Bercht. & J.Presl
The Saxifragales are an order of flowering plants. Their closest relatives are a large eudicot group known as the rosids, following the definition of the Angiosperm Phylogeny Group classification system. Saxifragales is one of the groups that compose the core eudicots, which consists of the Dilleniaceae, superasterids and superrosids, the latter consisting of the Saxifragales and rosids.
The Saxifragales order 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 Saxifragales is very morphologically 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 variation in sepal, petal, stamen, and carpel number, as well as ovary position. This degree of diversity makes synapomorphy unclear, but partially fused bicarpellate gynoecia (unfused at least toward the apex), a hypanthium, and glandular leaf teeth are potential synapomorphies. Common traits include flowers that are usually radially symmetric, petals that are free, and seeds with abundant endosperm. 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.
Saxifragales was first described in 1820 by Berchtold and Presl in 1820 as a group of plants, Saxifrageae, with five genera, including Saxifraga, an therefore bear 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.
The order Saxifragales has undergone considerable revision in both placement and composition, since the use of molecular phylogenetics, and the use of the modern Angiosperm Phylogeny Group (APG) classification. They are identified as a strongly monophyletic group.
In the initial APG publication (1998), the Saxifragales were identified within the core eudicots clade but its relationship to other clades was uncertain. Later (2003) the order was described as "one of the major surprises of molecular phylogenetic analyses of the angiosperms", having elements previously placed in three or four separate subclasses based on morphology. This was eventually resolved in the third APG system (2009) placing Saxifragales as a sister group to the all the rosids (Rosidae). This large combination has subsequently been given the name superrosids (Superrosidae), representing part of an early diversification of the angiosperms. Among the rosids, they share a number of similarities with the Rosales, particularly Rosaceae, including a hypanthium, five part flowers and free floral parts.
|Cladogram of Saxifragales relationships among core eudicots|
The first APG classification (1998) placed 13 families with the order Saxifragales:
This was subsequently revised to 15, in the fourth version (2016).
The Saxifragales families have been grouped into a number of informally named suprafamilial subclades, with the exception of the basal split of Peridiscaceae, which thus forms a sister group with the rest of Saxifragales. The two major ones are (Paeoniaceae + the woody clade of primarily woody families) and the "core" Saxifragales (i.e. the primarily herbaceous families), with the latter subdivided into two further subclades, (Haloragaceae sensu lato + Crassulaceae) and the Saxifragaceae alliance.
In the clade Haloragaceae sensu lato (s.l.) + Crassulaceae the genera constituting Haloragaceae s.l. are all small, and APG II (2003) proposed merging them into a single larger Haloragaceae s.l., but transferred Aphanopetalum from Cunoniaceae to this group. The Saxifragaceae alliance represents Saxifragaceae together with a number of woody members of the traditional Saxifragaceae sensu Engler (1930). Within this, APG II (2003) proposed placing the two species of Pterostemon that constitute Pterostemonaceae within Iteaceae, and all subsequent versions have maintained this practice. Thus Saxifragales sensu APG II consisted of only 10 families. The third version (2009) added Peridiscaceae (from Malpighiales), as sister to all other families, but re-expanded Haloragaceae to provide for a narrower circumscription, Haloragaceae sensu strictu (s.s.) to give a total of 14 families. APG IV (2016) added the parasitic family Cynomoriaceae to provide 15 families, although its placement within the order remained unclear.
Of the families included in APG IV, the basal divergence Peridiscaceae underwent radical shifting and recircumscription from 2003 to 2009. Originally, it consisted of two closely related genera, Peridiscus and Whittonia. The APG II system placed the family in Malpighiales, based on a DNA sequence for the rbcL gene from Whittonia. This sequence turned out to be not from Whittonia, but from other plants whose DNA had contaminated the sample. After placement in Saxifragales, it was expanded to include Soyauxia in 2007, and Medusandra in 2009.
In the first of the subclades of the remaining Saxifragales, Paeoniaceae possesses many unique features and its taxonomic position was controversial for a long time, and Paeonia was placed in Ranunculales, close to Glaucidium, prior to transfer to Saxifragales as sister to the woody clade.
In the woody clade, the genus Liquidamber was included in Hamamelidaceae until molecular phylogenetic studies showed that its inclusion might make Hamamelidaceae paraphyletic, and was segregated as a separate monotypic family, Altingiaceae in 2008. Cercidiphyllaceae was for a long time associated with Hamamelidaceae and Trochodendraceae and was often thought to be closer to the latter, which is now in the basal eudicot order Trochodendrales.Daphniphyllum was always thought to have an anomalous combination of characters and was placed in several different orders before molecular phylogenetic analysis showed it to belong to Saxifragales.
