1. Aldous, A. E., and Shantz, H. L. Types of vegetation in the semi-arid portion of the United States and their economic significance. Jour. Agr. Res. 28: 99–128. 1924.
2. Beckett, R. E., and Stitt, R. S. The desert milkweed (Asclepias subulata) as a possible source of rubber. U. S. Dept. Agr., Tech. Bull. 472. 1935.
3. Beckett, R. E. Rubber content and habits of a second desert milkweed (Asclepias crosa) of southern California and Arizona. U. S. Dept. Agr., Tech. Bull. 604. 1938.
4. Bell, E. H., and Castetter, E. F. The utilization of mesquite and screwbean by the aborigines of the southwest. N. M. Univ., Bull. 314. 1937.
5. Benson, L., and Darrow, R. A. A. manual of southwestern desert trees and shrubs. Univ. Ariz., Biol. Soc, No. 6. 1945.