Gymnocalycium

Gymnocalycium is a genus of plants in the family Cactaceae.
All of these plants are succulents without leaves. The plants grows in South America (Argentina, Bolivia, Brazil, Paraguay, Uruguay). In total, there are 50-80 species.
Stems of these plants are round, their diameter is 7–30 cm, their height is 3–25 cm. The color of stems is grey-green or brown-green or grey-brown.
Plants of this genus are popular houseplants.