A brief historical account of the Joshua tree (Yucca brevifolia, Agavaceae) is presented. The geographical distribution, morphological and pollinator differences between subsp. brevifolia and subsp. jaegeriana are examined and the latter is raised to speciﬁc rank. The fossil Protoyucca and its possible relationship to the Joshua trees are considered together with implications as to the possible origin of the genus.
Lenz, Lee W.
"Reassessment of Yucca brevifolia and Recognition of Y. jaegeriana as a Distinct Species,"
Aliso: A Journal of Systematic and Evolutionary Botany:
1, Article 7.
Available at: http://scholarship.claremont.edu/aliso/vol24/iss1/7