Evolutionary Lability of Symmetry in Early Floral Development.

Floral symmetry is the product of organogeny plus subsequent organ development. In terms of floral apical meristem activity, radial symmetry results from equilateral organ initiation on a radially symmetrical apical meristem followed by equilateral development among like organs. Bilateral symmetry results from restriction of organ initiation to two planes that are perpendicular to one another. Zygomorphy can result either from zygomorphic organogeny (successive from one side of the flower) or from differential enlargement among like organs in a flower that has radial symmetry in earlier stages. Asymmetry results in rare instances when (a) the floral apex is asymmetric and initiates organs in asymmetric order or (b) when the floral apex is radially symmetrical but when differentiation among the organs of a whorl produces unlike structures. Evolutionary lability in symmetry has usually not involved the shape of the floral apex, except in a few examples, such as Labichea in Fabaceae. More frequently, such lability has resulted from postinitiation events, such as selective organ differentiation or heterochrony.