The Galium aparine complex in Canada

The occurrence in Canada of Galium aparine sensu lato has been studied. It is concluded that two species are present: G. aparine L. sensu stricto (Cleavers) and G. spurium L. (False Cleavers). Although closely related and morphologically similar, these species differ morphologically, cytologically, and physiologically. The chromosome number of collections of G. aparine from Canada and the United States was found to be 2n = 66, that of G. spurium from the prairie provinces of Canada was 2n = 20. The latter species is an introduction from Eurasia and seems to be the more serious weed in fields of grain and rapeseed in western Canada. Galium aparine is believed to be native, as well as introduced from Eurasia, in North America. Galium aparine occurs usually in moist, shaded habitats, whereas G. spurium favors sunnier habitats. Both species have spiny or smooth fruits and these fruit variants have been recognized at various taxonomic levels. In this paper they are reduced to the rank of form.