In Monina salicifolia the histology of a gall in the stem is analyzed, whose secondary conducting tissues are altered by the stimulation of the inductive agent, which is found in the larval chamber. The vascular change is hyperactivated and the number of cellular layera increases, from the implantation site in the larval chamber until the cortical region. With the consequent inhibition of the differentiation in the elements of the secondary phloem and xylem in the affected region, these tiasues are replaced by cells of the parenchyma.