Distribution and taxonomic circumscription
Frailea castanea is an accepted species distributed in southern Brazil, Uruguay and northeastern Argentina. Its current taxonomic circumscription also includes plants previously described as F. asterioides and F. perumbilicata. When older studies are interpreted, the name applied to their material preserves its precise identity within the more broadly circumscribed species [2-WWW].
The material used to study seed microsculpture came from seven populations in these three countries. This sampling allowed comparison of seeds from different parts of the range and identification of a recurrent surface character. The study examined testa structure using scanning electron microscopy [Metzing and Thiede 2001].
Diagnostic seed characters
The microsculpture of the seed coat is a diagnostic feature of F. castanea. Its outer cell walls form papillae bearing short, rounded or more elongated branches. Metzing and Thiede documented this structure in material from seven populations in northeastern Argentina, southern Brazil and Uruguay. Its recurrence in material from different localities supports its value for identifying the species [Metzing and Thiede 2001].
Papilla form varies across an individual seed. Projections near the region containing the hilum and micropyle remain unbranched, whereas branched forms are conspicuous elsewhere on the surface. Seed comparisons therefore require examination of corresponding testa regions. Older literature also uses the name F. asterioides, which the study treats as a synonym of F. castanea [Metzing and Thiede 2001].
Synonyms and names in the literature
Astrophytum asterioides · Astrophytum castaneum · Astrophytum perumbilicatum · Echinocactus asterioides · Frailea asterioides · Frailea asterioides var. backebergii · Frailea asterioides var. harmoniana · Frailea castanea subsp. harmoniana · Frailea castanea subvar. spinosior · Frailea castanea var. perumbilicata · Frailea perumbilicata · Frailea perumbilicata var. spinosior [2-WWW]