To big doubles full of petals like this?
(photo from jungle seeds)Well, it starts with flowers like this:
Look closely (click on the image to expand it) in the center of each flower, and you can see little tiny specks of extra petals -- they're actually called "petaloids"Here is what they look like carefully pulled out of each flower: (I highly recommend clicking on the photo to see them much larger)
At far right is a normal stamen -- the male part of the flower that produces pollen. The others are in various stages of conversion into petals. Keep going with this, and you get the full, frilly, double flowers you see in the second pictures. It only takes a very small change in the expression of a single gene to switch an anther to a petal -- so double versions of flowers pop up randomly all the time in nature due to random mutations, or in the genetic confusion of hybrids between species (which is what lead to the petaloids in these pictures). In the wild, of course, double flowers die out because they are usually virtually sterile -- and even when they aren't how is a bee supposed to get in there and pollinate them? But in the garden, we treasure them and keep them alive -- so much so that many people don't even realize that the classic rose:
Is a double version of a very simple flower with only five petals like this: