North American black-fruited hawthorns, 1: Variation in floral construction, breeding system correlates, and their possible evolutionary significance in Crataegus sect. Douglasii

Differences in the numbers of stamens and styles per flower are conspicuous features of variation in North American hawthorns (Crataegus). Variation in stamen number between individuals is discontinuous, with modes of approximately 20 and 10 (or fewer). The 10-stamen morphotype is exclusively polyploid, whereas the 20-stamen morphotype comprises both diploids and polyploids. Polyploidy is associated with apospory, self-fertility, and varying degrees of pollen sterility. Variation in stamen number may also be correlated with variation in distribution, phenology, leaf shape, and other features of floral morphology, leading to recognition of taxa at the specific or intraspecific level.