Floral Evolution, Development, and Convergence: The Hierarchical- Significance Hypothesis

Abstract

In 1984, I proposed a hypothesis that, in flowers, generalized character states are expressed in early stages of ontogeny and that specialized character states are expressed late in ontogeny. The early expressed states (e.g., floral symmetry and number and position of organs) are usually stable and characterize suprageneric levels of hierarchy (subfamilies, tribes), while specialized states (e.g., fusions among organs, heterogeneity within an organ whorl) usually characterize genera or species. This hypothesis is currently being tested in various ways and for various plant groups. The distribution of character states for a character can be seen among a set of related taxa, for such characters as order of organ initiation, floral symmetry, position of the median sepal, and loss of floral organs. Character states for unisexuality can be expressed by differing mechanisms at any of several stages of development, from early stage (organogeny) through midstage (organ suppression or transference of function) to late stage (organ differentiation). Pollination mechanisms generally are expressed late in floral development. Characters that are difficult to interpret in light of the earlier hypothesis include those concerning inflorescence, bracteoles, and petal aestivation. The hypothesis is used to test for convergence in three segregate genera of Cassiinae and supports that convergence has occurred. Floral organogeny differs markedly among the three genera, but the resultant flowers are similar at anthesis.