Crataegus (hawthorn) and the closely related genus Mespilus (medlar) pose challenging problems for taxonomists and evolutionary biologists because of the occurrence of hybridization, polyploidy, and apomixis. Here they will also serve to exemplify similar problems also encountered in other Maloideae such as Amelanchier, Cotoneaster, and Sorbus, among others. Hawthorns colonize sites with high light levels and exposed soil (erosion surfaces, abandoned agricultural land) and resist grazing by vertebrates because of their thorns. They can reproduce prolifically, and in all respects can be thought of as long-lived, woody weeds. Click HERE to visit a page that discusses the interrelationships between these issues as they relate to hawthorn taxonomy.
|Like most Maloideae, Crataegus has unspecialized pentamerous flowers with a hypanthial ovary. Virtually all Eurasian Crataegus species, regardless of ploidy level, and all known North American diploids have 20 stamens per flower. Style and locule number varies from one to five, but some individuals may produce some flowers lacking gynoecia altogether. Within each locule the two ovules are superposed, so that only the micropyle of the lower one is associated with the funicular obturator. North American hawthorns are unusual in that stamen numbers also vary, with modes of 5, 10, 15, and 20 depending on species. Variation in stamen number may represent selection for optimal pollen dispersal (click HERE to read a paper on this topic). Lower numbers of stamens apparently result from the loss, rather than suppression, of stamen whorls (Evans & Dickinson 1996).|
Hawthorns reproduce not only sexually but also asexually, by means of gametophytic apomixis (vegetative reproduction from root sprouts also occurs in some species).
The polypyrenous drupes of Crataegus can be an important food source for wildlife, including grouse, pheasant, pine grosbeak, fox sparrow, gray fox, pine mouse, and white-tailed deer. Hawthorn fruits are used for food by humans as well (e.g. tejocote, C. pubescens, in Mexico and Guatemala; C. azarolus, in southern Europe and Asia Minor; C. pinnatifida, in China). Hawthorn fruits are also used in herbal remedies for circulatory system problems because of their flavonoid content.
Photo: R. Presgrave, © 2000, Royal Ontario Museum.
|Open-pollinated fruits typically contain only a single filled seed, regardless of pyrene number. Seeds require cold stratification to germinate.|
|Click HERE to link to a page describing current studies of Crataegus and Mespilus evolution and systematics. Click HERE to link to a brief report on recent studies of nuclear DNA sequence variation in Crataegus and Mespilus.||
|Medlars (Mespilus germanica) are almost indistinguishable from hawthorns and are known outside of cultivation only in Asia Minor and western Asia. Recently, a second species, M. canescens, was described based on material from a single population in Arkansas (Phipps 1990).|
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All photos © 2000, T. A. Dickinson except as noted.
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