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Catling, P. M., Mitrow, G., Haber, E., Posluszny, U. and Charlton, W. A. 203. The biology of Canadian weeds. 124. Hydrocharis morsus-ranae L. Can. J. Plant Sci. 83: 1001-1016. European frog-bit (Hydrocharis morsus-ranae L.), a free-floating, stoloniferous aquatic, is native to Europe and parts of Asia and Africa. It was discovered in Canada in 1932 in Ottawa, but earlier introduction is possible. By 1955 its North American distribution extended from Ottawa to Montreal. By 1980 it had extended southwest to Lake Ontario and northeast to Quebec City. Recently it has spread throughout much of the central and southwestern parts of southern Ontario, and further into northern New York and Vermont and eastern Michigan. The maximum rate of spread has been 15.6 km yr -1 . Reproduction by seeds is rarely reported but vegetative reproduction is very important in spread and colonization. In the fall, turions separate from the plant, sinking to the bottom where they overwinter. In the spring, these turions grow into small floating rosettes. Extremely rapid stoloniferous growth during the summer months results in the formation of large masses of interlocking plants that diminish native submerged aquatic plant communities by reducing available light. It is also of importance in limiting water flow in irrigation systems and restricting water traffic, thereby hindering recreational activity. Management has been largely mechanical. Hydrocharis morsus-ranae is a food plant for several water birds, rodents, fish and insects.

A reevaluation of the generic limits within the Pyroloideae is presented, taking into consideration the more recent evidence of chromosome number within the group. Chromosome number is correlated with more obvious morphological features, many of which have been the prime basis for the delimitation of generic boundaries in earlier treatments. Brief consideration is given also to the implications of data obtained from a thin-layer chromatographic analysis of the subfamily.Three generic segregates from within the original broad concept of the genus Pyrola are recognized. The diploid chromosome numbers which have been reported for Orthilia secunda (L.) House (2n = 38) and Moneses uniflora (L.) Gray and Chimaphila spp. (2n = 26) in conjunction with discrete differences in inflorescence types and floral modifications among these taxa serve as useful characteristics on which to base their recognition as genera distinct from Pyrola s.str. (2n = 46).

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Circaea × intermedia Ehrh. in North America is an interspecific hybrid between C. alpina L. and C. lutetiana L. subsp. canadensis Aschers. & Magnus. In spite of the morphological differences that exist between the European and North American subspecies of C. lutetiana, hybrids from both continents are morphologically identical. Documentation of the intermediacy of the hybrid taxon is presented based on the evaluation of the means of 22 characters of specimens from an Ontario locality at which all three taxa are found.Diploid chromosome counts of 2n = 22 are reported for Ontario populations of the hybrid and parental species. The presence of irregular, somatic chromosome numbers are also reported for all three taxa.Distribution maps for all three taxa in eastern North America are included. In the case of C. lutetiana subsp. canadensis, the northern range is sharply delimited by the Precambrian–Paleozoic bedrock boundary.A table of character comparisons and a key to the three taxa summarizes the salient characteristics of the hybrid and the parental species in eastern North America.