The spiral patterns of growth seen in many plants, such as cacti and sunflowers, follow a precise mathematical sequence, but just why they do has always been a mystery. Now the problem has been solved: these patterns minimise the amount of mechanical stress in a growing plant. The spirals are easy to spot. For example, a cactus head is full of bumps that each sport a pointed tip or "sticker". In some cacti, one can start at the centre and draw spirals connecting each sticker to its nearest neighbour. What one gets are three sets of spirals, one with three, another with five, and the third with eight members. These are consecutive numbers in a mathematical sequence known as the Fibonacci series in which each number is the sum of the previous two. As the growing tip puts out new plant material, new primordia appear. "The question is: why do you see Fibonacci numbers of spirals," says Patrick Shipman, a graduate student at the University of Arizona in Tucson, Arizona.