There are mainly six types of outbreeding devices that ensure cross-pollination.
1. Dicliny:
In unisexuality, the stamens and carpels occur in different flowers.
2. Dichogamy:
In some bisexual flowers, the stamens and carpels don’t mature at the same time, thereby self-pollination is inhibited.
3. Prepotency:
Pollen grains of another flower germinate on the stigma more rapidly than the pollen grains of the same flower.
4. Self-Sterility:
Pollen grains of a flower don’t germinate on the stigma of the same flower due to the presence of similar self-sterile genes.
5. Heterostyly:
Flowers have different lengths of styles and stamens.
6. Herkogamy:
It is a mechanical device to prevent self-pollination and promotes cross-pollination. In orchids and members of Asclepiadaceae like Calotropis Procera, where the pollens are aggregated in pollinia, the pollination completely depends on the mercy of the insects.