Abstract: Asteraceae are distinguishable by their head-like inflorescence called a capitulum. There are a variety of capitulum types due to different combinations of different composing florets. Morphological evolution of capitula must involve homoplasy under selective constraints. Therefore, a comparative examination of ontogenies of divergent capitula among phylogenetically close taxa is important to investigate the genetic and molecular bases of this trait. In the subtribe Artemisiinae, tribe Anthemideae, three closely related genera have different flower heads: Chrysanthemum with the radiate, Ajania with the disciform, and Stilpnolepis with the discoid capitula. Through observations using scanning electron microscopy, we examined the morphogenetic processes of flower heads of representative species of these genera. The morphological differentiation of the discoid from the radiate/disciform capitula occurred early in the floral initiation stage. The development of the ray/marginal florets of the radiate/disciform capitula lagged behind the outermost disc florets throughout the phases of floral initiation and floral organogenesis. Primordia of their 1–2 dorsal corolla lobes and all stamens ceased to grow soon after initiation. In contrast, all florets on a discoid capitulum developed equally in rates and modes into bisexual actinomorphic flowers. Differentiation between the radiate and the disciform capitula occurred late during the floral maturation stage. Our data provided evidence to the interpretation that uniformly acropetal development only occurs in species with homogamous flower heads, and to the idea that the timing of ontogenetic divergence between taxa is positively correlated with the phylogenetic distance of the taxa.

Key words: Asteraceae, capitulum, floral symmetry, morphogenesis, scanning electron microscope (SEM)