The genus Mammillaria is one of the largest in the cactus family (Cactaceae), with currently 171 known species and varieties recognized. Most of the mammillarias are native to Mexico, but some come from the southwest USA, the Caribbean, Colombia, Venezuela, Guatemala and Honduras. The common name “pincushion cactus” refers to this and the closely related genus Escobaria.
The first species was described by Carolus Linnaeus as Cactus mammillaris in 1753, deriving its name from Latin mammilla, “nipple”, referring to the tubercles that are among the distinctive features of the genus. In 1812, the cactus specialist Adrian Haworth described the genus Mammillaria to contain this and related species. Numerous species are commonly known as nipple cactus, fishhook cactus or pincushion though such terms may also be used for related taxa, particularly Escobaria.
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 1 cm to 20 cm in diameter and from 1 cm to 40 cm 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 7 mm to 40 mm 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, from 1 to 3 mm in size.
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.
Photograph used under a Creative Commons license from kakteenklaus.