How does the rostellum prevent self-pollination in many orchids?

In most orchids, the rostellum acts as a physical barrier that prevents self-pollination by separating the stigma from the anther.12

The rostellum is a specialized flap of tissue derived from one of the three stigmatic lobes that projects downward in front of the anther. By maintaining this separation, the rostellum ensures that the pollinia—the masses of pollen grains—cannot easily come into contact with the flower's own stigma.12

This mechanism is highly effective in promoting cross-pollination, as it typically requires an external agent, such as an insect, to visit the flower. When an insect interacts with the flower, it brushes against the rostellum and the anther, causing the pollinia to adhere to the insect's body for transport to another plant. In some species, however, the rostellum may degenerate or become stigmatic, which can lead to self-pollination if the pollen germinates on the stigmatic fluid.12