Abstract:The floral morphology, sexual system, and pollination biology of Dunnia sinensis Tutch. Were studied. The stigma and anther heights, morphology of pollen grains and stigma, as well as the fruit set after artificial pollination were evaluated. The results showed that individuals of D. sinensis within a population exhibited two floral morphs, viz., long-styled (pin) and short-styled (thrum) morphs. Stigma and anther heights in long-styled flowers are 9.61±0.36 mm and 6.79±0.38 mm, while in the short-styled flowers are 6.11±0.56 mm and 8.96±0.59 mm respectively, exhibiting reciprocal herkogamy. No fruit was produced in treatments with self-pollination and intra-morph pollination, suggesting the occurrence of strong self and intra-morph incompatibility in the species. Ancillary dimorphisms were found in pollen and stigma of the two morphs: the pollen number in pin flowers is significantly smaller than in thrum flowers(38555.6±6258.4 vs. 52145.45±8924.5; t=4.009, P<0.01), the pollen size of pins is also significantly smaller (21.27±0.70 µm vs. 21.92±0.79 µm, t=3.890, P<0.01), although the ovule numbers of the two morphs are similar (38.73±1.85 vs. 39.1±2.25, t=-4.13, P>0.01, pin vs. thrum); while the stigma papillae is larger in long-styled flowers than in short-styled flowers. It was concluded that D. sinensis is a typically distylous species. Floral visitors and their behaviors were also observed, showing that bees (Bombus sp. and Xylocopa magnifica) and butterflies (Graphium sp.) were the main visitors and could efficiently pollinate between morphs in D. sinensis.