Small Arizona(?) mystery cactus

[aavery]

Hello,

I am hoping that someone can help me identify a cactus that I received as a gift a few years ago. I have looked at many of the pictures here at CactusGuide.com and elsewhere, but I am not sure which features to use to make an identification.

I have included photos of the mystery cactus. (Two of them are older and show the cactus in its original pot.) The cactus did not come with any form of taxonomic identification, but it was sold as a tourist souvenir and came in a pot with "Arizona" written on it. So, perhaps it is a species native to Arizona. I also suspect it was purchased in Tucson.

Here are some details:

The above-ground portion is about 25mm tall and 35mm in diameter. That is probably about 3 times as tall as when I received it three years ago. (The diameter has not changed much in that time.)

The tubercles are arranged in 13 spirals.

Each areole has 2-3 central spines, each of which has a reddish tip and is about 1mm long. One spine on each areole points straight out, while the other(s) is/are pointed down. I count 20-24 radial spines per areole. Each radial spine is about 2mm long.

When I first received it, there did not seem to be any fuzz on the areoles, but some of the newer areoles have some fuzz just below the radial spines.

The cactus has never flowered.

Can anyone help identify this cactus? Thanks in advance.

[iann]

Looks like Mammillaria formosa ssp microthele. Needs more light. Tall isn't good, it should be wider than tall. The areoles near the top look a little distorted, did it flower?

[Onzuka]

Well spotted that the tubercles on your Mammillaria form 13 spirals, but they are only the left hand ones! If you study it closely, and it's very apparent from your first pic, there are also 21 others going in the opposite direction. The only other number that there could have been would have been 8. The numbers of spirals are dictated by Fibonacci numbers and are a key identifier for Mammillaria experts. You will have to do your own research if you are not familiar with the Fibonacci sequence, but it is often nature's way of packaging things very efficiently.

Steve

[DaveW]

Whether Mammillarias always retain the same Fibonacci numbers through their life as they get larger is open to question since of course we know other cacti can gain or loose ribs, and tubercles are often on ribs themselves, but the relationship of the left to right spiral usually is in the Fibonacci series?

http://www.psychologytoday.com/blog/mat ... athematics" onclick="window.open(this.href);return false;

[adetheproducer]

Its not just the mammilarias its pretty much all of nature when you see a spiral, ferns, the seed arrangement in sunflowers, pine cones, cacti ribs/tubercles, pineapples, agave ... they are everywhere. They are not always perfect but alway present.