Abstract: The floral morphogenesis of Caltha palustris L. and Trollius buddae Schipcz. was observed with a scanning electron microscope (SEM). The primordia of all floral organs initiate spirally and centripetally and develop centripetally. The spiral initiation sequence may be a basic pattern in Ranunculaceae. The primordia of bracts, sepals, and other floral organs are different in shape: the bract primordia are triangle, the sepal primordia crescent, and the petal (in Trollius), stamen, and carpel primordia hemispheric. This may indicate that the bracts, the sepals and other floral organs are different in origin. The petals are retarded in early developmental stages in Trollius buddae Schipcz, and have purses at the base. The retarded petals are very common in Ranunculaceae and the purse of the petal is similar to that of some Aquilegia species. The microspores in a longitudinal series of stamens develop centripetally in Caltha and Trollius; this may be a basic pattern in Ranunculaceae. The carpel primordia are plicate. In the developmental process of the carpels, the stigmatic tissue appears from the apex of the style and is decurrent along the ventral suture in Caltha, but there is no obvious stigmatic tissue in Trollius. Based on floral morphogenesis characteristics as well as the results from molecular systematics, comparative morphology and palynology studies, we consider that Caltha is not closely related to Trollius and that these two genera should not be treated in the same tribe.

Key words: Caltha, Trollius, flower, morphogenensis, development, systematics