CONTRASTED POLLEN CAPTURE MECHANISMS IN PHYLLOCLADACEAE AND CERTAIN PODOCARPACEAE (CONIFERALES)

Comparative study shows that Phyllocladus and representative Podocarpa ceae differ in the mechanism by which pollen is introduced into the po llen chamber and onto the apex of the nucellus (''pollen capture''). B oth types involve a pollination drop, but only in Podocarpaceae is it consistently inverted and in contact with adjacent surfaces. Phyllocla dus has functionally nonsaccate pollen (although a vestigial saccus ha s been claimed): its pollen is wettable and sinks in water. Podocarpac eae (except Saxegothaea) have saccate pollen, which is nonwettable and floats on water. In Phyllocladus the pollination drop receives the po llen directly and presence of pollen stimulates complete drop withdraw al, which may be a metabolic process. Once pollinated, an ovule does n ot resecrete a pollination drop. Ln Podocarpaceae the drop usually rec eives the pollen indirectly via pollen scavenging and saccate pollen i s preferentially captured. The retraction of the drop appears to be th e result of evaporation and is presumably nonmetabolic. Drop secretion fan be repeated in the presence of pollen. A major consequence of the se contrasted mechanisms is that in Phyllocladus the entire contents o f the pollination drop are ingested, whereas in Podocarpaceae only tha t part of the drop that includes saccate pollen is ingested. Because o f differences in repeatability of the secretion process, Podocarpaceae are likely to capture more pollen. Ln neither mechanism does the proc ess favor ''own'' pollen, but in Podocarpaceae all but saccate pollen is excluded. We thus have further evidence for differences in pollen c apture mechanisms in conifers with a pollination drop. and differences in the behavior of the pollination drop itself.