From a mismatch in growth, a flower blooms

Physicists reveal forces at work in the blossoming of a lily

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FLOWERS ABLOOM Physicists have described how speedy growth at the outer margins of lily petals cause the edges to ruffle and the flower to unfurl. PNAS

While wandering lonely as a cloud, William Wordsworth should have stopped to wonder how those daffodils bloomed.

Not content to just watch flowers dance in the breeze, Harvard physicists have described for the first time how flowers generate the forces needed to curl open come springtime. In the Casa Blanca lily hybrid, this poetic blossoming is driven by skewed growth at the edges of petals, the team reports online March 21 in the Proceedings of the National Academy of Sciences.

Over four and a half days, the lily’s young buds slowly suck up water, growing until they’re ready to explode. The petals and sepals — the outer, greener portion of a flower — gradually invert, then peel open like a banana and form a blossom.

When it comes to plant motion, the slow emergence of the lily flower is a far cry from the quick closing of the Venus flytrap, says Jan Skotheim, a biologist at Stanford University. “In the blooming lily, you don’t have an explosive snap,” he says. Skotheim and L. Mahadevan, a Harvard physicist and coauthor of the new study on lilies, discovered the biophysical mechanism underlying the flytrap’s snare in 2005.

But both blooming and snapping work because plants build up “instabilities,” Skotheim says. Instabilities that shape roots, stems and lily blossoms often form when certain cells elongate more than others. Too much growth causes strain, which bends thin tissues like a fish tugging on a fishing pole.

In the studies of the lily, exactly which cells were tugging on which wasn’t clear. The Harvard team’s first clue to the mechanism was that the outer margins of petals and sepals ruffled during blooming, while inner surfaces stayed smooth. Those wavy patterns hinted that cells might be growing faster at the edges, similar to adding slack to a rope. That excess growth could, potentially, coax the petal to go from curving in inside the bud to curving out. “Because it’s only growing at the edge and not the middle,” Skotheim says, “you get a mismatch of strain.”

The Harvard team surgically removed the edges of lily petals and sepals, and found that the remaining flower parts didn’t curl out with their usual elegance. The researchers also developed a mathematical model to demonstrate how extra edge strain could warp thin materials like flower petals. This strain doesn’t only open petals up but also curls their edges up like a smile. This mechanism may hold for other lilies, suggests study coauthor Haiyi Liang. “But beyond that, say roses, we are not sure,” says Liang, who is now at the University of Science and Technology of China in Hefei.

Mathematical models have been a boon for researchers studying the inner workings of plants, says Wendy Silk, a plant physiologist at the University of California, Davis. Models similar to those employed in this study have shown how grass blades twist to protect themselves from the sun and how kelp fronds develop ruffled edges, too. To explore the processes that trigger blooming or grass twisting, scientists first need to grasp the basic architectural rules, she says.

Numbers work like this may have applications down the road, Skotheim says. But he thinks it’s exciting just to learn something new and basic about something so familiar. “You want to understand why flowers look the way flowers do,” he says.

In other words, Wordsworth may have been missing out; there’s a lot more to flowers than just their superficial appearance. “Infusing a scientific aesthetic into a thing of beauty only enhances our appreciation of it,” Mahadevan says. “This is what we try to do as scientists.”

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