Sepals form first, followed by petals, and then by stamen, and finally by carpels.
Below is an image of a wild type Arabidopsis thaliana flower, and a diagram of the flower outlining the four whorls. These scientists identified homeotic mutations that resulted in the misplacement of floral organs. Three classes of mutations were identified in Arabidopsis . Class A genes are required in whorls 1 and 2, class B genes are required in whorls 2 and 3, and class C genes are required in whorls 3 and 4. Both class A and class B genes are required in whorl 2, and both class B and class C genes are required in whorl 3.