Charles Peirce's development of his diagrammatic logic, his entitative and existential graphs, was significantly influenced by his affinity for tree graphs that were being used in chemistry. In his development of systems of natural deduction and sequent calculi, Gerhard Gentzen made use of the tree (tableau) method. In presenting the historical sources of both these tools, we draw on unpublished manuscripts from the Peirce Edition Project at the University of Indianapolis, where for many years the first author was a member of the research staff.