The rosid clade is a well supported, but the least resolved major clade of core eudicots, containing more than a quarter of all angiosperm species (Schönenberger and von Balthazar,2006). Saxifragales are generally linked with rosids, although support is not high and the order has been associated with caryophyllids (e.g. Soltis et al., 2003). It is clear that Saxifragales represent an ancient early diverging lineage in the core eudicots (Soltis et al., 2005; Magallón, 2007). A recent analysis incorporating a high number of genes has clarified the internal relationships of rosids with the recognition of two main clades, fabids (or Fabidae) and malvids (or Malvidae), and the inclusion of Saxifragales as a basal order and Vitaceae as sister to the rosid clade (Wang et al., 2009). Figure 10.1 represents a phylogenetic tree of the rosids based on Wang et al. (2009).

Saxifragales

Saxifragales sensu APG (2003) comprises an assemblage of highly diverse families (about ten), including core saxifrages and allies and part of the former Hamamelidae of Cronquist (1981) with strongly reduced flowers.

The general flower morphology fits well with the syndrome found in rosids. Flowers tend to be generally pentamerous or tetramerous and share a hypanthium, often with half-inferior ovary. Obdiplostemony is a common feature in families with two whorls of stamens (e.g. Saxifragaceae, Haloragaceae, Crassulaceae: Ronse De Craene and Smets, 1995b). Diplostemony is also reconstructed as the ancestral character state in the Saxifragales by Soltis et al. (2005).