Saxifragales is a small, but morphologically diverse, clade containing Saxifragaceae, Crassulaceae, Hamamelidaceae, Haloragaceae, Paeoniaceae, and related taxa. Of many diverse vegetative and reproductive characteristics, the range of ovary positions is notable. This variation, which is manifest at levels ranging from closely related species to families, appears to violate hypotheses of Stebbins and Grant that ovary position is an evolutionarily conservative feature. Apparent reversals from inferior ovaries (epigyny) to superior ovaries (hypogyny) also contradict widely-held beliefs in an evolutionary trend towards epigyny; the derivation of hypogyny from epigyny is considered to be exceedingly rare. We use ontogenetic criteria established by Kaplan to assess the homology of ovary position across Saxifragales. SEM and histological study of early floral buds were used to characterize the conformation of the floral apical meristem throughout organogenesis. We find that nearly all clades of Saxifragales are characterized by epigynous development, regardless of apparent ovary position in mature flowers, affirming the conservative nature of the developmental basis underlying ovary position. Apparent variation in ovary position in the clade is due largely to allometric shifts in the relative rate of growth in the upper and lower portions of the ovary late in development, rather than shifts in the conformation of the meristem during the earliest stages of floral development. However, at least one reversal to hypogynous development is documented unequivocally; this represents the first well-documented case of a reversal to hypogyny. We explore evolutionary changes in other floral characters that are correlated with changes in ovary position. Reversal to hypogyny in Saxifragales may be associated with the derivation of apocarpy in the clade, although the limited number of reversals precludes quantitative assessment of the association.

Key words: correlated characters, evolutionary reversal, homology, ontogeny, ovary position, Saxifragales