ORIGIN AND SIGNIFICANCE OF TRIMEROUS FLOWERS

Summary The purpose of this note is to bring into focus the morphological and evolutionary relationship between spiral anthotaxy and pentamerous, trimerous and dimerous whorls. The trimerous as well as the pentamerous condition has its morphological and evolutionary origin in spiral anthotaxy, and slight morphogenetic changes are responsible for the switch from spiral to whorled anthotaxy, the latter including pentamery, trimery, and rarely dimery. A pentamerous whorl is considered to be morphologically equivalent to two alternating trimerous or dimerous whorls. The transition from spiral to whorled anthotaxy is documented in several extant families of the Ranalean complex and must have taken place in parallel in various—albeit related—evolutionary lines. Except for its predominance in the monocotyledons and frequent occurrence in the Ranalean families trimery is relatively rare in other dicotyledons, and its morphogenetic origin may be different in these two major groups. It is proposed that trimery is a morphological constraint, which offers only very limited possibilities for meristic variation, with no return to pentamery or spiral anthotaxy possible.