The Metamorphosis of Flowers.

One of the unifying theories of plant biology is that the variety of plant forms are simply different modifications of a common growth plan. Different permutations of a few key features of plant growth can generate a bewildering array of seemingly distinct forms. There is perhaps no better illustration of this than the comparison of a flower and a shoot. The idea that these two apparently different structures might be fundamentally equivalent goes back to Goethe’s treatise on metamorphosis, published in 1790. He concluded, “Flowers which develop from lateral buds are to be regarded as entire plants, which are set in the mother plant, as the mother plant is set in the earth” (Goethe, 1790). In equating flowers and shoots, four key assertions need to be made. First, the different parts of the flower (sepals, petals, stamens, and carpels) are equivalent to the leaves of a shoot. Second, the organs of both shoot and flower are separated by internodes, but in the case of the flower these are so short as to be barely visible. Third, the organs of shoot and flower usually have a distinct phyllotaxy, or arrangement around the central axis. Finally, the indeterminate growth that so characterizes a shoot is suppressed in the case of a flower, both apically, because it eventually stops producing organs around the central axis, and laterally, because branches do not normally alise in the axils of floral organs. The comparison of flower and shoot therefore highlights four key variables: organ identity, internode length, phyllotaxy, and determinacy. The numerous forms and habits of plants simply reflect different variations and permutations of these four fundamental aspects of growth. What is their developmental basis? The development of shoots and flowers depends on the behavior of meristems. On the periphery of meristems, groups of cells are partitioned off to form either secondary meristems or organ primordia. Phyllotaxy depends on the precise pattern in which partitioning occurs. Determinacy also reflects partitioning, most importantly whether it favors meristems or primordia. Organ identity depends on the developmental characteristics of primordia. Finally, the growth of regions between primordia determines internode length. In the last few years, the systematic analysis of flower development has allowed genetic control of these key aspects of plant development to