Dandelions defy gravity by going with the flow
The dandelion is one of nature’s great fliers. When land is cleared it is the seeds of the dandelion, floating on the wind, that first turn barren mud green.
When the eruption of Krakatoa sterilised the entire island, biologists recorded how the plumed seeds of its cousins even blew across the ocean to colonise the volcanic soil.
But how do they do it, given that the parachute that holds them aloft is composed mainly of air? Scientists have now found the answer: dandelions use a form of flight never before recorded. The apparently useless gaps in its parachute are actually crucial, creating a delicate bubble that draws the seed up on thermals. Naomi Nakayama, of the University of Edinburgh, said: ‘‘The dandelion is an outstanding flier. It can fly over 100km.
‘‘But no-one would think of making a parachute that was 90 per cent air space,’’ she said.
Conventionally, scientists model the dandelion seed’s arms as independent cylinders, providing their own drag to slow its descent. But that would not explain why similar methods of dispersal seem to have developed in other plants.
‘‘This independent evolution of the same strategy shows it must be quite an effective way to fly,’’ Nakayama said.
For an experiment published in the journal Nature, she and her colleagues constructed an extremely gentle wind tunnel to see what advantage the bristles gave. They created a barely perceptible updraft, of the kind that sends dandelion seeds floating aloft, and then watched what happened.
Ignazio Viola, a fluid dynamicist, said that what they observed took them by surprise. ‘‘The flow goes through the bristles and while going through, the pressure drops,’’ he said.
‘‘After it has passed through it continues to flow upwards but because the flow further ahead has a higher pressure, it is slowed down until it stops, forming a spinning vortex above the dandelion.’’
What so surprised Viola was that the bristles were designed to keep this vortex stable, continually sucking it upwards and also to interact with each other to maintain the right flow.
The researchers realised it might explain far more than just how dandelions colonise volcanic islands.
‘‘There many similar bristly things, like the wings of very small insects,’’ said Nakayama. ‘‘They don’t have membranes – they can fly with bunch of hairs [by] using the same kind of mechanism. Once you notice this in the dandelion, you start to get the idea, ‘Wow, we should look into all these different examples in nature’.’’