Mammillaria duwei  forma mostruosa.
This is a true oddity and some say that this monster is nothing else than a abnormal root grafted upside down.
 

Description: This plant is very different from a normal Mammillaria duwei, it reveal its true identity only when an occasional normal areole produces a tuft of white feathery spines. It is thought by some that it is nothing else than a monstrous root grafted upside down, in fact it produces easily adventitious roots in any part of its body. It forms large low clusters of hard, corky-textured creamy-brownish, greenish and pinkish fissured protuberances, some only a few centimetres across.  The plant looks more like some non-cactus succulent or rocky material than a cactus.  This cultivar shows many growing variations, with different crested and monstrous shapes.
 

Cultivation: Mammillaria duwei "mostruosa" is almost always seen grafted on a stronger stock, but seems it can grows well on its own roots too. The plants on their roots are quite difficult to grow long enough to produce clumps. Need regular water but do not water again until dry as it is specially sensitive to over watering; plants should be allowed to become crowded in their pot. Keep in shallow pot.   It require high level of sun light but can be sunburned if moved from shade/greenhouse into full sun too quickly. During the spring it may be able to take full sun until the heat arrives at the end of spring. In an area that has hot afternoon sun, it may be able to take full morning sun, but requires afternoon shade or afternoon light shade. If grown correctly, it will reward the grower with stronger and healthy growth. Keep well ventilated, as it is prone to root rot. Also, it is a species that is dormant in the winter and require very little water (maybe even none) during the cold months. Protect from frosts and freezes. Minimum of 5º C for safe growing. 

Propagation: Grafting or cuttings; leave them attached to form a cluster, or wait until they are 1/3 the size of the parent and then detach and plant.

Photo of conspecific taxa, varieties, forms and cultivars of mammillaria duwei.

• Mammillaria duwei
• Mammillaria duwei forma mostruosa

  
  

  

Conservation status: Listed in CITES appendix 2.

It is thought by some that it is nothing else than a monstrous root grafted upside down, in fact it produces easily adventitious roots in any part of its body.