Does pollen form follow function? Effects of stigma type, pollination type and habitat on pollen morphological traits

Pollen grains exhibit striking morphological diversity, yet the ecological drivers of this variation remain poorly understood. We tested whether pollination mode, habitat moisture, and stigma type shape pollen traits related to dispersal, desiccation tolerance, and hydration. Using 13 morphological traits from 1,151 European angiosperms and phylogenetically informed models, we found that insect-pollinated species produce larger, coated grains with thicker exine, while wind-pollinated species had small, smooth, lightweight grains enhancing aerial dispersal. Habitat moisture influenced traits linked to protection and hydration, though some long-standing predictions (e.g. variation in aperture number, simplified exine ornamentation) were not supported, possibly due to phylogenetic constraints or trait generalisation. Pollen morphology also reflected stigma type: dry-stigma species showed more surface coatings, whereas wet-stigma species produced larger, thinner-walled grains suited for rapid hydration. Our results demonstrate that pollen morphology reflects adaptation to ecological and reproductive conditions, supporting a trait-based framework for studying pollen function and evolution.