Inflorescence and floral development in Ranunculus and three allied genera in Ranunculeae (Ranunculoideae, Ranunculaceae)

Abstract

The development of inflorescences and flowers of three species of Ranunculus and three allied genera (Ceratocephala, Halerpestes, and Oxygraphis) (Ranunculeae, Ranunculaceae) was studied comparatively using scanning electron microscopy (SEM). Our results showed that both the inflorescence branching patterns and the floral morphogenesis in Ranunculeae are extremely labile. Inflorescences range from a thyrsoid to a solitary flower, with intermediate branching patterns in which sometimes only the terminal flower of the inflorescence main axis fully develops, such as in Ceratocephala and Halerpestes. Only in Oxygraphis is the flower truly solitary. In all species, sepal primordia are broad, crescent-shaped, and truncate, and the primordia of petals, stamens, and carpels are rather small and hemispherical, and very similar to each other. The plastochron between the last sepal and the first petal is relatively long. At the onset of androecium or gynoecium development, the regular spiral pattern of the perianth may become irregular in Ceratocephala and Oxygraphis, whereas spiral and whorled patterns may co-occur in Ranunculus and Halerpestes. The development of the petals is delayed with regard to that of the other floral organs in all species studied here, except for Oxygraphis. Our study confirms that, for solitary flowers, increased number of petals and the shape of the ventral nectary may have evolved independently many times, as suggested by earlier molecular phylogenetic studies and ancestral character reconstructions. With respect to the remainder of the tribe, Oxygraphis displays several autapomorphies, such as nondelayed development and increased number of petals, which will be of interest for comparative molecular developmental studies.