Gymnocalycium is the most popular cacti for hobbyists—and certainly exciting to see in the field. A vast genus of globose cactus from eastern South America, east of the Andes, is called Gymnocalycium.
Gymnocalycium has low-growing, typically solitary stems with multiple ribs that are occasionally tuberculate. It has no differentiated fertile zone, and the flowers simply arise from the usually depressed stem tip. Flowers are open during the day, funnel-shaped to bell-shaped, and white or pale pink.
The pericarpal and floral tubes have a few broad, obtuse scales with membranous margins; their areoles are naked. Fruits are oblong to globose, either dry or fleshy, and dehiscent. Gymnocalycium Pfeiffer ex-Mittler 1844, as “Gymnocalicium” Brachycalycium Backeberg 1942 Subfamily Cactoideae, tribe Trichocereeae.
Plants are slow-growing, usually solitary but sometimes forming clusters. Stems globose to depressed globose to short cylindrical. The ribs are about 3.5–15.5, sometimes more, usually broadly rounded, often spiraling, and at times tuberculate, often with “chins” just below the areoles. Tubercles are often present and variable.
Nevertheless, the areoles are large. The spines are present and variable. Differentiated fertile zones are absent. Flowers are funnel-shaped to bell-shaped and borne at or near the stem tips. Normally, it opens during the day in spring and summer.
The flowers come in many colors: white or pink, sometimes yellow or vivid red; pericarpel and floral tubes with a few large, broad, obtuse scales with membranous margins. The areoles are naked; the perianth is usually spreading; and the stamens are in two groups, one surrounding the nectar chamber and the other inserted near the mouth of the floral tube.
Fruits: The fruits are typically rectangular to globose, dry or meaty, dehiscing in a variety of ways; floral vestiges persist; and seeds are quite varied.
Distribution: Gymnocalycium is mostly found in South American countries, i.e., Bolivia, southern Brazil, Paraguay, Uruguay, and Argentina. Based mostly on seed features, a number of infrageneric taxa of Gymnocalycium have been defined; Schutz (1986) and Pilbeam (1995) have done so.
Gymnocalycium (type, Echinocactus denudatus—G. denudatum) was described by Ludwig Pfeiffer in 1844 for three species; the name derived from the Greek gymnos, naked, and calyx, bud, referring to the smooth flower bud.
There has been little debate as to the placement of Gymnocalycium in the tribe Trichocereeae, but determining which of the many species is acceptable is another matter. Metzing et al., and others, have greatly assisted the International Cactaceae Systematics Group in developing its list of 71 accepted species.
Six subgenera are often recognized:
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Gymnocalycium,
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Macrosemineum,
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Microsemineum,
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Muscosemineum,
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Pirisemineum,
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Trichomosemineum
The great popularity of cacti has resulted in quite a few names appearing in several books. Nonetheless, important studies on Gymnocalycium have been published by John Pilbeam (1995), Bohumil Schutz (1986), and Gerhart Frank (1976–1977). However, one of the most valuable in assessing the status of the multitude of species names is the work by Detlev Metzing, Massimo Meregalli, and Roberto Kiesling (1995).
