Mammillaria is one of the largest genera in the cactus family (Cactaceae), with currently 200 known species and varieties recognized.[1] Most of the mammillarias are native to Mexico, while some come from the Southwestern United States, the Caribbean, Colombia, Guatemala, Honduras and Venezuela.[2] The common name "pincushion cactus" refers to this genus and the closely-related Escobaria.
The first species was described by Carl Linnaeus as Cactus mammillaris in 1753, deriving its name from the Latin mammilla, "nipple", referring to the tubercles that are among the distinctive features of the genus. Numerous species are commonly known as globe cactus, nipple cactus, birthday cake cactus, fishhook cactus or pincushion cactus (though such terms are also commonly used for related taxa, such as Escobaria or Ferocactus).
The distinctive feature of the genus is the possession of an areole split into two clearly separated parts, one occurring at the apex of the tubercle, the other at its base. The apex part is spine bearing, and the base part is always spineless, but usually bears some bristles or wool. The base part of the areole bears the 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.
The plants are usually small, globose to elongated, the stems from NaNcm (-2,147,483,648inches) in diameter and from NaNcm (-2,147,483,648inches) tall, clearly tuberculate, solitary to clumping forming mounds of up to 100 heads and with radial symmetry. Tubercles can be conical, cylindrical, pyramidal or round. The roots are fibrous, fleshy or tuberous. The flowers are funnel-shaped and range from NaNmm and more in length and in diameter, from white and greenish to yellow, pink and red in colour, often with a darker mid-stripe; the reddish hues are due to betalain pigments as usual for Caryophyllales. The fruit is berry-like, club-shaped or elongated, usually red but sometimes white, magenta, yellow or green. Some species have the fruit embedded into the plant body. The seeds are black or brown, ranging from NaNmm in size.
The genus Mammillaria in the family Cactaceae was proposed by Adrian Hardy Haworth in 1812. Initial spellings varied by authors but Mammillaria is now recognized as the accepted spelling. The first species in the genus was described by Carl Linnaeus in 1753 as Cactus mammillaris. The name Cactus became so confused that the 1905 Vienna botanical congress rejected Cactus as a genus name, and conserved Mammillaria.
Mammillaria is a large and diverse genus with many species often exhibiting variations due to the nature of terrain, weather, soil and other ecological factors. As a result, subdivisions within the species has been rather inconsistent over time. Initially, some investigators were more inclined to consider each variation as a unique species, creating confusion and long synonymy-lists for some of the species. Over time, new investigators began grouping closely related forms under the same name to attempt to more accurately define the species.
Several systems for classification began to emerge. The first of note, created by Schumann and modified by Berger, divided the species into ten named groups. However, the criteria for these divisions was somewhat indefinite and flexible. In the early 1923, cactologists Nathaniel Lord Britton and Joseph Nelson Rose developed the Britton & Rose system which arranged the classification characteristics in a system of keys with tangible separation factors, resulting in a much more workable system of identification.
Later classification was performed by the cactus specialists Hunt, Reppenhagen and Luthy, with much work focusing on researching the meanings and value of the original plant descriptions, synchronizing them with modern taxonomic requirements and studying the morphology of plants and seeds, as well as ecological aspects of the genus. These works helped to expand the understanding of Mammillaria taxa.
Currently the classification of Mammillaria is in a state where few newly discovered species are likely, though some new species may yet be found when the chaos of names created earlier by commercial plant collectors is sorted out. Many names that were introduced for plants barely differentiated by a shade of flower colour or variation in spination were eliminated in attempt to make the use of names consistent with the rest of the botanical world. The number of taxa, which at one time numbered above 500, is now below 200. Some genera (Dolichothele, Mammillopsis, Krainzia and others) have been merged back into Mammillaria, and others like Coryphantha, Escobaria and Mammilloydia were confirmed as separate.
Intense studies of DNA of the genus are being conducted, with preliminary results published for over a hundred taxa, and this promising approach might soon end the arguments. Based on DNA research results, the genus does not seem to be monophyletic and is likely to be split into two large genera, one of them possibly including certain species of other closely related genera like Coryphantha, Ortegocactus and the formerly recognized Neolloydia.
As noted above, some might not belong in this genus.
Mammillarias is predominantly found in Mexico but also have a wide range of distribution in neighboring regions north of the equator including the southwest United States, the Caribbean, Guatemala and Honduras. The southernmost limits of its range appears to be Colombia, and Venezuela, where only two known species are found.[3] Within this wide distribution, some species will exhibit large variations depending on the locality, sometimes even within just a few hundred feet. Some of these variations are so extreme that they have resulted in classifications of new species, many of which are so limited to one locality that they are considered critically endangered.
Mammillarias have extremely variable spination from species to species, and attractive flowers, making them attractive for cactus hobbyists. Most mammillarias are considered easy to cultivate, though some species are among the hardest cacti to grow. Several taxa are threatened with extinction at least in the wild, due to habitat destruction and especially overcollecting for the pot plant trade. Cactus fanciers can assist conservation of these rare plants by choosing nursery-bred specimens (wild-collected ones are illegal to possess for the rarest species). Several mammillarias are relatively easy (for cacti) to grow from seeds. One such species, popular and widely available from nursery stock but endangered in the wild, is Mammillaria zeilmanniana.
Water can be extracted from the cacti.[4]