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A small genus of twining or laxly-stemmed shrubs, ranging from W. Africa and the Canary Islands through S. Europe and the Near East to China. Eleven species are recognised by the Polish botanist K. Browicz in his monograph on the genus (Arb. Kornickie, No. XI (1966), pp. 5-104, in English).
Leaves opposite, entire (though some species, not treated here, are almost leafless). Inflorescence a terminal or lateral cyme. Calyx five-lobed. Corolla with five spreading lobes, the inner surface of which is heavily marked in some shade of brown or purple and more or less viscid except at the paler margins. Stamens five, free from each other. Corona-lobes five, linear, each springing from the base of a stamen and arching inward at the apex. Styles two, stigmas united. Carpels two, each developing into a many-seeded follicle (‘pod’). For the remarkable pollination mechanism see: Willis, Dict. Fl. Plants & Ferns, ed. 7, p. 852, ed. 8, p. 874.