Species normally comprise harmonious systems for which developmental processes, including phytohormone metabolism, are integrated to produce normal growth. However, in 31 interspecific combinations of Nicotiana studied, particularly with Nicotiana langsdorffii, as one parent, certain tissues were stimulated by genetic factors carried by one or both* parents to grow abnormally without definite cell differentiation. Evidence presented indicated that this abnormal growth was associated with a disturbance in the growth regulatory system with the result that the plant was no longer able to metabolize phytohormones normally, and this unbalanced condition resulted ultimately in the stimulation of plant tissues to form tumors. Tumorous growth was likewise associated with the loss of N. tabacum chromosomes in a segregating population produced by selfing the plant. (N. debneyi-tabacum) x N. debneyi.