Different patterns of floral development in whorled flowers, exemplified by Apiaceae and Brassicaceae

Studies of the initiation sequence of floral organs are valuable not only in polymerous flowers but also in whorled flowers with a fixed number of organs. They show the sequence of the different whorls as well as the sequence of organs within each whorl. In the whorled flowers of the eudicotyledons there is little floral plasticity in the number and position of floral organs. It will be shown in this study that a number of different developmental pathways can lead to a relatively invariant mature floral morphology. In the flowers of the Apiaceae, the same number and position of floral organs is maintained in spite of major differences in the early sequence pattern. Remarkable are the temporal overlaps between subsequent whorls, i.e., overlap among initiations of different kinds of organs. In the flowers of the Brassicaceae, the uniform floral phyllotaxis is likewise brought about by sequences that vary in detail. Nearly all possibilities in the sequence of the four petals, the two shorter outer stamens, and the four longer inner stamens are realized (e.g., 4 petals --> 2 stamens --> 4 stamens or 2 stamens --> 4 petals --> 4 stamens or 4 petals --> 4 stamens --> 2 stamens, etc.). The following morphogenetical conclusions can be drawn from the detailed sequence studies: (1) A floral apex can produce more than one organ category at the same time. (2) The sequence of whorls must not be strictly acropetal. (3) Organ position and sequence of organ initiation, on the one hand, and organ identity, on the other hand, are regulated on different genetic levels.