1. S. G. Blythe, 1931 . Taming the wild guayule. Saturday Evening Post, 2 May, 28 .
2. E. K. Bolton, 1937 . Letter to J. E. Crane, 22 January, Series II, Part 2, Box 1035, records of the E. I. Du Pont de Nemours and Company, Hagley Museum and Library, Wilmington, DE.