C. Degano, M. Alonso, J. Ochoa, and A. Catan, “Seed Characterization and Scanning Electron Microscope (SEM) Morphology of the Testa of Three Groups of Argentine Opuntia ficus-indica (Cactaceae)”, J PROF ASSOC CACTUS, vol. 2, pp. 103–113, Jan. 1997.