A taxonomic revision of Crataegus (Rosaceae) in Ontario

This taxonomic revision of Ontario Crataegus (Rosaceae) indicates the presence of 39 good species in 14 series in the province. This appears to represent about one-third of the taxa of North American Crataegus. Additionally, six "little-known" species are briefly described. The reasons for taxonomic difficulty in Crataegus in eastern North America are fully discussed and attention is drawn to the care with which specimens must be collected and the characteristics known to be of diagnostic value. A key to and descriptions of the series are provided. A key to species is followed by descriptions of each. Line drawings of 25 species at anthesis are provided to indicate the range of variation in Ontario Crataegus and to underline discriminating features. This is the only pictorial record of this detail for Ontario Crataegus. Thirty-two colour figures emphasize the differences among fruiting Crataegus, which have hitherto caused much confusion to collectors. Thirty-five species are mapped for their Ontario distribution and exemplar specimens are cited for all taxa. Seven new combinations have been made. Our species concepts are not consistent because of the range of variation patterns found in the genus, from ordinary sexual outbreeders through polytypic forms with various cytotypes to situations where hybrid swarms may be suspected. The taxonomic situation is known to be complicated by apomixis and polyploidy.