Ethnobotany is the scientific study of the relationships that exist between people and plants.
Ethnobotanists aim to document, describe and explain complex relationships between cultures and (uses of) plants: focusing, primarily, on how plants are used, managed and perceived across human societies (e.g. as foods; as medicines; in divination; in cosmetics; in dyeing; as textiles; in construction; as tools; as currency; as clothing; in literature; in rituals; and in social life.)
http://en.wikipedia.org/wiki/Ethnobotany
Dr. Daniel Moerman's Ethnobotany Database
The definitive catalog of Native American uses.
http://herb.umd.umich.edu/