Pollen Transfer within Flowers: How Pollen Is Secondarily Presented

In several flowering plants, pollen is not delivered directly from the anthers to the pollinators but is transferred to and presented by another floral structure, a phenomenon known as secondary pollen presentation (2PP). The new pollen-presenting structure, that is, the pollen presenter, is usually the style, but other floral organs may also be involved in this within-flower pollen transfer. This transfer usually happens during late bud development when pollen is already released and thus is associated with extreme protandry. The evolutionary significance of 2PP is reflected by its occurrence in at least 29 families of angiosperms, suggesting many instances of independent evolution. The functional role of 2PP includes pollen saving and pollination accuracy, by reducing pollen loss and increasing the precision of pollen transfer to the stigma. Many types of 2PP have been described, distinguished either by the structure of the pollen-presenting organs or by the different processes involved in pollen transfer and presentation to pollinators. Herein three main mechanisms of 2PP are distinguished: (1) The deposition mechanism is the most common type of 2PP among angiosperms. Pollen is transferred to the presenter, often with the assistance of sticky secretions or hair tracts. (2) In the brush mechanism, pollen is wiped off of the anthers by attached hairs as the style elongates or moves relative to the anthers. (3) In the pump mechanism, the style acts as a piston, pushing the pollen out of a cylinder formed by laterally contiguous floral organs. In this article, we present a short introduction into the diversity and functional significance of 2PP, providing examples as case studies that can be easily observed in the lab or field courses. Potential avenues for future research on 2PP are also discussed.