Floral structure hindering natural pollination in Vanilla is

The floral structure hindering natural pollination in Vanilla is Rostellum.
The flowers are borne in axillary racemes and each inflorescence consists of 15 - 20 flowers. The flowers are to be artificially pollinated (hand pollination) for the fruit set.
The pollen of the vanilla flower is produced in a mass called pollinia, and is covered by a hood or anther cap. The stigma is protected with a lip known as 'rostellum'. For pollination, the stamen cap is removed by a needle exposing the pollinia. Then the flap-like rostellum is pushed up and the pollinia are brought into contact with the stigma.