Introduction

The widespread occurrence of centrifugal stamen formation in some members of the group that we now term eudicots (tricolpates) led the influential tropical botanist E. J. H. Corner to suggest that ‘so profound a disturbance in floral development as the reversal of the androecium’ (Corner, 1946, p. 435) must be of considerable phylogenetic significance. Has Corner’s prediction proved correct, when viewed in a modern phylogenetic context? One reason that this question remains relevant today is that there is a considerable shortage of useful morphological synapomorphies that are taxonomically applicable at deep nodes within angiosperms. Most major groupings above the family level are defined by molecular, rather than morphological, differences, making them difficult to teach to students or to recognize in the field. In angiosperm taxonomy, the majority of characters relating to the flower are traditionally employed primarily at the species and genus level, or at best the family level. For example, differences in the degree of fusion between floral organs, both within and between floral whorls (especially petals and stamens), are often used to delimit species and genera. With the exception of syncarpy, characters relating to floral organ fusion are mostly too homoplastic to be taxonomically applicable at deeper nodes in angiosperms.

The character state ‘centrifugal stamens’, as defined by Corner (1946) and others, represents one aspect of centrifugal organ growth. Centrifugal growth in flowers was first documented by early morphologists (Payer, 1857; Eichler, 1875, 1878) and has been reviewed by several authors (e.g. Leins, 1964; Sattler, 1972b; Tucker, 1972, 1984; Rudall, 2010). It represents a complex set of morphogenetic phenomena, rather than a single developmental process (Rudall, 2010). Consistent with the Tree of Life theme of this volume, this chapter explores the taxonomic significance of this feature in the context of recent angiosperm classifications based on molecular data (e.g. Angiosperm Phylogeny Group III, 2009).