The general problem studied in this work was the control of the formation of different types of cells from the cambium in ordered spatial arrangements. Young Phaseolus vulgaris plants were used, and the cells which were studied were the vessels and fibres of the secondary xylem. It was possible to control the type of xylem which was formed in the stem by selectively removing leaves of different ages. Many vessels were formed below young leaves while a single mature leaf caused the formation of a large volume of xylem, often more than that formed in an intact plant, and this xylem consisted almost entirely of fibres. Evidence is presented that the stimuli of young and mature leaves differ by at least one component. Quantitative results show that young leaves can be imitated, in terms of the xylem which is formed, by a source of auxin while a mature leaf can be imitated by a combination of auxin and gibberellin.