Chimney Sweep

the extraordinary flight of the dandelion

Dandelion seeds float thanks to a previously unknown source of buoyancy.

The English poet and artist William Blake was not a fan of Isaac Newton’s reductionism. True discovery and thus knowledge, Blake insisted in his poem “Auguries of Innocence”, was to be found in everyday life, where a world could be seen in a grain of sand and “Heaven in a wild flower”.

Today we know exotic states of matter that can slow down the enormous speed of light to a mere sprint. And astronomers have discovered more than 3,800 planets in more than 2,800 distant star systems: an astounding rate of discovery since the first confirmed discovery of a planet orbiting another Sun-like star was not made until 1995.

None of this should blind us to the fact that, as Blake suggested, some of the most surprising discoveries come from the world of the familiar. Nobody has visited an exoplanet, but most people know what a dandelion looks like. This flower (Taraxacum officinale) is common all over the world. And as some children are delighted to discover, the flower (actually hundreds of tiny florets) transforms into a mass of seeds known as the dandelion clock when a dandelion sets seeds. Each seed hangs on a parachute-like stem – easily released with a breath.

The parachute is a bundle of bristles called a pappus. Each pappus carries about 100 filaments, each attached to a central point, similar to the head of a chimney brush. Just like a parachute, it increases air resistance, slowing down the descent of each seed and allowing it to hover in the air for miles from the mother plant. We know that much.

Here’s the surprising part – the mechanism by which this spread was previously unknown. As researchers write in Nature this week (C. Cummins et al. Nature https://doi.org/10.1038/s41586-018-0604-2; 2018), the bristles are arranged in such a way that air flows between them when the pappus falls it and creates a low pressure vortex, like a smoke ring. This vortex moves above the pappus and yet is not attached to it, an invisible but loyal confidante that creates buoyancy and prolongs the descent of the seed.

The key is not in the bristles of the pappus, but in the spaces between them. When projected onto a disc, the bristles together take up almost 10% of the area of ​​the pappus and still generate four times the air resistance that a solid disc with the same radius would create. The study shows that air currents entrained by each bristle interact with air bubbles held by their neighbors, creating maximum drag with minimal bulk. The porosity of the papus – a measure of the amount of air it lets through – determines the shape and type of the low-pressure vortex.

All falling objects, from feathers to cannon balls, create turbulence. However, it takes a rare combination of size, mass, shape and, above all, porosity for the pappus to create this vortex ring. Size is also especially important as the air is noticeably viscous from the point of view of something as small as a pappus. On such a scale, a parachute made from a bundle of bristles is as effective as the wing profile found in larger seeds scattered from larger plants – like the winged seeds of the maple. Likewise, the smallest insects do not fly with fixed wings, but rather swim through the air with “paddles” made of bristles.

It’s an example of how evolution can produce ingenious solutions to the most difficult problems like seed distribution. There are many unknown things that are smaller than atoms, or larger than galaxies, or billions of years away. But there are secrets of things that we take for granted – things on a human or near-human level – that seem all the more precious to them. Even the sky in a wild flower.

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