Plant sex in many species involves the transfer of pollen from one individual to another, often with the help of animals. There are only so many pollen binding sites on the pollinator, though, suggesting that there might be competition amongst plants for those pollen-carrying sites. Pollen sacs of the milkweed relative, Oxypetalum, have a horn-like appendage on them, surprisingly, since other members in the Apocynaceae have hooks on the pollen sacs. The “horn” appears to ensure that once the pollen sac is picked up by the pollinator, no other types of pollen can latch onto it in a daisy chain fashion and usurp the benefit of a quick drop off at the next plant.
New Phytologist, published online, March 20, 2014