Ewe — leaves — is the operative material of Yoruba religious and medical practice, and the two are one discipline. A practitioner may work with several hundred plants, and the pharmacological interest of this corpus is real: Yoruba materia medica has been the subject of sustained ethnobotanical research and a number of its plants have documented activity. The religious element is not separable in the tradition's own account, because the leaf is held to be inert without the ofo, the incantation spoken when gathering and preparing it — the words are regarded as the activating component rather than as accompanying ritual. This is why Osanyin's permission matters and why herbalists, onisegun, are religious practitioners. It also creates a transmission problem: the knowledge is oral, held by ageing practitioners, and being lost at the same time as the forests the plants grow in.