Two‐dimensional Pollination in Hydrophilous Plants: Convergent Evolution in the Genera Halodule (Cymodoceaceae), Halophila (Hydrocharitaceae), Ruppia (Ruppiaceae), and Lepilaena (Zannichelliaceae)

In most plant species with abiotic pollination systems, pollen is dispersed in three dimensions. Theoretical considerations suggest, however, that there are significant advantages for two‐dimensional pollination systems over three‐dimensional systems, especially if pollen is dispersed in conveyances or aggregations of large diameter. We report that two‐dimensional pollination systems occur in the genera Halodule, Halophila, Lepilaena, and Ruppia, where pollen grains are not transported through the water singly, but in rafts or search vehicles. These genera possess unusual pollen morphologies which facilitate assemblage of pollen grains into search vehicles. These floating search vehicles have large diameters, thus greatly increasing probability of encountering a stigma. The grains have a hydrophobic surface that appears to mediate adhesion by an external coating of proteins and carbohydrates. Similar adaptations to two‐dimensional pollination are found in the target organs, the stigmas. The long filamentous stigmas of the marine genera float, as do the indusiate stigmas of the freshwater genera, creating small depressions in the water surface. Pollination occurs through the collision of a moving search vehicle with a floating stigma. The existence of similar pollen search vehicles, stigma morphologies, and flowering phenologies in these unrelated hydrophilous genera evidences convergent evolution towards efficient search strategies in surface‐pollinated aquatic plants.