Morphological and anatomical features were observed during the formation of adventitious roots of Populus tremula×P.tremuloides by micro-cuttage multiplication with paraffin method,and the relationship between root primordia site and rooting was also analyzed.The results showed that ontogeny process of adventitious roots can be divided into four stages.The stage of adventitious root primordium induction:the cells of vascular cambium divided into parenchyma cell groups which contained the characteristics of meristem.The stage of adventitious root origination:the vascular cambium cell kept dividing and form adventitious root primordium initial cells which showed strong mitogenic ability.The stage of adventitious root expression:the adventitious root primordium initial cells kept dividing and formed root primordium with directivity.The stage of adventitious root elongation growth:the root primordium stretched out from lenticel and nick,inner vascular system well matured,then the adventitious root formed.The study showed that Populus tremula×P.tremuloides was bark inductive root formation type,adventitious root primordia originated in the vascular cambium area,single site of origin,was hard rooting type.