Hawthorn: Crataegus monogyna
- Photographs of this tree, its buds, bark, leaves and inflorescence.
- http://www.plant-identification.co.uk/skye/rosaceae/crataegus-monogyna.htm

Crataegus douglasii (Black Hawthorn)
- Photos and brief description.
- http://www.cwnp.org/photopgs/cdoc/crdouglasii.html

Crataegus douglasii
- Includes distribution and occurrence, botanical and ecological characteristics, value and use, and fire ecology.
- http://www.fs.fed.us/database/feis/plants/shrub/cradou/

Crataegus monogyna
- images
- http://www.dipbot.unict.it/orto/0298-1.html