The blossoming of flowers, a moment of physics

Qwhen the James-Webb telescope gradually activated, at the beginning of 2022, the most critical period was the deployment of five superimposed veils of aluminized polymer constituting the thermal shield which protects the instrument from solar radiation. As vast as a tennis court for thicknesses of a few tens of microns, this fragile structure constituted a serious challenge for the engineers. It was a question, in fact, of tightening it folded up in the fairing of Ariane-5, and of sending this gigantic kite to flourish a million kilometers from the Earth, using a hundred motors, cylinders and expanders, operated in an ultra-precise ballet.

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This birth of a new genus resembles the blooming of flowers in our gardens, a banal phenomenon after all, but which, by comparison, seems to be a miracle: how, for example, can the delicate structure of a flower deploy without damage from the button? Is there a program that would run in a millimetric sequence and be a sort of biological equivalent of a NASA checklist?

Physicists have long been interested in these questions of the growth of living organisms, and have taken a relevant look at this matter, which mainly concerns biology, by showing that a large number of characteristics of the growth process and the final forms of flowers are often the emergent result of subtle physical phenomena. For example, in an important article from 2012, two Harvard researchers studied the growth of the flower of the oriental lily “Casablanca”, whose floral apparatus is quite simple. The petals of this flower are initially enclosed in a tapered capsule-shaped bud that curves them inward. After hatching, they unfold, completely reversing their curves and adopting that slender, open form which makes flowers so much resemble landing strips for bees in need of pollen.

Mechanical deformation

Previous work had hypothesized that the midrib of each petal, which is woody, acts as a guide to help the flower find its final shape, much like a tent stiffener. However, the physicists show here that these veins do not play any decisive role, and that one obtains a barely modified flower if they are removed with care at the very beginning of growth. According to their observations, supplemented by theoretical and numerical modeling, the main cause of the unfolding-turning over of each petal comes from a greater cell growth rate on the edge than in the center.

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