Abstract: In the aquatic order Najadales, severe structural reduction, particularly the floral structure reduction, and strong convergence of morphological characters occurring in the Najadaceae and Zannichelliaceae have greatly obscured their phylogenetic relationships and presented systematists with difficult problems in understanding their phylogenetics. To gather more basic phylogenetic information regarding Najadaceae and Zannichelliaceae, pollen morphology of 4 genera of these two families was examined with light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Our results provide the following insights: (1) Pollen grains of Zannichelliaceae are inaperturate, subspheroidal or subprolate, with ornate or reticulate exine sculpturing. Pollen grains of Vleisia and Zannichellia are smaller, usually with ornate sculpturing, while those of Lepilaena are larger, with crispate, ornate or reticulate sculpturing. (2) Pollen grains of some species, e.g. Najas oguraensis Miki and N. flexilis (Willd.) Rostk. et Schmidt. in Najadaceae, are monosulcate, but in some other species studied here they are inaperturate. Pollen grains of this family usually have not exine sculpturing, but those of N. oguraensis Miki and N. flexilis (Willd.)Rostk et Schmidt have verrucate or granular sculpturing. (3) Evidence from the aperture, wall sculpturing and structure of pollen grains does not support the viewpoint that Najadaceae is closely related to Zannichelliaceae and Potamogetonaceae, but shows that Zannichelliaceae and Pota-mogetonaceae are near relatives.

Key words: Najadaceae, Zannichelliaceae, Pollen morphology