"To know the rose, someone uses geometry and another uses the butterfly," wrote Paul Claudel in L'Oiseau noir dans le soleil levant (The Black Bird in the Rising Sun, 1927). In other words, in botany as elsewhere, it all depends on your point of view. The perspective of the geometer – less whimsical but more methodical – is clearly the one that has been chosen by the laboratory that described, in the May 1 issue of Nature, how the petals of this floral jewel form.

This May, rose lovers can see it for themselves: in its youthful prime, the rose displays petals with smooth, curved edges. But as the flower unfurls the folds of its purple (or white, yellow or red) dress, its petals undergo a surprising metamorphosis. They take on a polygonal shape with sharp points, called "cusps," whose number and definition increase as the rose matures.

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