Testa sculpture in the genus Frailea (Cactaceae)

The primary sculpture of the seed surface in the genus Frailea (Cactaceae) is characterized by a ± flat outer periclinal wall and the presence of an usually eccentric papilla. Papilla morphology differs mainly in size and breadth:length ratio. Seeds of different species (groups) are characterized by special patterns in the arrangement of short and long papillae on the surface, thus providing a useful tool for taxonomic studies in Frailea. The seeds of Frailea castanea possess branched papillae, a character restricted in this genus to this species. Moreover, such papilla morphology seems to be unique for the seed epidermis in spermatophytes. The possible function of the papillae is discussed.