@article {27369,
	title = {A taxonomic study of western Canadian species in the genus Betula},
	volume = {44},
	year = {1966},
	pages = {929-1007},
	publisher = {Canadian Journal of Botany},
	abstract = {Five western North American birches, Betula fontinalis, B. glandulosa, B. glandulifera, B. resinifera, and B. papyrifera, are studied taxonomically. Four hybrids, B. {\texttimes} winteri, B. {\texttimes} sargentii, B. {\texttimes} arbuscula, and B. {\texttimes} uliginosa, are described for the first time and three others, B. {\texttimes} utahensis, B. {\texttimes} eastwoodae, and B. {\texttimes} sandbergii, are also included. Populations formed when species are in contact have been analyzed and the nature of several hybrid entities determined. B. {\texttimes} uliginosa results from B. resinifera {\texttimes} B. glandulifera. In this population, introgression is toward B. glandulifera, which has the higher chromosome number. B. glandulifera also hybridizes with B. glandulosa producing B. {\texttimes} sargentii. Most gene flow is toward B. glandulifera, which has the higher chromosome number. In B. glandulosa {\texttimes} B. fontinalis (B. {\texttimes} eastwoodae) introgression was little influenced by environmental selection, the hybrid was as common as either parental species, and any gene flow was in either direction. B. {\texttimes} utahensis results from B. papyrifera {\texttimes} B. fontinalis. Introgression is toward B. papyrifera. Hybridization of B. {\texttimes} sargentii and B. papyrifera produces B. {\texttimes} arbuscula, a rare hybrid. Hybridization of B. resinifera and B. papyrifera results in B. {\texttimes} winteri, with introgression in both directions. B. {\texttimes} sandbergii results from B. papyrifera {\texttimes} B. glandulifera. Probably as a result of environmental influence in the analysis area, introgression was toward B. glandulifera, with the lower chromosome number.},
	url = {http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet\&calyLang=eng\&journal=cjb\&volume=44\&articleFile=b66-106.pdf},
	author = {Dugle, J. R.}
}
