MammillariaMammillaria is a large genus with about 140–180 species, depending on who's listing them. So if you're stuck on that hypothetical desert island, you won't be limiting your options very much.
Pilbeam (1999) recognizes 181 Mammillaria species and of these Hunt (1999) accepts 145. Any way you split this genus, it is one of the most varied in the Cactaceae, and it also has wide distribution: southward as far as Colombia and Venezuela and northward extending into the American Southwest. The greatest richness and diversity of Mammillaria varieties is in Mexico.
Mammillaria carmenae is native to Tamaulipas, Mexico.
Mammillaria is a very diverse group; however none of these cacti are giant trees. They are all small-to-medium-size "globular" (roundish) cacti. Some are solitary; others grow into large clumps. They all have distinctive bumps which remind scientists of breasts enough to earn the name mammillae—thus the genus name (or as they say in the lingo, the generic epithet).
The plant pictured is fully grown at around 3 inches tall and 2 inches in diameter. This species is known to grow in clusters, so it's probably time for me to move it to a larger pot where it can spread out and grow more bodies.
Mammillaria carmenae has pale yellow to white spines, and each areole (at the tip of each mammilla) has over 100 spines, obscuring the surface of the plant almost entirely. It reminds me a bit of Mammillaria candida (profiled recently) or Mammillaria lasiacantha (in the collection but not profiled yet). Surprisingly, these similar-looking cousins are not its closest relatives.
Instead, according to molecular studies by Butterworth and Wallace (2002), Mammillaria carmenae is most closely related to M pectinifera, a bizarre subminiature which is about to bloom in my yard. I hope to profile it soon. You'd never guess these two are so closely related. M pectinifera (means "comb-bearing") resembles a strange, round top with spine-beds (areoles) like tiny, multilegged creatures. You'll see!
See AlsoANDERSON, E. F. 2001. The cactus family. Timber Press, Portland, Oregon, USA.
HUNT, D. 1999. CITES Cactaceae checklist. Royal Botanic Gardens, Kew, Richmond, UK.
BUTTERWORTH, C.; WALLACE, R. 2002. "Phylogenetic studies of Mammillaria (Cactaceae)—Insights from chloroplast sequence variation and hyptothesis testing using the parametric bootstrap". American Journal of Botany 91(7): 1086–1098. 2004.
PILBEAM, J. 1999. Mammillaria. Nuffield Press, Oxford, UK.
Also... coming soon, Mr Sentient Meat, chief plant profiler for Succulent Sunday, is very excited to be upgrading his library with the top, current, go-to reference for cactus:
HUNT, D. 2006. New cactus lexicon. dh books. Sherborne, England, UK.
[I can practically feel your excitement from here. —Mr S M]