In recent phylogenetic studies Acorus came out in the significant position as sister to all monocots; after exclusion from Araceae it forms its own monogeneric family. Floral development was studied in order to assemble more data for comparison with potential relatives. The inflorescence is an indeterminate spadix. However, terminal pelorias occur, which are composed of reduced flowers. The pelorias in Acorus are compared with similar forms in Araceae, Juncaginaceae and Saururaceae. Prominently unidirectional early floral development does not equalize later and results in monosymmetric flowers. Unidirectionality begins with an enlarged abaxial organ, which is either a tepal or a complex organ composed of tepal plus incorporated subtending bract. Seemingly bract-less flowers are also present in potential relatives of Acorus: Juncaginacae, Potamogetonaceae, Araceae, Nymphaeaceae and Saururaceae. In anthetic flowers of Acorus the placenta is distinctly apical. In early development, however, it is basal and its apical position is attained secondarily by a late elongation of the ovary base. Despite the prominent syncarpy at anthesis due to basal ovary elongation, early stages of gynoecium development show a considerable degree of apocarpy. This free apical part is a conspicuous feature and should be compared with that in Araceae and Juncaginaceae.

Key words: Acoraceae, Acorus, flower develpment, Monocotyledons, morphology