The genome sizes of plants represent a key character often associated with other traits. Bai et al. use flow cytometry to provide prime C-values for 514 taxa. Most of these (407) are new reports, including 129 genera and five families extending over 390 angiosperms, two gymnosperms, ten monilophytes and five lycophytes. New family C-value maxima or minima are reported for Betulaceae, Ericaceae, Ranunculaceae and Sapindaceae. These data provide the basis to explore phylogenetic patterns of C-value variation and relationships between genome size and other functional traits with the goal of linking genome size variation to other patterns of diversity and drivers of ecological change.