Aerodynamic Characteristics of Saccate Pollen Grains

The pollen grains of several gymnosperm groups consist of a main body and one to three air-filled bladders, or sacci. Although sacci may serve a buoyancy function to orient the grain on the ovular pollination droplet in some taxa, sacci have also been shown to increase pollen volume while adding minimal mass, thus decreasing density and thereby increasing the aerodynamic efficiency of wind pollination. However, no published studies have quantitatively addressed the effects of grain geometry or surface ornamentation at the low Reynolds numbers that pollen grains demonstrate. The objectives of this study were to empirically investigate the effects of varying geometries and surface ornamentation on the aerodynamic properties of saccate pollen grains through the experimental determination of drag coefficients and shape factors. Structurally different grains of two extant conifers (Pinus and Falcatifolium) were studied, and using electron microscopy, mathematical modeling, and solid modeling, we created scaled-up physical models of the pollen types. Models were produced with and without sacci, as well as with and without surface texture on the main body. Sacci increased the shape factor, or resistance coefficient, in all pollen types studied, compared to the same types that had been modeled without sacci. The presence of surface ornamentation also decreased the drag coefficients for saccate pollen grains of Pinus. This study is the first to experimentally demonstrate the effect of surface texture on drag for any biological or nonbiological particle at low Reynolds numbers. This study also provides additional empirical evidence for the aerodynamic role of sacci, supporting their adaptive significance for anemophily.