Metzing D., Thiede J., Testa sculpture in the genus Frailea (Cactaceae), Botanical Journal of the Linnean Society, 2001, 137(1):65-70, doi: 10.1006/bojl.2001.0454
Scanning electron microscopy study of Frailea seed surfaces. The paper compares the distribution, length and branching of testa-cell papillae and assesses these characters for distinguishing species. Frailea castanea material represented seven populations.