Abstract: Pollen morphology of 15 species in nine genera of Hydrocharitaceae was examined with light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The pollen grains are usually inaperturate, spheroidal to subellipsoidal in shape, but those of Stratiotes aloides and Halophila ovalis are monosulcate. The ornamentation of exine is spinulose with tuberculae, verrueae, granules or rugulae on pollen surface under SEM. The exine is composed of three indistinct layers, e.g. tectum, columellae and foot layer, but the columellae are only slightly developed. Some characters, such as the size of pollen grain, shape and length of spinules and wall structure are of some systematic value. The treatment of Stratiotes and Hydrilla as independent tribes respectively is supported. The ornamentation and structure of exine in the Hydrocharitaceae are similar to those in the Alismataceae, Aponogetonaceae and Butomaceae, but the presenceof inaperturate pollen grains in most taxa of Hydrocharitaceae shows its affinity with Najadales.

Key words: Hydrocharitaceae, Pollen morphology, Systematics