Systematics of Crataegus series Coccineae (Rosaceae)

The reality of six putative taxa usually considered to be members of Crataegus series Coccineae (Rosaceae) based on late 20th century treatments was investigated primarily by morphometric techniques. Two major samples were acquired to examine morphological variation; one from field collections localized in southern Ontario and the second from herbarium specimens loaned by major herbaria of northeastern North America representing the entire morphological and geographical range of the series. A morphometric study using 43 characters validated five of the six putative taxa and indicated one taxon not currently recognized. Outgroup analysis showed that series Coccineae was a natural group separated by a few correlated characters from series Molles and series Dilatatae. A breeding study suggested that all the taxa studied were sexual self-compatible or, if apomictic, required a pseudogamous trigger. Chromosome counts made for the series were all tetraploid. Isozyme studies indicated a close relationship between members of series Coccineae and to other North American Crataegus species. The typification of the 65 names associated with series Coccineae enabled the proper names and synonyms thereof to be assigned to the taxa recognized in this thesis. Based on this evidence, I suggest that three species be recognized in series Coccineae: C. coccinea (C. pedicellata), C. holmesiana and C. magniflora with some further divisions at the varietal rank. Crataegus pennsylvanica, which has been confused with C. ellwangeriana, is transferred to series Molles, but may represent a stable hybrid between series Molles and Coccineae. Keys, illustrations and distribution maps of taxa studied are included for identification purposes.