Adenium arabicum Balf. f. occurs along the southern and western margin of the Arabian peninsula. The plants are caudiciform in the more arid habitats and treelike in wetter ones. The Saudi form may produce an erect trunk to four meters (12 feet) tall (Vincett, 1984). Higher in the mountains the trunk is reduced to a massive caudex (Collenette, 1985). The leaves are generally larger than those of A.obesum, tend to be pubescent, and have very rounded tips. Populations in southern Yemen (Figs.19 and 21) are characterized by short, broad caudexes and the largest leaves in the genus-up to 20cm long and 12 cm wide (8 x 4.5", larger than those of A.boehmianum, which was erroneously reported to have the largest leaves in Dimmitt and Hanson, 1992). The caudex can attain a meter in diameter (Collenette, 1985; Miller and Morris, 1988).
In cultivation this species has a definite winter dormancy and seems to leaf out several weeks after A.obesum cultivated under the same conditions. The Saudi form retains leaves year round but still grows only during the hot season. Plants begin flowering at the end of dormancy in late winter and continue into early summer. The Saudi form has a major flush in spring but flowers sporadically all year. The flowers vary in size and pubescence. Those of the Saudi form are about 4cm (1.5") in diameter. The form from southern Yemen has flowers up to 8.5 cm (3") with very prominent pubescence in the throat (Fig.20). The petals are bright pink with some fading toward the white throat, which has usually one nectar guide per petal. The follicles (at least of the Yemeni form) are much larger than those of A.obesum and dark mahogany in color; the seeds are correspondingly large.
Both the Yemeni and Saudi forms of this species have only recently become readily available. They seem to be as easy to grow as the other taxa. The common cultivar sold as A.obesum 'Singapore' may be a Yemeni A.arabicum (see the next installment).
Adenium socotranum Vierh.(Fig.22) is endemic to the island of Socotra in the Indian Ocean south of the Arabian peninsula. It is the giant of the genus, forming a conical trunk/caudex several metres tall and up to 2.4 m (eight feet) in diameter (Balfour, 1888; Rowley, 1983). It resembles a miniature baobab. The stems of the single clone available to us are strongly vertical and distinctly striated, a unique character in the genus. The leaves are about 12 cm (4.7") long, are widest (4 cm, 1.6") near the tip, and are dark green with a white midrib and light major veins. Balfour described the flowers as bright pink and twice the size of those of "mainland A.multiflorum" (he considered the Socotran population to be of this species), which would make them 10-13 cm (4-5") in diameter.
This magnificent species is virtually unknown in cultivation, so its performance cannot be described with confidence. The six-foot tall specimen in Hanson's collection is in leaf only during the summer months, leafing out even later than A.boehmianum. It has not flowered in seven years, although it did once when Frank Horwood owned it. A specimen at the Huntington Botanical Gardens has also not flowered to date. The only other cultivated specimen known to us is in the collection of the Botanical Research Institute, Pretoria, South Africa (Myron Kimnach, written comm.).
We express our gratitude to John Lavranos and Gerald Barad for lending us their photos for publication. They are among a tiny group of botanists who have visited some of the nearly inaccessible areas where adeniums grow.
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