Two reciprocal F1-families comprising 15 and 8 individuals, respectively, were obtained from the cross of two plants growing at 3 m distance from one another in the garden of the Royal Veterinary and Agricultural University in Copenhagen. Self-pollinated F1-plants had almost complete lack of seed-set: cross-pollinations were generally successful among the 489 cross combinations conducted (506 possible). Delayed cross-pollination—after self-pollination—was equally successful, which indicates that lack of seed-set after self-pollination is not due to a postzygotic mechanism. Studies of pollen tube growth, by ultraviolet microscopy, did not, however, indicate different growth rates, 24 hours after pollination, between self-pollinated and cross-pollinated pistils; The ovary was reached in both cases. No cases of reciprocal incompatibility being obtained among the 23 F1-sibs, a complex complementary 5-gene mechanism is presumed, with gametophytic pollen control and involving at least 3 or, more probably, 4 loci. The genetic analysis in this species is hampered by the long generation cycle; in the present case seven years were required to obtain flowering F1-plants.