Functional Morphology of Saccate Pollen in Conifers with Special Reference to Podocarpaceae

The distribution of saccate and nonsaccate pollen in modern conifers (nomenclature after Page 1990) shows a correlation with the method of pollen capture by the micropyle, although this has been little emphasized. Saccate pollen is restricted to, but not ubiquitous in, Phyllocladaceae, Pinaceae, and Podocarpaceae and is associated with the development of an inverted pollination drop, except in Phyllocladus. Saccate pollen is also nonwettable, in the sense that it floats on water, and it functions as a distinctive antigravity device. Pollen entry into the micropyle is dependent on resorption of the pollination drop and selectively excludes foreign particles. Nonsaccate pollen is wettable and sinks in water, but with two contrasted behaviors that discriminate two types of pollen reception. In Cephalotaxaceae, Cupressaceae, Sciadopityaceae, Taxaceae, and Taxodiaceae there is a pollination drop without preferred orientation. Pollen swells and bursts in water, shedding the exine by a mechanism determined by the apparently simple wall structure. In the remaining conifers with nonsaccate pollen-Agathis, Araucaria, i.e., Araucariaceae, together with Larix, Pseudotsuga, and Tsuga (Pinaceae) and Saxegothaea (Podocarpaceae)-there is no pollination drop, pollen is wettable but does not burst in water, and there is either extended siphonogamy or micropylar invagination. These observations show the adaptive features of pollen structure and ultrastructure. Despite their ubiquitous wind pollination, conifers thus have some diversity of pollination syndromes with reference to water and the introduction of pollen into the pollen chamber of the ovule, at which time pollination is complete.