a genus of herbaceous plants of the family Leguminosae. The plants are herbs that lignify at the base, or lianas. There are two to four species, distributed in tropical Africa and on Madagascar. The woody liana P. venenosum, which is native to tropical Africa, is cultivated in India, Brazil, and—to a lesser extent—other tropical countries. Its seeds, known as Calabar beans, contain several alkaloids, including physostigmine.