AERODYNAMICS OF SACCATE POLLEN AND ITS IMPLICATIONS FOR WIND POLLINATION

The pollen grains of many anemophilious gymnosperms have one to three air-filled sacci, which have been thought to add surface area, yet add minimal weight, thereby increasing dispersal distance. However, no published studies have tested this hypothesis. Using the saccate pollen grains of three extant conifers (Pinus, Falcatifolium, Dacrydium), electron microscopy, and mathematical modeling, a computational model has been developed to study pollen flight. The model uses structural characters of pollen grains to calculate terminal settling velocity. Examples of characters utilized in the model include: lengths, widths, and depths of the main body and sacci; angle of saccus rotation; thicknesses of the saccus wall, endoreticulations, intine, and exine; and surface ornamentation. Settling speeds predicted by the model have been compared and validated with terminal settling velocity data obtained by other methods, such as stroboscopic photography. Modeling pollen both with and without sacci indicates that sacci can increase dispersal range. The mathematical model, based on structural characters, permits flight properties to be measured without physically testing pollen, allowing the flight dynamics of fossil pollen to be investigated. Several fossils have been studied, including non-saccate (Monoletes), mono-saccate (Gothania), and bi-saccate (Pteruchus, Caytonanthus, Pinus) pollen types. Although some studies of extant conifers indicate that sacci have a buoyancy function once grains reach a pollination drop, the present study provides the opportunity to further evaluate the adaptive significance of saccate pollen by correlating structural and aerodynamic features, and measuring these characters during the evolution of the seed habit in various groups of early plants.