Summary After a first research (Meletti 1954), done in order to study the reactions of Cereus validus Haw. to treatments with various concentrations of 2,4-dichloropheno-xyacetic acid (2,4-D) in acqueous solution, given for injection, particular attention has been dedicated to the study of the concentration of 2,4-D 100 mgr/l, which determines, as more interesting reaction, the formation of tumours, macroscopically revealed by the swelling of the treated parts. The histological analysis of the tumours has permitted to emphasize their origin and structure: initial hypertrophical phenomena interest the parenchyma cells (cortical or medullary parenchyma, for injections made respectively in the ribs or in the central cylinder), which successively undergo a clear necrotic process, followed by the proliferation of cells placed near the periphery of the region in necrosis, cells which originate neoformation tissues, by which derive adventitious roots. The general development of the reactions permits to attribute to Cereus a characteristic behaviour and different from that of tests usually employed for experiments with growth substances. The peculiar reactivity of the plant is demonstrated by: 1° - resistance to elevated concentrations of 2,4-D in acqueous solution; 2° - constant occurrence of reactions always limited to the zone of diffusion of the iniected substance (“zone D”); 3° - difference of the phenomena of hypertrophy and hyperplasia, separated by a notable interval of time, what has led the Author to consider hyperplasia and connected reactions due not to 2,4-D, but to the action of hormones which derive by the necrosis of the parenchyma cells. On the ground of this last consideration one arrives to the important conclusion that 2,4-D is unable to produce formative effects on Cereus: the only reaction, which occurs under its direct control, is represented by the hypertrophical phenomenon, generally not followed by hyperplasia, when repeated treatments are made, which determine the intervention of poisonous effects, most probably due not to accumulation of 2,4-D, but to the excess of injected liquid.