In this paper, the writer has described the morphological and histological structure of Achlys japonica MAXIM., comparing it with that of A. triphylla DC., and some points of difference have been found between the two.The scale leaves of the rhizome were described by CITERNE and TISCHLER as being attached to it in the 2/5 divergence, and the xylem of the aerial stem by _HIMMERBAUR, as being the typical V-shaped in the American species, while the writer has found that the scale leaves are attached in the accurate 1/2 divergence and the xylem is not V-shaped in the Japanese species.The irregular arrangement of the bundles in the aerial stem and the presence of the cortical bundles in the rhizome have been also comfirmed in the Japanese species. the cauline bundles of the rhizome are usually four in number in the Japanese species, although they were described by CITERNE as usually five, rarely four or six.According to CITERNE, the cortical bundles in the rhizome, which are originated from the trace bundles of the scale leaves, are not united into the cauline bundles; in the Japanese species, however, they united into the cauline bundles laterally without exception, after running downwards through one or two internodes.The vascular course in the aerial stem and in the axis of the inflorescence had been hitherto unknown, and so the writer traced it by the successive microtom-sections. Most of the large bundles, situating in the rather medullary part of the aerial stem, are the cauline bundles and some of them are trace bundles from the flowers which are attached at the lower parts of the inflorescence. The peripheral small bundles found in the aerial stem are the branches of the trace bundles from the flowers or those of the cauline bundles themselves, and they are not found in the apical part of the inflorescence.The rhizome is divided morphologically into two categories; the one is considered as identical with the long shoot of other plants, the other as identical with the short shoot. The rhizome of the former type with the long internodes elongates quite monopodially, producing usually no aerial leaf or stem, while that of the latter type with the short internodes elongates sympodially, each monopodial axis being terminated in the aerial stem every year.