The cells in whorl 1 develop into a whorl of sepals . These form at the lowest level. Collectively they make up the calyx . Whorl 2 forms above the calyx, forming the petals . Whorl 3 develops into the stamens , the male reproductive organs.
Carpels often fuse to form a single structure, which some botanists call the pistil .
This model postulates a group of genes that encode the transcription factors needed to turn on the genes for sepal, petal, etc. development. This occurs at the lowest level of the floral meristem. Cells in which both A and B genes are expressed develop into petals . This occurs at the next higher level. Expression of B and C genes turns on the developmental program to form stamens .