You can find any information about Fibonacci on the web. This mathematic discovered that Nature uses any number series to optimize the space, so, you will find a serie that is repeated. 1, 1, 2, 3, 5, 8, 13...
Here I post two photos, the first show how the citrus optimize the space and use the number 5, the next is a "racimo de pomelos" grapefruit bunch with 8 pomelos.
(http://img176.imageshack.us/img176/4462/albertonc0.jpg)
(http://img59.imageshack.us/img59/4048/dsc04055ge2.jpg)