In the core Saxifragales, Crassulaceae and Tetracarpaeaceae have been associated with Saxifragaceae, while Penthorum has been associated both with Crassulaceae and Saxifragaceae, before being placed here. Aphanopetalum was often placed in Cunoniaceae, a family in Oxalidales, even though there were good reasons to put it in Saxifragales, and it was subsequently transferred. Haloragaceae was included in Myrtales, before being placed in Saxifragales.
The other "core" group, the Saxifragaceae alliance comprises four families: Pterostemonaceae, Iteaceae, Grossulariaceae, and Saxifragaceae, which have long been known to be related to each other, but the circumscription of Saxifragaceae has been much reduced and Pterostemonaceae submerged as Pterostemon in Iteaceae.
Most of the families are monogeneric. Choristylis is now considered a synonym of Itea, but the addition of Pterostemon, gives Iteaceae two genera.Liquidambar and Semiliquidambar are also submerged into Altingia, making Altingiaceae monogeneric. About 95% of the species are in five families: Crassulaceae (1400), Saxifragaceae (500), Grossulariaceae (150-200), Haloragaceae (150), and Hamamelidaceae (100). The number of genera in each family is:
The phylogeny in this cladogram still has uncertainty as to the exact relationships, and the phylogenetic tree is subject to further revision.Cynomoriaceae, previously placed in Santales or Rosales is included in Saxifragales, but unplaced within it. Li et al.(2019) have slightly different relationships, and also place Cynomoriaceae as the first branch in the Crassulaceae+Haloragaceae s.l. tree, i.e. as sister to those two families.
|Cladogram of Saxifragales families|
|100% maximum likelihood bootstrap support except where labeled with bootstrap percentage|
Monogeneric families are represented by genus names, otherwise the number of genera is in (parentheses)
Cynomorium (Cynomoriaceae) remains unplaced within this tree
The Peridiscaceae (Ringflower family) are a small tropical family of 4 genera and 11-12 species of small trees and shrubs found in the Guiana Shield of S America (2 genera) and West and Central Africa (2 genera), one genus of which (Whittonia) is thought to be extinct. The majority of species occur in the African genus Soyauxia. The name comes from the Greek, peri (around) discos (ring).
The Paeoniaceae (Peony family) consist of a single genus (Paeonia) with about 33 species of perennial herbs and shrubs with showy flowers, found in N America and from the Mediterranean to Japan. They are popular garden ornamentals, cultivated since antiquity, and have been used medicinally. The botanical name comes from its Greek name, paionia, named in turn for the God Pan.
The Altingiaceae (Sweetgum family) consist of a single genus (Liquidambar) with 15 species of trees with unisexual flowers found in N America and Eurasia. Liquidamber is used for its resin and timber, as well being ornamental trees. The nominative genus and family are named after Willem Alting, and Liquidambar for liquid ambar, Arabic for the resin.
The Hamamelidaceae (Witch-hazel family) consists of trees and shrubs with a widespread distribution, 26 genera and about 80-100 species, in five subfamilies, of which the nominative, Hamamelidoideae, contains over 75% of the genera. The species have uses as medicaments, timber and ornamental plants. The family and nominative genus is named for the Greek hamamelis, the wych elm.
The Cercidiphyllaceae (Caramel-tree family) are a small family of deciduous trees found in China and Japan, with a single genus, Cercidiphyllum and two species. The trees are valued for their wood (katsura) and as ornamentals. C. japonicum is the largest deciduous tree in Japan. The name is derived from the Greek words kerkis (poplar) and fyllon (leaf), from a supposed similarity in leaves.
The Daphniphyllaceae (Laurel-leaf family) consist of a single genus, Daphniphyllum, with about 30 species. They are evergreen unisexual trees and shrubs distributed in SE Asia and the Solomon Islands. The dried leaves of D. humile have been used for smoking in Japan and Siberia. The name is derived from the Greek words dafne (laurel) and fyllon (leaf), from a supposed resemblance to the leaves of the former (Laurus nobilis).
The Crassulaceae (Stonecrop family) are a medium size diverse and cosmopolitan family, that are mainly succulent. Genera vary from 7 to 35, depending on the circumscription of the large genus Sedum, and there are about 1,400 species. Uses are diverse, including spices, medicaments and roof coverings as well as ornamental garden and household plants. The name is derived from the Latin, crassus (thick), referring to the fleshy leaves.
The Aphanopetalaceae (Gum-vine family) consists of a single genus of Australian climbing shrubs, Aphanopetalum, which has two species. The name is derived from the Greek words afanos (inconspicuous) and petalon (petal).
The Tetracarpaeaceae (Delicate-laurel family) is a very small Australian shrub family with a single genus, Tetracarpaea and a single species, T. tasmannica. The name is derived from the Greek words tetra (four) and carpos (fruit), referring to the ovaries which have four carpels.
The Penthoraceae (Ditch-stonecrop family) is a very small family of perennial herbs found in eastern N America and E Asia. It consists of a single genus, Penthorum with two species. The name is derived from the Greek word pente (five) referring to the five-part fruit.
