Abstract:The early stages of graft union, when male branch was grafted onto female branch in Ginkgo biloba L. by cleft graft, have been observed under light microscope in order to determine the origin of callus cells between the stock and scion. Pith parenchyma cells near the graft interface were the earliest cells in response to such method of grafting. These cells dedifferentiated and then divided within 7～ 12 days after grafting. A large number of callus cells extended from the pith into the space between the graft interface linking the stock with the scion about 18～ 20 days after grafting; and then continued to proliferate and extend outwards along the space. Cambium cells and immature vascular tissue near the graft interface dedifferentiated into callus rather late. Theover all link between the stock and scion was completed in the sites 30 days after grafting. Callus cells were also produced from corticai parenchyma cells, but they were much limited in quantity. In conclusion, the graft interface may be considered as a "natural culture bed" after grafting, in which all undamaged, living cells are capable of dedifferentiation and producing callus cells for compatible graft union. In the case of G. biloba (male/female) it were the pith parenchyma cells that appeared first to form the callus cells and later extend to link the stock with the scion.