Chacruna, Psychotria viridis, is a modest shrub of the coffee family, glossy leaved and unremarkable to look at, and it is where the visions in ayahuasca actually come from. Its leaves carry dimethyltryptamine, a compound that is present in trace amounts in a great many plants and, unusually, in the human body itself.
Swallowed on its own it is destroyed almost immediately by monoamine oxidase in the gut, which is why chacruna tea is inert and why the plant has no solitary tradition anywhere. Combined with the beta carbolines of the ayahuasca vine, which suppress that enzyme, the same leaf becomes active for several hours.
The traditions are precise about the division of labour and consistent across a wide area. The vine is the teacher, the one with the intelligence and the authority, the plant that is addressed and dieted and sung to. Chacruna is the light, what allows the teaching to be seen. Ethnographers working separately among indigenous and mestizo practitioners record the same distinction, which is a stronger kind of evidence than any single account.
Regional practice varies in the admixture rather than the vine. In parts of Colombia and Ecuador chagropanga, a Diplopterys species, is used instead, and other leaves appear in smaller quantities. The vine is the constant. The leaf is chosen, and that choice is a matter of place, availability and lineage rather than of doctrine.