How can falling pollinia lead to self-pollination?

In many orchid species, self-pollination can occur as a result of the simple falling of old pollinia. This process serves as a mechanism to avert sterility at the end of a period during which the flower has remained open but failed to attract a pollinator.1

Typically, the rostellum acts as a physical barrier that keeps the pollen separate from the stigma. However, when this barrier is absent or when the flower reaches the end of its receptive life without successful cross-pollination, the falling of the pollinia allows the pollen to come into contact with the stigmatic surface, thereby facilitating self-fertilization. This adaptation is particularly significant for plants growing in environments where their customary pollinators are absent, helping to prevent extinction.1