The genus Mammillaria is one of the largest in the Cactaceae family with about 200 species and varieties recognized. The first mammillaria was described by Carl Linnaeus as Cactus mammillaris in 1753, deriving name from Latin mammilla = nipple, referring to tubercules that are one of the plant specific features. Mammillaria is also one of a few genera which are without ribs. In 1812 Adrian Haworth described the genus Mammillaria seeds.
The distinctive feature of the genus is the specific development of an areole, that is split into two clearly separated parts, one occurring at the tubercules apex, another – at its base. The apex part is bearing spines, and the base part is always spineless, but usually bearing some bristles or wool. The base part of the areole is carrying flowers and fruits, and is a branching point. The apex part of the areole does not carry flowers, but in certain conditions can function as a branching point as well.
Plants are usually small, globose to elongated, stems from 1 cm to 20 cm in diameter and from 1 cm to 40 cm tall, solitary to clumping forming mounds of up to 100 heads. Roots are fibrous, fleshy or tuberous. Fruits are berry-like, club-shaped or elongated, usually red but also white, yellow and green. Some species have fruits embedded into the plant body. Flowers are funnel-shaped and range from 7 mm to 40 mm and more in length and in diameter, from white and greenish to yellow, pink and red in color, often with a darker mid-stripe. Seeds are black or brown, from 1 to 3 mm in size.
Mammillarias have extremely variable spination from species to species and attractive flowers, making them specifically attractive for hobbyists. Mammillaria plants are considered easy in cultivation, though some species are among the hardest cacti to grow.
Most of the mammillarias are native to Mexico, but some come from South-West USA, Caribbean islands, Columbia, Venezuela, Guatemala and Honduras.