Floral morphology of 16 species of Gunnera representing all six subgenera is examined and described using scanning electron microscopy (SEM). The evolution of 19 flower morphological characters is discussed. Eight of these characters are optimized on a current phylogenetic tree. Character evolution tends to be generally continuous over different clades and several parallel evolutionary patterns of floral reduction can be identified within the family. At one extreme stand species such as G. manicata and G. perpensa with mainly bisexual flowers bearing well-developed sepals and petals, and at the other stand species such as G. magellanica and G. herteri with perianthless staminate flowers, and pistillate flowers with only gynoecia and sepals. In between one finds a range of flower morphologies, bisexual or not, with petals and sepals present or absent, or discernable as reduced organs. Character-state reconstruction and morphological evidence suggest that flowers in Gunneraceae were bisexual in origin with a reduction leading to simple unisexual flowers. Among the examined characters, absence of reduced organs in the other gender, spathulate petals in the staminate flower, long styles, and the presence of well-developed bracts are typical for species of subg. Milligania. Basally connected sepal lobes with a strongly swollen base and reduced blade is found in all species of Panke and in G. magellanica (subg. Misandra). Selected synapomorphies are consistent with the deep phylogenetic division of Gunnera between the New and Old Worlds.