Mammillaria hernandezii
 

With the uniqueness of the spine patterns this tiny plant is a real gem.

Mammillaria hernandzii is one of the most sought-after and very distinctive species of Mammillaria, this little species is immediately recognisable by its spination even when not in flower.

Description:  Tiny globular succulent plants, usually solitary (Slowly clumping in cultivation)
Stems: Depressed-globose to globose, soft, dark green, usually not more than 2,5 cm in diameter and height (but in cultivation it can slowly grow up to 4,5 cm in diameter). Without latex.
Roots: Somewhat fleshy, thickened root
Tubercles: Pyramidal with short white wool in the axil.
Radial spine: 17 - 25, white, cream coloured or tan, radiating, somewhat pectinate and curved backwards, not interlacing, 1.2 - 2.2 mm long.
Central spine: Absent.
Flower: Cherry red to fuschia-red (or occasionally white) with a paler throat, relatively large in relation to the stems size, up to 20 mm long, 2,5 cm large. The flowers are diurnal and close at night.
Blooming season (Europe): A characteristic of this species is that, at least when cultivated northern Europe and US, the flowers appears during Autumn and Winter, and often fails to develop properly due to cold, damp, and lack of light in a temperate climate. A sufficiently sunny October day is needed to prompt them to open.
Fruit: Remains embedded in the stem.
Seed: Large, black.

Cryptocarpic fruit: M. hernandezii is one of the few species of cactus with cryptocarpic fruits. That is, the fruit and seeds are produced and retained inside the stem of the plant. After the flower is finished and dropped off, the stem closes over the fruit and the fruit/seed gradually ripens within. The following years the fruit may remain within the body at the axil, or may protrude a bit.  A thin membrane will be above the part where ripe seed can emerge. As the plant swells with the new growing season, the membrane fractures, and some seeds from the past years can little by little drop down and germinate in the close proximity, forming small colonies.
But usually the seeds remain within the plant body for several years (usually about 9) or for the whole life cycle, and frequently  they will be released only at the death of the plant, after the disintegration of the old  stem.
It is possible to collect fruit and seeds only by means of a thin pointed forceps. The seeds' vitality lasts for many years, and  moreover  seeds contain inhibitors that preserve them from premature germination.  Generally fresh seeds won’t germinate very well; only old seeds do. The complete germination of this kind of seed may take several years (Some will sprout unexpectedly after 5 or 8 years!) Because of the above peculiarity, seeds and plants of cryptocarpic Mammillaria (Series Longiflorae)  are seldom available from commercial sources. Viability, measured as the proportion of germinating seeds, varies widely and is not correlated with seed’s age (e.g. 77% in 8-year old M. hernandezii seeds). Germination speed, however, increases with age. That is, given the opportunity (humidity) to germinate, older seeds do it faster. Slow release of fruits in these species introduces a time-lag mechanism that has important consequences on long-term population dynamics. It reduces both population growth and its temporal variability. This may account both for the rarity of these species, and for their long-term population stability. This makes them highly vulnerable to habitat disturbance and has implications for the strategies required for their conservation.
 

Distribution: Central Mexico, Oaxaca, Tehuacan-Cuicatlan Valley, Altitude 2.300 m.

This attractive plant from Oaxaca is still not common in cultivation.
Surprising large pink  flowers (2,5cm) completely dwarf its tiny body.

Seasonal growth and contraction:
In the wild this plants contract considerably during the dry season, sometimes pulling down completely under the soil level.  Frequently the flowers push up through the dirt from the underground cactus body. In fact, even though this plant shows new growth each year, it hardly gets any larger, and its dimensions remain unvaried year after year, as the individual stems tend to contract at the base. The new growth produced during the vegetative season compacts considerably and retracts, sometimes pulling down  the plant completely under the soil in the hottest months of summer and coldest months of winter.
It should be noted that "when specimens are in this withdrawn state, it becomes almost impossible to find them in their natural state even though their exact locality is known"

See also: Dehydration/rehydration cycles
 

An unusually large specimen (grafted on Trichocereus to speed growth)