From a gross morphological view, picloram (4-amino-3,5,6-trichloropicolinic acid) acted much like 2,4-D. As soon as 2 hr. after treatment with higher concentrations of picloram (7.2 [mu]g/plant), the stems began to swell and bend, and the treated leaves began to shrivel. Apical growth was stopped if concentrations were too high (72.0-7.2 [mu]g picloram/plant). With lower concentrations (less than 0.72 [mu]g picloram/plant), however, apical growth was only retarded and flowering delayed. The light-microscope study of the aerial portions of the plant revealed that the vascular cambium and its initials remained meristematic when affected by picloram. Initial divisions were periclinal, but later divisions were anticlinal and transverse, forming an adventitious root. As the adventitious root developed, the cortical cells were torn around the area of the root, and the epidermis was stretched until fissures developed.