Abstract: The excised stem tips of Mentha haplocalyx Briq. L. had been cultured in vitro. Seven days after culturing, various aqueous concentrations of MH were added to make 0.1 ppm., 1 ppm., 10 ppm., and 100 ppm, in the culture solution, as soon as some of the adventitious roots had been established at the base. In low concentrations of MH (0.1 ppm, and 1 ppm.), the explanted stem tipsnormally developed and, instead, the growth was slightly promoted. But under the higher concentrations of MH (esp. in 100 ppm.), the activities of apical meristem, after two weeks of treatment, had been greatly reduced or completely inhibited and no further extention in length could be discerned. The distal ends of already grown adventitious roots showed elliptical swellings and simultaneously the roots ceased toelongate. Longitudinal sections of the tumefacient part exhibited remarkable increment in size of the cortical cells which were separated from the stele. Descriptions of the morphological and anatomical changes of the young leaves treated with the higher concentrations of MH were also presented.