Missouri Botanical Garden
Missouri Botanical Garden: Plants of the Desert
The Evergreen Project reveals such desert plants as the dragon tree, the saguaro cactus, the prickly pear, the desert spoon, the aloe plant, and the like. Illustrated.
Other
Branimir Gjetvaj: Sonoran Desert, Arizona
This site has several pictures and links on the flora of the Sonoran Desert in Arizona.