Gymnocalycium

Gymnocalycium, commonly called chin cactus, is a genus of about 70 South American species of cactus. The genus name Gymnocalycium (from Greek, "naked calyx") refers to the flower buds bearing no hair or spines.

Description
The species of the genus Gymnocalycium are low-growing, usually solitary or sometimes small cushion-forming plants with globose, depressed-globular to short-cylindrical stems. The 4 to 15 (rarely more) ribs are usually broadly rounded, often sinuous, occasionally warty, and often have a "chin" just below the areoles. The thorns are very variable.

The diurnal, funnel- or bell-shaped flowers appear at or near the apex. They are white or pink, sometimes yellow or bright red. The flower cup and the corolla are covered with a few large, broad and obtuse scales that have membranous edges. The areoles are bare. The perianth is usually spread.

The stamens form two circles. The first surrounds the nectar chamber, the second is attached near the opening of the corolla. The oblong to spherical fruits are dry or fleshy and spring up in various ways. A permanent flower residue adheres to them. The seeds are very variable. Their color ranges from brown to black, the size from quite small to relatively very large.

In cultivation they are popular for their easy flowering habits, and the flowers are generally brightly colored.

Species
Species of the genus Gymnocalycium according to Plants of the World Online :

Distribution
Their main area of distribution is Argentina, part of Uruguay, Paraguay, Bolivia and part of Brazil.