By special request (from a conversation with a friend on the weekend), here’s an infrared photograph of Yucca brevifolia (along with a non-IR photograph of the inflorescence).
Joshua trees have made a previous appearance on BPotD in a brief entry on Joshua Tree National Park. In the comments section, Bill Hooker of Open Reading Frame suggested this article by Chris Clarke on Creek Running North: Joshua trees and extinction. In a bit of coincidence, during the same month that Chris wrote his piece, a journal article came out that suggested things are a bit more hopeful for the Joshua tree than was thought at the time. See: Vander Wall et al. 2006. Joshua tree (Yucca brevifolia) seeds are dispersed by seed-caching rodents. Ecoscience. 13(4): 539-543.
Since it’s a well-known plant from California (and also Arizona, Utah, Nevada and Baja California), excellent resources for more information exist: Calphotos, the Fire Effects Information System factsheet, and the Flora of North America: Yucca brevifolia.