The Haloragaceae (Water-milfoil family) is a small family of trees, shrubs, perennial, annual terrestial and aquatic herbs with global distribution. It consists of 11 genera and about 145 species. The largest genus is Gonocarpus with about 40 species. Some Myriophyllum (watermilfoil) species are valued as aquatic and pond plants but may escape and naturalise, becoming invasive. The family and nominative genus, Haloragis are named from the Greek words halas (salt) and rhoges (berries).
The Iteaceae (Sweetspire family) is a widespread small family of trees and shrubs, with 2 genera and 18-21 species, found in tropical to northern temperate regions. The larger genus, Itea (c. 16 spp.) is more widespread whereas Pterostemon (c. 2 spp) is confined to Oaxaca, Mexico. The name is derived from the Greek word itea (willow) for its rapid growth and similar leaf form.
The Grossulariaceae (Gooseberry family) are shrubs that are usually deciduous. The single genus, Ribes, has about 150 species that are widely cultivated for their fruit and also grown as ornamentals. They are found in temperate northern hemisphere regions but extending through the Andes into S America. The family name is derived from the Latin word grossulus (an unripe fig), and Ribes is latinised from the semitic word ribas (acid taste).
The Saxifragaceae (Saxifrage family) are mainly perennial herbs distributed throughout the Northern Hemisphere and Andes, and New Guinea. The family, greatly reduced, includes 35 genera and about 640 species. The largest of these is the type genus Saxifraga (370 species) though several genera are monotypic. They provide foodstuffs and include many ornamentals such as Astilbe. The family and type genus name are derived from the two Latin words saxum (rock), and frango (to break), but the exact origin is unknown, although surmised to be either growing in crevices in rocks or medicinal use for kidney stones.
The Cynomoriaceae (Tarthuth family) consists of a single genus, Cynomorium with two species. They are perennial bisexual herbaceous parasitic plants lacking chlorophyll, from deserts and arid regions of the Mediterranean basin and central Asia. They have been harvested for food, as a dye and in traditional medicine. The name is derived from two Greek words kynos (dog), and morion (penis), for its shape.
Diversification among Saxifragales was rapid, with the extensive fossil record indicating that the order was more diverse and more widespread than an examination of the extant members suggests, with considerable phenotypic diversity occurring early. The earliest fossil evidence is found in the Turonian-Campanian (late Cretaceous), suggesting a minimum age of 89.5 Myr. However, molecular divergence time estimation suggest an earlier time of 102-108 Myr, into the early Cretaceous, for the crown and stem groups respectively. Within the order Saxifragales, the molecular data imply a very rapid initial diversification between 112-120 Myr, with major lineages appearing within 3-6 Myr.
The ancestral state appears to be woody, as in Peridiscaceae and the woody clade, but also ancestral to Grossulariaceae. A number of independent transitions to a herbaceous habit in the ancestors of Crassulaceae, Saxifragaceae and the base of the Haloragaceae-Penthoraceae clade, the other two families in Haloragaceae s.l. remaining woody, while other taxa reverted to a woody habit, especially Crassulaceae. Most of Saxifragaleshave a superior ovary, but some families show frequent transition with inferior or subinferior position, particularly Saxifragaceae and to a lesser extent Hamamelidaceae. Almost all Grossulariaceae have an inferior ovary. The ancestral carpel number is two, with transition to higher numbers, such as four in Haloragaceae s.l. and Peridiscaceae with five in Penthoraceae. The ancestral number for Crassulaceae is two, decreasing to Kalanchoe, where it is synapomorphic, though the most frequent transition in this family is 6-10, but only where stamen number is increased above five. Some Macronesian taxa (Aeonieae) have 8-12, with up to 32 carpels for Aeonium.
The ancestral petal number is five, with three major transitions; 5 to 0, 5 to 4, 5 to 6-10. Increased petal number is seen in Paeoniaceae and Crassulaceae, particularly where stamen number is also increased. Cercidiphyllum + Daphniphyllum, Chrysosplenium and Altingia are examples of the complete loss of petals. The ancestral stamen:petal ratio is 1, with transitions characterising several clades, e.g. Paeonicaceae+woody clade >2, Crassulaceae 2 (but Crassula 1). Overall there has been a decrease over evolution, but independent of a decrease in petal number, so that it is the stamen number that has decreased.
Saxifragales are found worldwide, though primarily in temperate zones and rarely in the tropics. They occupy a wide variety of habitats from arid desert (Crassulaceae) to aquatic conditions (Haloragaceae), with 6 families including North American species that are obligate aquatic (fully dependant on an aquatic environment).
Plants in the order Saxifragales have found a wide variety of uses, including traditional medicines, ornamental household, aquarium, pond and garden plants, spices, foodstuffs (fruit and greens), dyestuffs, smoking, resin, timber and roof coverings.