Aerodynamics and Pollen Grain Depositional Patterns on Cycad Megastrobili: Implications on the Reproduction of Three Cycad Genera (Cycas, Dioon, and Zamia)

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Abstract
Characteristic air-disturbance patterns around megastrobili of Cycas, Dioon and 2 spp. of Zamia (Z. pumila and Z. furfuracea) influence the pattern and quantity of wind-borne pollen grain deposition on ovules and surfaces of megasporophylls. The greatest number of adhering pollen grains is found on the windward profiles of cycad cones, with the density of pollen grains generally diminishing along cone surfaces oriented parallel to airflow. Cycas megastrobili preferentially deflect airflow passing over the cone toward leeward, distal surfaces, where pollen grains accumulate. Dioon cones deflect airflow and pollen grains toward leeward, proximal surfaces, where pollen grains preferentially impact with or settle on the surfaces of sterile bracts. Z. pumila megastrobili have a fairly uniform distal to proximal distribution of pollen grain impactions. Z. furfuracea megastrobili split longitudinally along the contact zones of orthostichous megasporophylls. Air currents carrying pollen grains into the cone through these openings circulate within the cone, centrifugally depositing pollen on ovules and megasporophyll surfaces. Statistical analyses of pollen distribution in Cycas, Dioon and Z. pumila indicate no significant differences between the number of pollen grains adhering to surfaces of ovules and adjacent megasporophylls. Although the general pattern of pollen grains adhering to megastrobili of Cycas and Dioon is the result of nonrandom aerodynamic processes, direct wind pollination in all 4 taxa appears to be stochastic. The preferential concentration of pollen on the distal portions of Cycas megastrobili may aid pollination. Water, dislodging adhering pollen grains, flows along the glabrous ovule-bearing margins of megasporophylls and accumulates on or near micropyles. Similarly, air eddies may sift pollen into the cone onto ovules. By contrast, the preferential accumulation of pollen on the proximal, sterile bracts of Dioon, in conjunction with insect foraging and semidestructive burrowing into the cone, may effect pollination. Cycad pollination may have 2 phases: the transport of wind-borne pollen grains to megastrobili (Cycas, Dioon and Zamia), and the subsequent transport of adhering pollen to ovules by water and/or passive sifting (Cycas) or insect activity (Dioon, and possibly Zamia).