Their genesis is described with flowery eloquence in the Geoponica, a collection of twenty books of agricultural lore compiled during the tenth century: as the goddess Hera was suckling Heracles, drops of milk fell onto the ground and grew into lilies (the milk spilt into the sky became the Milky Way). Over the centuries, lilies — and in particular their blooms — have held mystical appeal, as a symbol of purity or rendered emblematically as the fleur de lys in coats of arms and flags. They've received less attention, however, from science, even if the mechanisms of how lilies and other flowers bloom, and thus how they reveal the bright colours of their petals and adopt their characteristic shape, are not fully understood.

Haiyi Liang and L. Mahadevan have taken a close look at the physical process of blooming and present a compact theory for the movements involved (Proc. Natl Acad. Sci. USA 108, 5516–5521; 2011). The metamorphosis from bud to flower takes, depending on the flower, somewhere between a few hours and several days, suggesting that the process is driven by growth, rather than, for example, by flow of water.

Credit: © ISTOCKPHOTO.COM / LUBOMIR JENDROL

For the common lily Lilium Casablanca (pictured), which Liang and Mahadevan have studied, the process typically spans four and a half days. During that time, pressure builds up inside the bud, as the three inner petals grow inside the three outer sepals that embrace them. A locking mechanism between petals and sepals ensures that the bud remains intact during this period, but once a critical pressure is reached, the flower blooms relatively rapidly, as petals and sepals reverse their curvature and at the same time wrinkle around the edges.

These wrinkles hint at differential growth being part of the blooming process. It had been proposed that the relevant difference in growth rate is between the upper and lower sides of the petals and sepals, and that the midrib has an important role. But by shaving the midrib off petals, Liang and Mahadevan proved that it is not necessary for blooming. Furthermore, they find in their observations and through modelling that growth at the edges alone can induce the shape change.

So it's all about forces and stresses then, rather than mystique and elegance? Not quite, say Liang and Mahadevan. They see their study as “infusing a scientific aesthetic into a thing of beauty”, but also expect, more pragmatically, that these findings could inspire new designs for artificial edge-activated bimorphs.