Saxifragales

The Saxifragales are an order of flowering plants. Their closest relatives are a large eudicot group known as the rosids by the definition of rosids given in the APG II classification system. Some authors define the rosids more widely, including Saxifragales as their most basal group. Saxifragales is one of the eight groups that compose the core eudicots. The others are Gunnerales, Dilleniaceae, Rosids, Santalales, Berberidopsidales, Caryophyllales, and Asterids. Saxifragales have an extensive fossil record. The extant members are apparently remnants of a formerly diverse and widespread order. The Saxifragales order, as it is now understood, is based upon the results of molecular phylogenetic studies of DNA sequences. It is not part of any of the classification systems based on plant morphology. The group is much in need of comparative anatomical study, especially in light of the recent expansion of the family Peridiscaceae to include Medusandra, a genus that before 2009 had usually not been placed in Saxifragales. The order is divided into suprafamilial groups as shown on the phylogenetic tree below. These groups are informal and are not understood to have any particular taxonomic rank. Saxifragales contain about 2470 species. These are distributed into 15 families, or into 12 families if Haloragaceae sensu lato is recognized as a family consisting of Haloragaceae sensu stricto, Penthorum, Tetracarpaea, and Aphanopetalum. About 95% of the species are in five families: Crassulaceae (1400), Saxifragaceae (500), Grossulariaceae (150–200), Haloragaceae (150), and Hamamelidaceae (100). Most of the families are monogeneric. The number of genera in each family is: Some authors do not recognize Choristylis as a separate genus from Itea. Similarly, some authors sink Liquidambar and Semiliquidambar into Altingia. Thus Altingiaceae and Iteaceae are monogeneric in some classifications. Within the Saxifragales is a suprafamilial group known as the Saxifragaceae alliance. It comprises four families: Pterostemonaceae, Iteaceae, Grossulariaceae, and Saxifragaceae. These have long been known to be related to each other, but the circumscription of Saxifragaceae has changed dramatically. It is now a much smaller family than it had been. Crassulaceae and Tetracarpaeaceae have long been associated with Saxifragaceae. Penthorum has usually been associated with Crassulaceae, but sometimes with Saxifragaceae. Two members of the core Saxifragales had sometimes been placed near Saxifragaceae, but usually elsewhere. Aphanopetalum was often placed in Cunoniaceae, a family in Oxalidales, even though there were good reasons to put it in Saxifragales. Aphanopetalum is now excluded from Cunoniaceae. Haloragaceae was often thought to be a family in Myrtales, but it is no longer included in that order.