Have you ever gone out into your yard and been surprised by a particular plant or tree that seems to have sprung up out of nowhere? It certainly wasn’t planted by you, so how did it get there? There is more than one possible way it may have happened – a seed could have been dropped or excreted by a bird flying overhead, but the seed also may have come directly from a parent plant, even if that plant was miles away. Blow on a dandelion puff and watch how far the seeds float, especially on a windy day, and it’s easy to see how dandelions end up absolutely everywhere. The dandelion is far from the only plant that sends out flying seeds to be spread on the wind, however. This is the focus of a paper entitled “Minimal terminal descent velocity of autorotating seeds, fruits and other diaspores with curved wings.“
“Wind dispersion of seeds is a widespread evolutionary adaptation found in plants, which allows them to multiply in numbers and to colonize new geographical areas,” the researchers explain. “Seeds, fruits and other diaspores spores (dispersal units) are equipped with appendages that help generate a lift force to counteract gravity as they are passively transported with the wind. Seeds with a low terminal descent velocity increase their flight time and the opportunity to be transported horizontally by the wind before reaching the ground. Many plant species are today unfortunately under severe stress and on the verge of becoming extinct due to climate change, timber extraction and agricultural development. The terminal velocity of the seed is a necessary prerequisite for accurate predictions from dispersion models, which can help predict their wind dispersion and influence policy-makers in their conservation and reforestation plans.”
The researchers describe several shapes of windborne seeds and fruits, including single- and multi-winged seeds, many of which are autorotating or autogyrating – think of the whirly seeds that drop from maple trees. In order to better understand the relationship between wing geometry and terminal descent velocity, the researchers 3D printed several models of winged seeds and fruits using a Formlabs Form 2 3D printer. A series of experiments was performed in a large water tank; the 3D printed seeds were immeresed in the water and then released to drift to the bottom. A camera recorded the motion of the seeds from the side of the tank, and images were extracted from the video to track the seed’s lowest point and the wing tips.
The researchers also performed measurements from the top and bottom of the tank, which were found to be in excellent agreement with the measurements taken from the sides. They then developed formulas that showed the optimum shapes for the seeds’ wings.
“Our results point to geometrical shapes of the wings of multi-winged seeds, fruits and diaspores, which provide them with an optimal dispersion potential i.e. maximal flight time, and compares favourably with wing geometries found in the wild,” the researchers conclude. “For whirling fruits to maximize the time they are airborne, their appendages that function as wings must not curve too much or too little.”
Authors of the paper include Richard A. Fauli, Jean Rabault and Andreas Carlson.
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