Xenogamy

• Herkogamy: Flowers possess some mechanical barrier on their stigmatic surface to avoid self-pollination, e.g. presence of gynostegium and pollinia in Calotropis.
• Dichogamy: Pollen and stigma of the flower mature at different times to avoid self-pollination.
• Self-incompatibility: In same plants, the mature pollen fall on the receptive stigma of the same flower but fail to bring about self-pollination.
• Male sterility: The pollen grains of some plants are not functional. Such plants set seeds only after cross-pollination.
• Dioecism: Cross-pollination always occurs when the plants are unisexual and dioecious, i.e., male and female flowers occur on separate plants, e.g., papaya, some cucurbits, etc.
• Heterostyly: The flowers of some plants have different lengths of stamens and styles so that self-pollination is not possible, e.g., Primula, Linum, etc.