Adenium socotranum.
By far one of the most rare and desirable of all succulents it forms in age more of a bush than a small tree (like a dwarf baobab)

Description: It is the giant of the genus, forming a massive conical trunk/caudex several metres tall and up to 2.4 m in diameter, the whole plant up to 3,5 meter in height. It resembles a miniature baobab with surrealistic forms. Some turn into tall trees while some may grow with their swollen bases more flat on the soil, This species - virtually unknown in cultivation - is frequently indicated to be a winter grower but the few plant observed in European collection are in leaf only during the summer months, leafing out even later than A. boehmianum.
Stem: Solitary or multiple bottle-shaped trunk/caudex, swollen, vertical, conical more or less distinctly striated (a unique character in the genus) the twiggy branches are few short and thin.
Leaves: About 12 cm long, usually widest near the tip, dark green with a white midrib and light major veins. They are glabrous in dense terminal rosettes.
Flowers: Pale to bright pink 10-13 cm in diameter. Very rarely seen flowered in collection.
Seeds: Big, about 1.5-2 centimeter.

If placed in a large container the differences between species will become more obvious. A. socotranum develop a swollen base while the plant is still quite young more rapidly than it strictly relatives (such as A. arabicum and A. multiflorum) that branch more freely but stay thinner, while some (A. obesum ssp.  boehmianum) don't develop much of a caudex at all.

Cultivation: Needs lots of light and fresh air. It will be an indoor bonsai in all but the warmest climates. Keep in a bright location in winter (minimum temperature 10° C). However, in the winter, keeping it cool gives the plant a needed rest. In summer, if possible, move outdoors to a sunny or partly shaded location. Needs little water during winter, especially when kept cool. Increase water during growing and blooming periods. The total watering needed is similar to other succulents, and it will lose leaves if overwatered. Feed monthly during spring and summer with liquid bonsai fertilizer. It has a large caudex with few short thin branches and do not need a heavy pruning after the plant's rest period as requested by other species. Eventually pruning should be kept to a minimum. The sap is poisonous, so clean hands after pruning, and avoid getting sap into open wounds.
Repotting: Root prune and repot every two years, after the winter rest period, in a mix of 2 parts bonsai soil, 2 parts peat, and one part sand. Can tolerate being pot-bound.

Propagation: Usually by seeds. It can also propagated by cutting of branch or stem rooted in a sand-peat mix. (Wait until 2-3 day before you plant. It will reduce rot.)

Photo of conspecific taxa, varieties, forms and cultivars of plants belonging to theAdenium obesum complex :

• Adenium boehmianum (A. obesum ssp. boehmianum)
• Adenium socotranum (A. obesum ssp. socotranum)
• Adenium swazicum (A. obesum ssp. swazicum)

Origin:  Endemic from from the isolated island of Socotra in the Indian Ocean south of the Arabian peninsula and east of the Horn of Africa. For many years Socotra hosted a Soviet naval port and was off limits to most everyone, restricting the availability of plants and seeds. In recent years it is accessible, but the authorities are very protective of the natural resources and it is illegal to collect plant material of any type.

Habitat and ecology: Grows among stones in grit or on other well drained soil. It displays several morphological and physiological adaptations to cope with the dry climate and fierce monsoonal winds. Adenium socotranum has a special cell sap cycling within the caudex which prevents overheating. Plant bodies are globular or columnar, with reduced surface areas that decrease transpiration. Glaucous wax surfaces and micro-anatomical epidermal emergences reflect radiation.
 

Synonyms:

• Adenium obesum ssp. socotranum (Vierhapper) Lavranos
  In: CSJA 38(1): 23, 1996

A. socotranum is currently the most beautiful and rarest specie in the genus and is virtually unknown in cultivation.  Occasionally seed becomes available from the few plants that are in cultivation, and specialized nurseries are starting to propagate this desirable rarity. But when the few plants do come on the market, they are expensive!