Saxifragales is an order of flowering plants in the superrosid clade of the eudicots. It contains 15 families and around 100 genera, with nearly 2,500 species. Well-known and economically important members of this order include saxifrages (after which the order is named), blackcurrants, redcurrants, gooseberries, peonies, liquidambars, witch-hazel, Persian ironwood, katsura, jade plant, houseleeks, and water milfoil. Of the 15 families, many are small, with eight of them being monotypic (having only a single genus). The largest family is the Crassulaceae (stonecrops), a diverse group of mostly succulent plants, with about 35 genera. Saxifragales are found worldwide, primarily in temperate to subtropical zones, rarely being encountered growing wild in the tropics; however, many species are now cultivated throughout the world as knowledge of plant husbandry has improved. They can be found in a wide variety of environments, from deserts to fully aquatic habitats, with species adapted to alpine, forested or fully-aquatic habitats. Many are epiphytic or lithophytic, growing on exposed cliff faces, on trees or on rocks, and not requiring a highly organic or nutrient-dense substrate to thrive. Globally, the saxifrages have a wide variety of uses by humans, ranging from textiles and timber to foodstuffs. Several families—such as the aforementioned Crassulaceae—and genera are of significant commercial importance in some countries and economies, being cultivated on a large scale for sale as ornamental plants. Apart from ornamentals, another highly prized group are the Grossulariaceae (currants and gooseberries), particularly blackcurrants, redcurrants and white currants. Overall, the order is extremely diverse, encompassing numerous trees, shrubs, perennial herbs and succulent plants, as well as aquatic and semi-aquatic species. The order's high degree of diversity, in terms of vegetative and reproductive traits (and sheer amount of species), can make it challenging to find any common or unifying features amongst the respective genera. 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 had been 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 phase of the late Cretaceous, dating to about 90 million years ago (Myr). However, molecular studies may suggest an older origin, from the early Cretaceous (102–108 Myr), with rapid and early diversification to more modern forms.
Description 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). The flowers demonstrate major variations in sepal, petal, stamen, and carpel number, as well as ovary position (see Biogeography and evolution).
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.
Taxonomy Saxifragales is a relatively small angiosperm order, having only 15 families, about 100 genera and about 2,470 species.
History
Saxifragales was first described in 1820 by Berchtold and Presl as a group of plants, Saxifrageae, with five genera, including Saxifraga, lending their names as the botanical authority (Bercht. & J.Presl). At times, that authority has also been given to Dumortier, due to a later publication (1829). Dumortier first used the word Saxifragaceae. By the time of John Lindley's The Vegetable Kingdom (1853), the term Saxifragales was in use, which Lindley called an Alliance, containing five families. Later, the Saxifragales were placed in the angiosperm class Dicotyledons, also called Magnoliopsida.
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