Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Crataegus crus-galli L. sensu lato is one of the most widespread groups of hawthorn taxa in eastern North America. Early twentieth century classifications of this group have recognized a considerable degree of dissection into rather narrowly defined species. However, the status of many of these species has been unclear in view of the widespread belief that apomixis is frequent throughout the entire genus. Crataegus crus-galli s. l, was examined at 11 sites across southern Ontario in order to characterize the pattern of morphological variation present in one part of its range. Cluster analyses that employed a selection of resemblance functions and sorting algorithms confirmed the breakdown of the entire complex sampled at these sites into a total of four morphotypes. Two of these are more or less widely distributed and differ from each other most obviously in phenology and stamen number. The other two are much more restricted in their occurrence in Ontario and differ from each other and from the two more common morphotypes in features of flower, foliage and thorn morphology. The degree of multivariate variability exhibited within individual stands of the two common morphotypes was found to be quite low. However, stands of each morphotype were differentiated from one another by a number of correlated flower, fruit and foliage characteristics. Only triploid (pollen-infertile or completely male-sterile) and tetraploid (pollen-fertile) individuals of C. crus-galli s.l. have been found so far in Ontario (x = 17). All four morphotypes produce apparently unreduced, aposporous embryo-sacs. Results of pollination experiments showed that three more or less pollen-fertile morphotypes are self-compatible; in all four agamospermy is evidently pseudogamous. Together, these results suggest that stands of a given morphotype of C. crus-galli s. l. frequently consist largely of sibling individuals. This may result from apomixis and self-fertility, together with certain characteristics of Crataegus seed dispersal. Unnecessarily narrow species concepts applied to this group in the past were probably a result of confounding variation among such stands with that among taxa.



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