Abstract: Pollen morphology of 27 species representing 11 genera of the Alismataceae was studied by light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Based on the observed pollen characters, three types of pollen were distinguished: (A) The Caldesia-oligococca type. Disc- or lens-shaped, with 2 pores (pores probably equatorially orientated, opposite each other); pores situated in depressions, more or less circular with fimbriate, fold-like structures arising from the margins; the exine granulate or slightly spinulate under SEM. (B) The Sagittaria type. Spheroidal, pantopo rate; pores in hardly sunken depression, delimited or not sharply delimited in outline, covered with coarsly or sparesly granulate or spinulate membranes; the exine spinulate with spinules 1～2 μm long. (C) The Alisma type. Rounded to polyhedral pantoporate; pores covered with granular membranes and circumpolar ornamented area, situated in depressions; the exine smooth to finely spinulate with spinules less than 0.5 μm long. The exine structure of all examined materials under TEM is three-layered: tectum, columellae and footlayer. The endexine is neither distinguished nor separated from the footlayer. The most primitive alismataceous pollen grains might be boat-shaped, monosulcate, with hardly sunkenpores, smooth pore membranes and exine spinulate exine without tectum perforation. The pollen of the Alismataceae is most likely derived from the monosulcate ancestor (possibly via nonaperturate intermediate) by an increase in pore number and a reduction in supratectal ornamentation. The evolutionary trends of alismataceous pollen are described as follows: (A) Pollen shape: from boat-shaped type to polyhedral one via a spherical intermediate condition, then to bilateral pollen, the highly derived. (B) Aperture: from the primitive monosulcate type to the pantoporate one via a nonaperturate condiation. (C) Aperture membrane: from smooth to granulate and spinulate. (D) Aperture not sunken was primitive; while sunken condition derived. (E) The sculpture of exine: from spinulate to granulate or spinuleless. (F) Tectum with microchannels or channels might be derived, that without channels the most primitive.

Key words: Alismataceae, Pollen morphology