Apple rootstocks M26 and EMLA 9'COST' shoots propagated in vitro were used for the histological study of initiation and development of adventitious roots after a brief induction pretreatment. The results show that there are differences in mode and timing of initiation and development of adventitious roots between the two varieties. In M26, adventitious roots were directly initiated from the derivatives of the cambium, some of which were immediately transformed into meristemoids in situ 36 h after pretreatment. subsequently, meristemoids differentiated into root primordia. Development of adventitious roots were completed when they emerged at the surface of stem based 10 days after pretreatment. In EMLA 9, before the meristemoids formd, internal cell files were formed by continuous divisions of cambial cells. The cells were regularly arranged in files external to the cambium. On the fourth day after pretreatment, some cells in the outermost layers of these files became meristematic, started to divide and turned into meristemoids, which differentiated into root primordia. The cells of the files between the root primordium and the cambium were transformed into vascular tissues which ocnnected the vascular systems of the adventitious roots and stems.