The Saxifragales (saxifrages) are an order of flowering plants (Angiosperms). They are an extremely diverse group of plants which include trees, shrubs, perennial herbs, succulent and aquatic plants. The degree of diversity in terms of vegetative and floral features makes it difficult to define common features that unify the order.
In the Angiosperm Phylogeny Group classification system, the Saxifragales are placed within the major division of flowering plants referred to as eudicots, specifically the core eudicots. This subgroup consists of the Dilleniaceae, superasterids and superrosids. The superrosids in turn have two components, rosids and Saxifragales. The Saxifragales order has undergone considerable revision since its original classification, which was based purely on plant characteristics. The modern classification is based on genetic studies using molecular phylogenetics. There is an extensive fossil record from the Turonian-Campanian (late Cretaceous) time, about 90 million years ago (Myr). However, molecular studies suggest an earlier origin in the early Cretaceous (102–108 Myr) with rapid early diversification to more modern forms.
The order Saxifragales consists of 15 families, about 100 genera and 2,470 species. Of the 15 families, many are small, with 8 having only a single genus, the largest family being the Crassulaceae (stonecrops) with about 35 genera. Saxifragales are found worldwide, though rarely in the tropics, and in a wide variety of habitats from desert to aquatic. They also have a wide variety of uses, from timber to foodstuffs and ornamental plants. Apart from ornamentals, the major economically important group is the Grossulariaceae (currants and gooseberries), particularly blackcurrant.
The order Saxifragales is extremely morphologically diverse (hyper-diverse). It includes trees (e.g. witch hazel, witch alder in Hamamelidaceae), fruit bearing shrubs (e.g. currants, gooseberries in Grossulariaceae), lianas, annual and perennial herbs, rock garden plants (e.g. saxifrage in Saxifragaceae), ornamental garden plants (e.g. peonies in Paeoniaceae), succulents (e.g. stonecrop in Crassulaceae) and aquatics (e.g. watermilfoil in Haloragaceae).[2] The flowers demonstrate major variations in sepal, petal, stamen, and carpel number, as well as ovary position (see Biogeography and evolution).[3][4]
This degree of diversity makes defining synapomorphy (derived common characteristics) for the group extremely difficult, the order being defined on the basis of molecular affinity rather than morphology. However, some characteristics that are prevalent (common traits) represent potential or putative synapomorphies based on ancestral states. These include flowers that are usually radially symmetric and petals that are free. The gynoecium (female reproductive part) generally consists of two carpels (ovary, style and stigma) that are free, at least toward the apex (partially fused bicarpellate gynoecium) and possess a hypanthium (cup shaped basal floral tube). In the androecium (male reproductive part), the stamen anthers are generally basifixed (attached at its base to the filament), sometimes dorsifixed (attached at centre) (see Carlsward et al (2011) Figure 2). Other commonly occurring features are fruit that is generally follicular (formed from a single carpel), seeds with abundant endosperm surrounding the embryo and leaves with glandular teeth at their margins (glandular dentate, see image). Within the Saxifragales, while the families of the woody clade are primarily woody, the primarily herbaceous families of Crassulaceae and Saxifragaceae exhibit woody features as a secondary transition.[5][6][7]
Saxifragales is a relatively small angiosperm order, having only 15 families, about 100 genera and about 2,470 species.[3]