The role of petal spurs and specialized pollinator interactions has been studied since Darwin. Aquilegia
petal spurs exhibit striking size and shape diversity, correlated with specialized pollinators ranging from
bees to hawkmoths in a textbook example of adaptive radiation. Despite the evolutionary significance
of spur length, remarkably little is known about Aquilegia spur morphogenesis and its evolution. Using
experimental measurements, both at tissue and cellular levels, combined with numerical modelling, we
have investigated the relative roles of cell divisions and cell shape in determining the morphology of
the Aquilegia petal spur. Contrary to decades-old hypotheses implicating a discrete meristematic zone
as the driver of spur growth, we find that Aquilegia petal spurs develop via anisotropic cell expansion.
Furthermore, changes in cell anisotropy account for 99 per cent of the spur-length variation in the
genus, suggesting that the true evolutionary innovation underlying the rapid radiation of Aquilegia was
the mechanism of tuning cell shape.