Geography and distribution
Dioscorea orangeana is native to northern Madagascar. It is restricted to the Forêt d’Orangea near Diego Suarez (Antsiranana) in Antsiranana Préfecture. It occurs in deciduous forest, on sand, up to 100 metres above sea level.
Dioscorea orangeana in the wild (Photo: Richard Randrianaivo)
A twining vine to no more than 5 m, with annual stems growing from a fleshy tuber. The tuber is up to 20 cm in diameter with several digitate (finger-like) young growths below (although only one tuber has been seen by scientists). The left-twining stems are quite woody and grow to about 5 mm in diameter. They bear small brown prickles at the extreme base.
The leaves are variable in shape and colour, but often have an undulate margin, and juvenile leaves are often variegated.
Like almost all yams (Dioscorea), D. orangeana is dioecious - it has separate male and female plants. The male inflorescences of relatively small flowers are pendent (hang down from the vine) and are up to about 20 cm long. The male flowers are lime-green to yellow-green and in clusters of up to four. The female inflorescences are also pendent and are about 18 cm long with solitary, lime-green flowers. The flower develops into a capsule which is pale brown with chestnut-brown flecking.
The seeds are matt dark brown and winged at the base. The wings are membranous and golden brown with slightly darker flecking.
Tubers of Dioscorea orangeana (Photo: Marcel Hladik)
Flowering takes place in January and February, and seeds are released from the resulting fruits in June.
Dioscorea orangeana differs from D. comorensis in having undulate leaf margins and a broader torus (the modified receptacle of the flower which bears the reproductive organs and tepals) and tepals in both the male and female flowers. In female flowers of D. orangeana the floral stipe between the ovary and the torus is shorter than in D. comorensis.
The tuber (fleshy underground stem) morphology of the species is atypical among Malagasy species in that there are several digitate lobes rather than a single tuber per growing season, although more research is needed on tuber morphology.
Threats and conservation
The conservation and sustainable use of Dioscorea orangeana are matters of concern, because its distribution is restricted to such a small area (its area of occurrence is 1.7 km²). The Forêt d’Orangea is not protected, and the nearest protected area is at least 20 km away.
D. orangeana urgently needs to be looked for in similar forests in the far north of Madagascar which is botanically poorly explored.
Dioscorea orangeana is reported to be edible. All the yams known as Angona in the Antsiranana region are favoured edible species and are heavily exploited as such.