Adenium is a genus of spectacular succulents from tropical Africa and Arabia. They range from shrublets with subterranean or above-ground caudexes to small trees with succulent stems to 15 feet tall. Their striking forms are further enhanced by some of the most beautiful flowers of all succulents, often bome in masses over a long season. Moreover, they adapt readily to container culture in the warmer parts of the world. This installment describes two very different taxa from Somalia and adjacent countries.
Adenium somalense Baif. f. occurs from Somalia south through the Rift Valley into Kenya and Tanzania (Rowley, 1983). The leaves arc 5 to 10cm long by 18 to 25 mm wide (2-4" by 0.75- 1"), bright green and usually with white veins. In habitat the most conspicuous form is the nominate variety somalense, a small tree to 15 feet tall with a very wide-based, distinctly conical caudex (Fig.11). The flowering branches are very thin and spreading to pendent. These giant-caudiciform populations occur in Somalia and northwestern Kenya. In most of Kenya they are shrubby with smaller caudexes, apparently from intergrading with A.obesum, which occurs on the coastal side of that country.
The flower is smaller than that of A.obesum, usually less than 5 cm (2") in diameter with narrower, pointed petals and prominent nectar guides that may extend slightly beyond the pubescent throat onto the petals (Fig.15). The flower color varies from pink to deep red, and, as in A. obesum, the color fades toward the throat. The flowering period of cultivated plants is usually from autumn through early summer but is considerably influenced by culture.
Adenium somalense is available in cultivation and easy to grow. Nearly all the plants are of the shrubby, presumed intergrade with A.obesum. This species has an obligate dormancy, usually beginning in November or December in Tucson. Plants do not releaf until flowering is past its peak in late spring.
In 1991 Dimmitt obtained seeds from an arborescent population in northwestem Kenya. The plants are extremely vigorous; seedlings produced mostly unbranched stems to six feet tall in a single growth spurt lasting 18 months. During the second and third growing seasons the plants scarcely increased in height, but the trunks thickcned substantially and grew numerous thin, ascending to horizontal flowering branches (Fig.13).
Adenium somalense var. crispum Chiov. is dramatically different from the nominate (first-described, i.e., the arborescent) variety and its intergrades with A.obesum. The plants are dwarfs with napiform (turnip-shaped) subterranean caudexes (Fig.14). The relatively thin roots originate almost exclusively from the top of the caudex, whjch is just below the soil surface, very unlike the arborescent form's thick, succulent roots that radiate from the base of the broad, above-ground caudex. The few above-ground stems are erect to ascending, scarcely succulent, and rarely more than a foot tall (Fig.18). The leaves are narrowly linear, usually strongly crisped (wavy-margined), and prominently white-veined.
The flowers of variety crispum are also distinctive (Fig.16). Compared to the arborescent variety, they have larger throats and smaller, narrower, white to pinkish petals. The margins of the squarish petals are often curled downward (quilled). The pink to red nectar guides in the throat may extend halfway to the tips of the petals, giving the flowers a distinctly striped aspect. In some plants the petals are solid red (Fig.17). Flowering in cultivation is sporadic; it seems to be most profuse during winter dormancy but may extend well into summer. Seedlings can flower in less than two years when only 6 inches tall.
This variety occurs in the same area as A.somalense somalense but apparently does not intergrade with it (Gerald Barad and Seymour Linden, pers. comm.). John Lavranos (pers. comm.) considers tbe two to belong to the same taxon. The variety crispum grows slowly in cultivation; it takes about five years to produce a specimen with its characteristic, though still small, caudex. The caudex can be exposed above the potting medium to create charismatic bonsai-like treelets (the base can produce roots in cultivation).
Fig.11. Adenium somalense var.somalense, Lake Baringo, Kenya. The conical caudex is diagnostic. Photo: Thomas A.Wiewandt.
Fig.12. Adenium somalense var.somalense east of Bargal, Somalia Photo: Myron Kimnach.
Fig.14. Collected plant of Adenium somalense var.crispum. In habitat the caudex would be underground. Stubs of the original feeding roots can be seen ariund the top of the caudex.
Fig.13. Three yera-old, six-foot-tall sapling of Adenium somalense somalense in cultivation from seed collected in northwestern Kenya. Note the incipient swelling of the entire trunk, which occured rapidly in the fall of the third season.
Fig.16. Flowers of Adenium somalense var.crispum, a selected clone with brighter than average color and only slightly quilled petals.
Fig.15. Flowers of a cultivated Adenium somalense somalense from northwestern Kenya.
Fig.17. Adenium somalense var crispum, red-flowered plant with nonquilled petals north of Mogadishu, Somalia. Photo: Gerald Barad.
Fig.18. Adenium somalense var.crispum. road to Warshak, Somalia. The caudex is typically underground, as (not) seen here. Photo: Myron Kimnach.
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