Cirsium japonicum is one of the gynodioecious herbs. We clarified the morphological differences between the female and hermaphrodite florets by an observation of their flowering stages in C. japonicum. Movement of stamen and pistil is basically same in both female and hermaphrodite fl orets, i.e., the fi laments shrank, and the styles expanded and contracted, as the fl owering stages progressed. In all the female fl orets, stamens are degenerated as to be not able to release the pollen. The degenerated stamens were recognized to divide into three types in the form. It was suggested that gynodioecy was robably evolved through several steps of degeneration of male organ. We observed that both the female and hermaphrodite flowers produced nectar from the nectar glands at their base of corollatube and also both were visited by insects. Therefore, insect-visitors can probably pollinate the fl owers without regard to plant sex. Female fl ower gave birth to both female and hermaphrodite fl ower of next generation. It was supported that male sterility was inherited by nuclear genes.
