Mammillaria microthele
(Syn: Mammillaria formosa)

Mammillaria microthele ( v. superfina ) SB34 Presa de Guadalupe, San Luis Potosi, Mexico
This is a relatively tough and user-friendly small cactus with tight, dense minute
white spines that lie flat against the plant so not that dangerous.
 

Description: Slow-growing flattened plants, clumping with tight, short white spines, form low mounds in time
Stems: Almost discoidal to globose, apically depressed, dividing dichotomously, each stem are up to 6 cm in diameter.
Radial spines: 22 - 24 tiny, white and bristle-like that almost obscure the stem.
Flowers: Rings of small (5 mm) nearly pure white (Or with light pink midstipe ) are produced in spring over a several weeks period
Remarks: The so called M. microthele “superfina” is a particular nice selected form with very small tubercles and spines.

Recognized subspecies, varieties and forms:

• Mammillaria formosa subsp. formosa: It has 20-22 radial spines and pink flowers. Origin: oahuila, Aguascalientes, Guanajuato, Zacatecas, Tamaulipas, Nuevo Leon and San Luis Potosi.

• Mammillaria formosa  subsp. microthele: It has 22-24 bristly, flattened spines with white flowers. Origin: Cohahuila, Guanajuato, Tamaulipas and San Luis Potosi. This form should be grown conservatively to preserve the flat habit that is present in the wild.

• Mammillaria formosa subsp. pseudocrucigera: It has about 18 radial spines, (sometime lacking in mature areoles) the blooms are pink with paler margins. Origin: Queretaro, Guanajuato.
   

Cultivation: It is a  slow growing  species .  Use pot with good drainage and a very porous mineral-based potting mix,  keep dry in winter. Pot plants are quite wet-sensitively. Care must be taken with watering (Rot prone) and needs good drainage. Water sparingly during the growing season, keep very dry in winter.  Feed with a high potassium fertilizer  in summer.
Usually it is recommended to overwinter this plant in a bright and warm greenhouse with at least 8-10° C , but it proved to be quite frost resistant (if kept dry it hardy as low as -10° C)Outside full sun or afternoon shade, inside needs bright light, and some direct sun.

Propagation: Direct  sow after last frost, cuttings (if available).

Scientific name:  Mammillaria microthele Muehlenpfordt
Published in: Allg. Gartenz. 16:11 (1848) First description

Nowadays regarded as: Mammillaria formosa ssp. microthele (Muehlenpfordt) D.R. Hunt

Origin:  Origin: Mexico (Coahuila, San Luis Potosi, Tamaulipas).

Habitat:  usually found in limestone slopes of Agave lecheguilla-Hechtia glomerata scrub. Altitude 1.000 - 2.200 m.

Conservation status: Listed in CITES appendix 2.

Synonyms:

• Mammillaria formosa Galeotti ex Scheidweiler
  1838
• Mammillaria pseudocrucigera
• Mammillaria arroyensis

Etymology: Microthele from Greek “micros”=small and Geek “thele”= tubercle, for the very small tubercles of the plant.

Notes: This is one of the Mammillaria commonly called "Owl Eye Cactus", known for dichotomous branching (forking or dividing into two parts). Although dichotomous branching is not a common occurrence in cacti in general, it happens for some reason in this particular subspecies.  What is interesting about this cactus is that it began as a single head, and it has now divided twice, forming what will be four separate branches. When the division process started, it was obvious that four heads would appear, but I don’t think the one head divided quadruply. Most probably, one head became two, and then those two immediately divided.  Other Owl Eye Cactus among others comprise:
 M. karwnskiana, M. formosa,  M. tlalocii and M. perbella.
 

Photo and © copyright by Mladen Turcinovic (Croatia)
Old plants with lots of branching are treasures.

Photo and © copyright by Mladen Turcinovic (Croatia)
 

Photo of conspecific taxa, varieties, forms and cultivars of  plants belonging to the Mammillaria formosa complex (This Taxon has lots of synonyms ( like many other cacti) whit several controversial varieties and subspecies and comprises a multitude of different forms, but where each form is linked to others by populations of plants with intermediate characteristics):

• Mammillaria formosa
• Mammillaria microthele (M. formosa ssp. microthele)