Structure and distribution
Frailea comprises small stem succulents with forms ranging from depressed globose to shortly cylindrical. The ribs may be distinctly divided into tubercles. The comparative account of the genus describes apical yellow or pale yellow flowers, with hairs and bristles on their external surface. The detailed character combination varies among species and also includes fruit and seed structure [Pontes et al. 2018].
Plants of the World Online currently recognises 20 accepted Frailea species. The genus extends from western Colombia to Bolivia, southern Brazil and northeastern Argentina, with occurrences also in Paraguay and Uruguay. Its species occupy varied habitats, and the rocky localities of F. erythracantha provide one documented example of the environments occupied by the genus [2-WWW] [Pontes et al. 2018].
Microscopic seed structure
Seed surfaces provide useful diagnostic characters within the genus. Scanning electron microscopy reveals testa cells with a flat or slightly convex outer wall and a papilla usually positioned away from its centre. The length of these projections varies across an individual seed. The longest generally occur on the dorsal and apical ridges, whereas shorter papillae occur near the region containing the hilum and micropyle. Taxon identification therefore considers both papilla form and its distribution [Metzing and Thiede 2001].
Microscopic structure also helps distinguish plants with similar general appearance. In the comparative material, F. pumila and F. schilinzkyana had long, erect testa papillae, whereas the broadly defined F. gracillima group had shorter, flaccid papillae. F. castanea showed a particularly distinctive character combination through its branched projections. These morphological traits assist identification and comparison of material from different localities [Metzing and Thiede 2001].
Identifying the species
The description of F. erythracantha from the basalt hills of Serra do Caverá illustrates the value of this approach. Its recognition combined stem form, spine arrangement, flower characters and seed microsculpture. The study also documented habitat, reproductive seasonality and a historical assessment of two small populations. Combining macroscopic characters with testa structure provides a fuller description of diversity within the genus than an individual feature of plant appearance [Metzing and Thiede 2001] [Pontes et al. 2018].
Scientific comparisons retain the names used for the studied material and relate them to the current classification. An example is F. perumbilicata, used in the 2018 diagnosis of F. erythracantha and currently included in the synonymy of F. castanea. This practice connects historical descriptions with accepted species while preserving information about the plants actually compared by the authors [Pontes et al. 2018] [2-WWW].