| The distylous Arnebia euchroma (Royle) Johnst (Boraginaceae) is one of the traditional Chinese medicines and belongs to the grade II protected plant in China. In this paper, its reproductive characteristics including the floral morphology, population structure, breeding system, pollination ecology and fruit-set patterns was observed and analyzed both in field and indoor. The main results were as follows:(1) A. euchroma is an herbaceous perennial and its population was composed of two kinds of plants with pin and thrum flowers respectively. The ratio of number of thrum to pin plants was approximately 1:1 fulfilling theoretical expectation. Altitude had an important effect on phenology, the date of germinating, the starting date of flowering, the date of the flowering peak and the finishing date of flowering were earlier and the flowering duration is longer in low altitude than in high altitude and the date of fruit mature and wilting were reverse.(2) A. euchroma exhibited a typical distylous morph. There were the vertical reciprocal placement of stigmas and anthers in pins and thrums and their spatial place had no overlaps. Thrum anthers were larger than pin's, but the conditon were reverse in stigma and there were significant differences under both conditions. Pins produced more pollen than thrums but the size of pollen in pins were smaller than that in thrums. Pollen grains in pins and thrums were all dumbbell-shaped, round at the ends and constricted in the center.(3) Flowering phenology were concurrent in pins and thrums plants and flowering duration was about 30 days in one population. The majority of flowers and anthers opened at 8: 00-12: 00. The duration of single flower was about 6-7 days, the colour of corolla changed from deep purple to pink and stigma kept receptivity during the whole flowering period.(4) Pollen production in pins and thrums was 624000±195640 and 270300±93570 respectively, and there were extremely significant differences between them, their large pollen production ensured the success of cross-pollination. P/O value of pins and thrums were 156000±48909 and 67583±23392 apart and the breeding system was xenogamy. Moreover, the outcrossing index and the results of bagging and artifical pollination also indicated that this species was self-incompatible intensively and xenogamy. The artifical pollination between intra-morphs was fertile but its fruiting percentage was lower than that between inter-morphs. This might be attributed to their compatibility between stigma and pollen in inter-morphs.(5)The pollination of A. euchroma was of entomophily. the main pollinators were Anthophora (Melea) plagiata Illiger, Bombus (Subterraneobombus) melanurus Lepeletier, Bombus (Megabombus) sushkini (Skorikov)and Bombus(Sibiricobombus)defector Skorikov. The climax of visitation come up 12:00-14: 00 and each flower was visited by insects about 2.9 times per hour during this interval.(6) In natural population, the fruiting characteristics of A. euchroma, such as the average fruit-set, the frequency distribution of flowers producing one, two or three seeds, the seeds yield per plant, the seed-set and 1000-seed weight, had some differences among different habitats and between different years in the same habitats, the total branches, flowers and fruit-set, the seeds yield per plant and the seed-set were higher in pins than that in thrums. The fruiting characteristics were also different between the artificial population and natural population. The above characteristics were relation to the size of two kinds of plants, the time of flowers differentiation, the fruits position, pollen type on stigma, pollination system, habitat conditions.
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