Mechano-eco-evo of the Common Dandelion: What the Notorious Weed Tells Us About Plant Dispersal Strategies

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• Dr Naomi Nakayama, Imperial College London
• Thursday 24 February 2022, 13:00-14:00
• Online.

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Hairs, whiskers, stems… plants are made of slender cylinders of various scales. Why are they so prevalent? We have started to answer this question, with an iconic example of hairy structures from nature: the dandelion parachute. The parachute (botanically named pappus or plume) is a bundle of about 100 filamentous organs, which is positioned above the seed. The pappus has over 90% empty space. Therefore, the fluid dynamic mechanism of a parachute cannot apply to pappus, and their flight mechanism remained unknown. By visualizing the air flow around the pappus in a bespoke vertical wind tunnel, we have found a previously unobserved type of whirlwind (vortex). It is a ring-shaped vortex that is physically separated from the pappus but stays at a constant distance downstream of the body. Such a separated vortex ring was first time to be observed, and it is likely to help the seed stay afloat. The formation of the vortex ring is dependent on the porous design of the pappus. Interestingly, the pappus porosity changes depending on the environment; the pappus closes reversibly depending on the moisture levels. In wet conditions, the dandelion seeds don’t disperse as much. This may be a form of ‘informed dispersal’ to tune their movement only when necessary. We then studied the mechanism behind the moisture-dependent morphing of the pappus, focusing on the structure at the base of the pappus bristles.

With this example of the dandelion diaspore, I will illustrate how biomechanical investigation embedded within the ecological context can provide new insights into key evolutionary innovations in plant structures.

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