From smooth and button-size to spiky and giant-size, why are cacti so diverse?

Cacti are among the most diverse plants, from the giant saguaro cactus that can grow up to 18 meters in height, all the way down to the tiny button cactus which is a few centimeters tall. A new study by the Milner Centre for Evolution at the University of Bath has shed light on the mystery as to why there are so many different types.