Abstract: Najas graminea is first reported to have a population of 2n = 12. A cytogeographic survey of this species reveals that the diploid (2n= 12) population is so far only found in Yunnan (You Jun 8804, in Herb. WH) and its adaptive radiation has probably resulted in the distribution pattern of N. graminea today in Asia. The polyploids are more widely distributed than the diploid, but no correlation was found between polyploids and geographic distribution, soil or climate types. The comparison of the karyotypes between diploid and hexaploid, lack of inter-specific hybridization in nature, and the occurrence of semi-cryptic polyploids in the populations suggest that polyploids of N. graminea are the result of autopolyploidy (at least in China). The highest polyploidy (almost 17X), the most asymmetrical karyotype, and the most reduced floral structure (the spathe is absent) support the suggestion that theN. graminea be the most advanced taxon in Najas.

Key words: Najas graminea, Polyploid series, Cytogeography, Autopolyploid, Evolution