|Etymology -From the Latin word mammilla, meaning "nipple" or "teat".
With nearly 200 recognized species, the genus Mammillaria is one of the largest of the cactus family. For the most part, these species are globose or ball-shaped plants which grow either solitary or in clumps. Some clumps may reach over 3 feet (1m) with many stems. Few species grow much over around 6-8 inches in height by 4-6 inches in diameter. All have nipple-like tubercles with dimorphic areoles on the ends. Spines may be stiff and stout, few or many, bristle-like, hair-like, pectinate (comb-like), and come in a wide range of colors. In the axils, that is between the tubercles, there may be wool or bristles or both or neither. Flowers come from second-year growth in these axils and often form a ring around the stem. In many species the flowers are small and pink at less than half-an-inch in diameter. Some are small and yellow or white, while a few species have showy flowers which stand on long floral tubes above the plant. In many cases, the petals will feature a darker midstripe. The fruits are typically red, tube-like structures resembling little candies and are edible. To describe so many different species in such general terms does not do justice to this genus.
In the wild, all but a few of the species are endemic to (only found in) Mexico. A handful of species reach up into the bordering United States and only a couple species find their way down through Central America and Northern South America as well as the islands of the Caribbean.
Mammillaria is arguably the most popular cactus genus in cultivation. The small-size and ease of growing and propagating for most species makes them perfect for mass distribution via large garden centers. The variation in spines are a novel selling feature and often, these plants fit the stereotypical expectations that most people have for a cactus plant. Although, some species are only going to be found among highly-dedicated cactus growers.
The popularity of this genus has led to a good deal of study and consequent literature on both growing and identifying Mammillaria species. Even so, disputes over nomenclature are still not eliminated and some species find their way back and forth between Mammillaria and other genera such as Cochemiea, Mammilloydia, Mammillopsis, and Solisia. Other similar genera such as Coryphantha and Escobaria which have both been included in Mammillaria in the past, seem to be more widely accepted as distinct by modern taxonomists.