina × C. canadensis. Although sexually sterile, like other hybrids in Circaea, it reproduces more vigorously vegetatively than either of the parents. Its occurrence in naturally disturbed places along streams no doubt results in portions of stolons being broken off and carried away during high water to establish new colonies some distance away; colonies often extend for great distances along streams. The hybrids persist many years and colonies discovered 100 or more years ago can still be found in the same locality, often in the absence of one or both parents. The hybrid also occurs outside the range of C. canadensis subsp. canadensis, most conspicuously on the Gaspé Peninsula, Quebec, and in northern New Brunswick, but also elsewhere. Those hybrids may indicate a once more northerly range for C. canadensis subsp. canadensis, or that seeds resulting from hybridization events can be transported outside the range of the parents by birds or mammals.

M. L. Fernald (1950) misapplied the name Circaea canadensis Hill to plants now known to be hybrids (C. ×sterilis) between C. canadensis and C. alpina in North America and placed C. intermedia Ehrhart, a name published later, in its synonymy. Circaea ×intermedia is the hybrid between the European C. lutetiana Linnaeus and C. alpina Linnaeus. Hill used the name C. canadensis for plants in North America, to distinguish them from the European C. lutetiana.

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