Heterostyly is widely distributed among the angiosperms, but detailed studies are limited to few taxonomic groups. Sánchez et al. (pp. 321–331) ascertain the presence of different stigma height polymorphisms among species of Nivenia, formerly considered either as monomorphic or distylous. Comparisons of the degree of reciprocity, pollen transfer and floral integration give results that are consistent with the hypothesis that pollinators are the main driving force behind the evolution of heterostyly.
