Disynstemon is a genus of flowering plants in the family Fabaceae (legume family), subfamily Faboideae, tribe Millettieae. It is a monotypic genus, containing only the species Disynstemon paullinioides.
Taxonomy and Nomenclature
The genus was established by the French botanist René Viguier and was published in Notulae Systematicae (Paris) volume 14, page 175 in 1952. The name was originally based on material collected in Madagascar. The sole species was first described as Lonchocarpus paullinioides by John Gilbert Baker in 1887 (Journal of the Linnean Society 22: 466–467). In 1977, Maurice A. G. Peltier recognized that Disynstemon madagascariense R.Vig. (published in 1951) was a later synonym of Baker's species and made the new combination Disynstemon paullinioides (Baker) M.Peltier. A variety, Disynstemon paullinioides var. hirsutus M.Peltier, has also been described.
Description
Disynstemon paullinioides is a liana (woody climbing vine) with long main stems that twine around supports, bearing short, leafy, and flowering branchlets. The leaves are deciduous, all trifoliolate-digitate (three leaflets arranged like fingers on a hand), with stipules and a pair of needle-like stipels at the petiole tip. The leaflets are variable in size, with the terminal leaflet elliptic to ovate and the lateral leaflets ovate and distinctly asymmetrical. A distinctive tuft of long white hairs is present at the base of the leaflet underside.
The inflorescences are terminal, racemiform-paniculate, with 2–6 few-flowered secondary axes near the base and a simple raceme toward the apex. The calyx is broadly campanulate. The corolla is violet to red. The androecium (stamen structure) is unique within the Papilionoideae: the five posterior stamens have short, broad filaments (about 2 mm), while the five anterior stamens have long, slender, curved filaments (about 7 mm), forming two distinct groups separated by a line perpendicular to the flower's symmetry axis. The anthers are all identical in shape and size. The ovary is sessile and tomentose, with 6–8 ovules. The style is glabrous, curved, and terminates in a thickened stigma.
The fruit is a cylindrical legume, 6–8 cm long and 1.3–1.5 cm in diameter, tapering at both ends, covered with felt-like hairs, and dehiscent. It contains 3–5 seeds, each about 8 × 6 × 5 mm, slightly compressed, with a carunculate hilum and a brown seed coat mottled with dark violet.
Distribution and Habitat
The species is native to Madagascar. Its distribution appears to be largely limited to the Isalo region and its surrounding areas, primarily on the western slope at elevations between 500 and 1,000 meters. Localities include the Col de Bepeho, Gorges du Rat, Mont Votaka, the western slope of Isalo, and Bois d'Anjohy. One specimen (Moral 3813) was collected at Andriambe on the banks of the Mangoky River at an elevation below 200 meters.
Classification
The genus is placed within the tribe Millettieae of the subfamily Faboideae. The distinctive androecium structure is considered unique among the Papilionoideae and is the primary diagnostic character for the genus.
