Three phases of cohesion were observable during the development of compatible autografts in Solanum pennellii. Phase I cohesion 1) lasted 4-5 d after grafting, 2) was characterized by an average increase in tensile strength of 4 g breaking weight (BW) mm-2 graft area (GA) d-1, and 3) correlated positively with cellular interdigitation at the graft interface. The fresh weight of the scion increased by approximately 5% d-1 during the first 2 d after grafting. Phase II cohesion occurred 5-15 d after grafting, during which time 1) the tensile strength of the graft union increased by 14 g BW mm-2 GA d-1, 2) vascular differentiation across the graft interface was completed, and 3) the fresh weight of the scion increased by 9% d-1. Phase III cohesion occurred subsequent to 15 d after grafting, during which time 1) the tensile strength of the graft union leveled off at a value similar to that of an ungrafted internode, and 2) the fresh weight of the scion increased by 14% d-1. These results are discussed relative to mechanisms underlying the formation of compatible grafts.