THE AERODYNAMICS OF PLUMED SEEDS

The aerodynamics of plumed seeds are examined using 4 species of Compositae and Asclepias syriaca (Asclepiadaceae). The hairs comprising the plume of the seeds are modelled as a single long cylinder experiencing an ambient wind velocity equivalent to the measured terminal velocity in still air. The relevant measure of area is the total project area of the plume rather than the plan area of the imaginary "disk' in which the hairs reside. The hairs of Asclepias begin to bend significantly at a wind speed of 0.5-0.9 m s-1, and have an angle of attack of almost 90° at wind speeds of 4.0-5.1 m s-1. Consequently, there is a strongly curvilinear increase in terminal velocity as mass is artificially added to specimens. Asymmetric samaras and plumed seeds are about equally "efficient' at reducing descent velocity when the ratio of mass to projected area is high. At lower ratios of mass to projected area, however, the plumed design is more "efficient' at reducing descent velocity. -Authors