Contrasted pollen capture mechanisms in Phyllocladaceae and certain Podocarpaceae (Coniferales)

Comparative study shows that Phyllocladus and representative Podocarpaceae differ in the mechanism by which pollen is introduced into the pollen chamber and onto the apex of the nucellus ("pollen capture"). Both types involve a pollination drop, but only in Podocarpaceae is it consistently inverted and in contact with adjacent surfaces. Phyllocladus has functionally nonsaccate pollen (although a vestigial saccus has been claimed); its pollen is wettable and sinks in water. Podocarpaceae (except Saxegorhaea) have saccate pollen, which is nonwettable and floats on water. In Phyllocladus the pollination drop receives the pollen directly and presence of pollen stimulates complete drop withdrawal, which may be a metabolic process. Once pollinated, an ovule does not resecrete a pollination drop. In Podocarpaceae the drop usually receives the pollen indirectly via pollen scavenging and saccate pollen is preferentially captured. The retraction of the drop appears to be the result of evaporation and is presumably nonmetabolic. Drop secretion can be repeated in the presence of pollen. A major consequence of these contrasted mechanisms is that in Phyllocladus the entire contents of the pollination drop are ingested, whereas in Podocarpaceae only that part of the drop that includes saccate pollen is ingested. Because of differences in repeatability of the secretion process, Podocarpaceae are likely to capture more pollen. In neither mechanism does the process favor 'own" pollen. but in Podocarpaceae all but saccate pollen is excluded. We thus have further evidence for differences in pollen capture mechanisms in conifers with a pollination drop, and differences in the behavior of the pollination drop itself.