Abstract
Both aerial and underwater pollination systems (hypohydrophily) occur in Callitriche (Callitrichaceae), and as such the genus can serve as a model system in which to probe questions concerning the evolution of hypohydrophily from aerial pollination systems. Evidence from scanning and transmission electron microscopy reveals strikingly different exine thickness in the pollen of four aerially flowering species (C. heterophylla var. bolanderi, C. marginata, C. peploides, and C. stagnalis), which have a distinct sexine layer (0.55—1.1 μm thick), in comparison with that of the hypohydrophilous C. hermaphroditica. The exine of the latter species is rudimentary.
