Abstract: The pollen grains of Salix matsudana can hydrate on the stigma of Populus simonii. Most of them germinate and many pollen tubes penetrate into the stigma with no serious barriers. The major prefertilization barriers are:① Most of the pollen tubes cease to grow at the basal part of the style of P. simonii in which the stylar cells are arranged more compactly than those of S. matsudana and contain a lot of starch grains. ② A large number of the pollen tubes which have entered the ovarian cavity coil and fail to reach the ovules. The frequency of fertilization is very low while the process is somehow normal if it takes place. The postfertilization barriers are; the development of some hybrid embryos appears abnormal and the incongruity between the hybrid embryos and their endosperms is quite obvious. Some hybrid embryos even abort before maturation. In vitro culture of immature embryos or ovules is proved to be effective to overcome postfertilization barriers, A successful culture of the 11th DAP ovules containing proembryos has been done and it is the first report that in a woody-plant intergeneric hybridization, the hybrid ovules at proembryo stage can be cultured into seedlings. The model for incongruity is discussed here. As an improvement, the concepts of random process and polyplacity are introduced into the model.