P1. Syst. Evol. 191: 83- 104 (1994) -, P l a n t ....... Systematics and Evolution © Springer-Verlag 1994 Printed in Austria Merosity in flowers: definition, origin, and taxonomic significance L. P. RONSE DECRAENE and E. F. SMETS Received August 3, 1993; in revised version November 24, 1993 Key words: Angiosperms, androecium. - Merosity, phyllotaxis, pseudowhorl, zygomorphy. Abstract: The term merosity stands for the number of parts within whorls of floral organs, leaves, or stems. Trimery is considered to be a basic condition that arose through the cyclisation of a spiral flower. Pentamery is mostly derived from trimery by the repetitive fusion of two different whorls. Dimery is either directly derived from trimery, or through pentamery as an intermediate stage. Tetramery is linked with pentamery and should not be confused with dimery. Possible causes for a change in merosity are the reduction of the number of carpels and zygomorphy in flowers. Derivations of different merosities have important consequences for the arrangement of the androecium (the insertion of stamen whorls, their identifications, and their number). It is concluded that two main groups can be identified within the angiosperms: magnolialean and monocotyledonean taxa are mostly trimerous or dimerous; non-magnolialean dicots are mostly pentamerous or tetramerous. By merosity (from the greek "m6ros") one understands the number of parts within whorls of floral organs, leaves, or stems (cf. RADFORD & al. 1974). This number can vary considerably in flowering plants, but tends to be more or less constantly distributed between different taxa; therefore some systematic significance can be attached to it. The highest frequency is five or three, but four or two are not uncommon. A "whorl" should be considered in a classical sense as a full cycle of organs that can be delimited from preceding and subsequent cycles (see ENDRESS 1992). The importance of the merosity for the configuration of flowers has mostly been overlooked in the past, but its impact has been brought to attention by EicHImp, (1875- 1878) and CEJe (1925) and more recently by DAI-ILCREN (1983) and Ku- BITZI<I (1987). In a remarkable article KU~ITZKI (1987) emphasised the importance of trimery in flowers and brought into focus the morphological and evolutionary relationship between different merosities. However, little is known of the origin and the relationships of the different merosities. That a characterisation of different merosities is badly needed, can be deduced from the often poor identification of specific cases. In considering the merosity of a flower all whorls have to be taken in consideration, and not only the petals or sepals. For example, dimery is all too often confused with tetramery, or all types of pentamery are considered as equiv- alent. Therefore, we will analyse the origin of different merosities, their nature and their evolutionary relationships in the following sections.