Formation and development of adventitious roots inOLEA EUROPAEAL.: Significance of the anatomical structure for the development of radicles. —A histological and anatomical study on olive-tree cuttings has been carried out in order to follow the structural modifications occurring during their stay in the rooting bed, and the processes of formation and development of adventitious roots. In the practice, the cuttings commonly used for the propagation of olive-tree are taken from 1-year old twigs; their rizogenous capacity is remarkably poor and the percentage of rooting may be increased only through special treatments (i. e. mist-propagation). The authors have studied the anatomical and histological causes of the different behaviour of coetaneous twigs taken from the same grown-up tree and kept under identical environment conditions. It has been ascertained the existence of a mechanical hindrance (the structure of the scleren-chymatous ring) to the development of the newborn radicles. When this hindrance is removed the newly formed radicles emerge along the internodes, and have no relation with the formation of a callus. To explain the different rhizogenous aptitude exhibited by the cuttings of the olive-tree a merely anatomical and histological study is not sufficient, but it is necessary to investigate also in the physiological field.