A dioecious flowering plant prevents both

Dioecious flowering plants prevent the occurrence of autogamy and geitonogamy in them. In dioecious condition, flowers occur on different plants, i.e. male and female flowers are present on different plants. Hence, it does not favour autogamy and geitonogamy because autogamy takes place in bisexual flowers and geitonogamy takes place between different flowers of the same plant.