The dynamics of a population and the potential of a species to spread are directly related to the reproduction of their individuals. The investigation of the reproductive mechanisms is therefore fundamental to the understanding of the characteristics of a species. To reach this goal, traditional methods as well as population genetics provide a variety of approaches. The work presented here tries to gain insight into the different aspects of the reproductive system of plants. Phenological patterns within a single flowering period of several Juncus species were investigated. These patterns can best described as synchronous pulsed flowering. Also, macro-ecological analyses of pollen/ovule-rates were performed. However, the main object of this thesis was the wind pollinated, in Germany rare, Black rush (Juncus atratus KROCK.). Of particular interest was the investigation of the close link between reproduction and population genetics. Appropriate neutral genetic markers (microsatellites) were developed to allow paternity and further genetic analysis. Using these markers the mating system as well as the population genetic structure of J. atratus was investigated. Although primarily autogamous the species maintains a high genetic diversity on individual and population level which can be attributed to strong inbreeding depression. These results are surprising for an herbaceous plant and challenge existing theories of the evolution of reproductive systems in plants.