Dandelions
Wind-dispersed plants evolved ingenious ways to lift their seeds. ~ Japanese biologist Naomi Nakayama et al
The flower of a dandelion turns into a mass of seeds known as a dandelion clock. Each seed is suspended from a parachute-like stalk which is easily released by a puff of wind.
The parachute is a bunch of bristles called a pappus. Each pappus carries ~100 filaments, each filament attached to a central point. The pappus provides aerodynamic drag, slowing the descent of each seed. A seed aloft may be wafted kilometers from its origin.
As a pappus falls air flows between the bristles to create a low-pressure vortex. This vortex travels above the pappus yet not attached to it: an invisible but faithful familiar that generates lift and prolongs the seed’s descent.
The key to sustaining lift lies not in the pappus bristles, but in the spaces between them. The bristles together occupy just under 10% of the pappus’s area and yet create 4 times the drag that would be generated by a solid disc of the same radius. Air currents entrained by each bristle interact with pockets of air held by its neighbors, creating maximal drag for minimal expenditure of mass. A pappus’s porosity – the air that it lets pass – determines the shape and dynamics of the low-pressure vortex which keeps a parachute aloft.
Only a precise combination of size, mass, shape, and pappus porosity could generate an optimal vortex ring. As air is appreciably viscous at the scale in which the pappus operates, size is significant. The tiniest insects do not fly with solid wings but swim through the air using paddles. The dandelion parachute is as effective an aerofoil as those in larger seeds that disperse from taller plants, such as the winged seeds of the maple.
