Phytochrome, a blue-green pigment, is part of a switching mechanism for phototropic responses to light of red and far-red wavelengths. Its active form, Pfr might trigger the secretion of one or more hormones that induce and inhibit flowering at different times of yeat. The main environmental cue for flowering is the length of night, i.e. the hours of darkness, which vary seasonally. Different kinds of plants from flowers at different times of years, depending on their phytochrome mechanism. Products of a set of master gene govern flower formation by selectively acting on meristematic cells of floral shoots.
The study of mutants of common wall cress, Arsbidopsis thaliana, supports an ABC model for flowering. By this model, three groups of master switch genes designated A, B and C control the formation of floral parts. Divisions of meristematic cells at the tip of a floral shoot yeild whorls of cells arranged one, inside the next. In the outermost whorl, only the A genes are activated. These cells give rise to sepals. Moving inward, cells in the next whorl express A and B genes; they give rise to petal. Father in, are cells that turn on B and C genes; their descendants from stamens. Innermost is whorl of cells that switch on C genes only. They give rise to two fused carpels.
Evidence supporting the model comes from plants with mutation in genes of the ABC group. For example, Mutation in one of the A group genes alters the two outermost whorls. The resulting flower has stamens and carpels but no petals. A mutation in the C group changes the innermost whorls. The result is a lovely but sterile flower., i.e. additional petals form in place of the stamens and carpels.
These ABC genes are analogous to the horreotic genes that control the mapping of body parts during the development of animals. ABC genes themselves are controlled by the product of another gene know as Leafy.