Reproduction is an important part of the life of plants. The high diversity in floral traits indicates the complex and impressive evolution of plants under environmental selection. The diversification of flowering shoot systems (FSSs) and flowers often reflect adaptions to pollination and seed dispersal. Studying the evolution of the FSSs and flowers can help us understand the causes and modes of diversification and adaptation of flowering plants, and also contribute to their taxonomy. Begoniaceae is a tropical family with more than 2100 species. It comprises the two genera Hillebrandia and Begonia, both with unisexual flowers. Hillebrandia is a monotypic genus endemic to the Hawaiian Islands differing from Begonia in a few floral traits. Although Begonia is better known for the high diversity of leaf morphology, its floral diversity is also high. Furthermore, the availability of detailed phylogenetic studies also makes it an ideal taxon for studying the evolution of floral traits. This thesis focuses on the evolution of the floral characters in Begoniaceae, with emphasis on the special floral traits of Hillebrandia and the merism diversity in Begonia. To interpret the origin and evolution of the floral diversity in Begoniaceae, a combination of ontogenetic and phylogenetic methods was used in this study. For the ontogeny part, scanning electron microscopy (SEM) and paraffin sectioning were used to observe the development of flowers and reproductive units (RUs) in H. sandwicensis and 12 representative Begonia species. The different development of the petaloid organs in carpellate and staminate flowers in H. sandwicensis, and the development of the gynoecium, was described in detail. The ovary becomes semi-inferior by hypanthial growth. For the 12 Begonia species, their whole floral development processes were observed, with particular attention to the origin of different merisms. To interpret the potential evolutionary history of the floral characters, ancestral state reconstructions on nine floral traits were conducted on a phylogenetic tree based on three cp sequences. Correlated evolution between the merisms of carpellate and staminate flowers was supported. The most common combination was between pentamerous carpellate flower and tetramerous staminate flower. This thesis discussed the evolution of floral diversity of Begoniaceae. The petaloid organs of the staminate flowers of H. sandwicensis are probably of staminodial origin, but the origin of that of carpellate flowers is unclear. The merism transitions were found to be achieved by the fusion and shift of floral organs. Pressure and spatial constraints contributed to the floral development and patterning in Begonia. Given that the carpellate flowers do not offer rewards, the pollinator of Begonia may not be very discerning, which explains the success of deceit pollination linked with sexual dimorphism. The combination of multiple floral traits may contribute to the pollination in different taxa in Begonia. Further pollination and ecological studies may help with understanding the mechanism of how these floral traits assist pollination and promote adaptation in Begoniaceae.