Seed size, dispersal, and aerodynamic constraints within the Bombacaceae

The aerodynamic constraints operating on the wind-dispersed, drag-producing diaspores of several species of the tropical family Bombacaceae were examined. Kapok (the drag-promoting appendage) was best characterized as a moderately flattened hemisphere impervious to air movement. The kapok shape was not isometric: kapok planform area was proportional to the kapok mass raised to the power 0.52 rather than to the 0.67 expected from isometry. Thus, necessarily, terminal velocity rises with seed mass much faster in this group than among taxa with winged seeds. Further, we derived the optimality argument to show that the kapok mass ought to be about 50% of the total diaspore mass (seed plus kapok). While seven of eight species had a lower kapok investment than this, and none were especially close to the theoretically optimal value, nonetheless the kapok investment values were hardly draws from a random distribution. Finally, the kapok fibers of these Bombacaceae species begin to bend at a drag of about 0.005 N, and this sets an upper limit on the efficient diaspore size of about 250 mg for the seed mass. This latter value is similar to the mass of the largest seed we know of in this family.