2023-08-28 06:00:18
A tumbleweed in Victorville, California, in April 2018. JAMES QUIGG/AP Find all the episodes of the series “Plants of genius” here.
The attack, the video of which has been uploaded to YouTube, is impressive. Carried away by a violent wind, hordes in fury break over the meadow, cross a deserted road, and pose a deaf threat. Light horses? No. A pack of dried bushes twirling on the ground.
They are also called “twirling”, or tumbleweed (“rolling herbs”) in the United States. Under this aerial name, these plants can cause real nuisance. In 2018, 150 homes in Victorville, California, were attacked by these prickly balls, reports biologist Katia Astafieff in The plants make their cinema (Dunod, 240 pages, 19.90 euros). And in New Years 2020, in Washington State, five cars and a tractor-trailer were buried under 4.5 meters of this demonic brush.
These wandering ghosts are so ubiquitous in the arid plains of the American West that they have become emblematic of westerns where, like faded ghosts, they spin through these windswept landscapes. They even inspired a song from the 1930s, Tumbling Tumbleweedsrecounting the wanderings of a cowboy.
Major evolutionary advantage
These wanderers belong to “a dozen different plant familiesnotes Germinal Rouhan, scientific manager of the herbarium of the National Museum of Natural History, in Paris. This testifies to a remarkable phenomenon of evolutionary convergence.. All come from plants whose aerial parts, once dried, detach from a block of roots (which remain in the ground) to twirl in the wind.
This original mode of movement – quite rare in the plant world – provides a major evolutionary advantage in an arid environment. Because, during their journey, these plants sow their seeds or their spores, ensuring their dispersal at little cost. They thus maximize the exploration of space, and therefore their chances of colonizing new territories. Some amaranthaceae twirl in this way, like “rolling soda” (Salsola tragus), a very cinegenic plant.
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This dispersal strategy has been selected, during evolution, in species growing in large open spaces, without too many obstacles. “Deserts or semi-deserts, coastal dunes, cultivated stubble, arid steppes with scanty vegetation, cultivated and overgrazed fields » are their favorite playgrounds, notes the naturalist Gérard Guillot on his Zoom Nature website. They also, of course, need strong enough winds to break up the dry plant at ground level and cause it to roll.
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