Saxifragaceae is a family ofherbaceousperennialflowering plants, within thecore eudicotorder Saxifragales. Thetaxonomy of the family has been greatly revised and the scope much reduced in the era ofmolecular phylogenetic analysis. The family is divided into tenclades, with about 640 known species in about 35 acceptedgenera. About half of these consist of a single species, but about 400 of the species are in the type genusSaxifraga. The family is predominantly distributed in the northern hemisphere, but also in the Andes in South America.
Species areherbaceousperennials (rarelyannual orbiennial), sometimessucculent orxerophytic, often with perennatingrhizomes. Theleaves are usually basally aggregated in alternate rosettes, sometimes oninflorescence stems. They are usually simple, rarely pinnately or palmately compound. Their margins may be entire, deeply lobed, cleft, crenate or dentate and petiolate with stipules.[1][2]
The inflorescences are bracteate racemes or cymes. Theflowers arehermaphroditic (bisexual), rarely unisexual (androdioecious),actinomorphic (rarelyzygomorphic). Theperianth is placed on a hypanthium that may be free or may be partly fused with theovary (which is then semi-inferior). There are usually fivesepals, but there may be three to ten, fused with the hypanthium, occasionally petaloid. Thepetals are clawed, sometimes cleft at the tip or finely dissected. Flowers have five to tenstamens, free and opposite the petals, with the anthers usually basifixed and opening by lengthwise slits. The ovary is inferior to semi-inferior with two (sometimes three)carpels usually fused at the base, sometimes free, each topped with stylodium and capitate stigma. Theovules are few to many, with axile or parietal placentation and two to threestyles. Thefruit is usually a septicidal capsule or follicle, with numerous smallseeds.[1][2]
Saxifragaceae has long been considered in a pivotal position in the evolution of angiosperm lineage, located in an ancestral "Saxifragalean stock". Historically the Saxifragaceae have included many verymorphologically disparatetaxa in systems based on morphology alone, and has been very difficult to classify and characterizephenotypically. Consequently many different classifications have been published, differing considerably in both the relationship of the family to other angiosperms, and its internal structure, for instance the systems ofCronquist (1981),Dahlgren (1983),Engler (1890),Hutchinson (1973), Schulze-Menz (1964),[3]Takhtajan (1980) andThorne (1992) (for a history, seeMorgan & Soltis (1993). In its broadestcircumscription, it included 17 subfamilies.[3] This construction is referred to as Saxifragaceaesensu lato (s.l.). Within those subfamilies, the large majority of genera (30) were located within subfamily Saxifragoideae, the core group, with all but two of the remainder only having 1-3 genera.[4][5]
The circumscription of the family has changed considerably in recent years, in large part due tomolecular phylogenetic work showing the familys.l. to bepolyphyletic, and probably represents the most extreme example, with at least 10 evolutionary lines. Consequently the circumscription has been considerably reduced, with many of the subfamilies being either elevated to separate families, or placed as components of other families, often quite distant. For instance subfamily Parnassioideae was raised to the level of familyParnassiaceae, and eventually a subfamily ofCelastraceae (orderCelastrales). Similarly the Hydrangoideae is now the familyHydrangeaceae (orderCornales).[4][6]
The reduced Saxifragaceae is distinguished by being referred to as Saxifragaceaesensu stricto (s.s.), corresponding to the Saxifragoideae a natural monophyletic group.[7] Historically, the Saxifragaceae has been placed in either theordersRosales orSaxifragales. This reduced Saxifragaceae, corresponding to subfamily Saxifragoideae and its 30 genera, is now placed within the Saxifragales. There, with three other subfamilies (Ribesoideaee, Iteoideae and Pterostemonoideae) it forms the Saxifragaceae alliance, while Penthoroideae and Tetracarpaeoideae are closely related within the core Saxifragales as shown in thecladogram. The remaining subfamilies are all transferred to more distant orders within therosid andasterid clades.[8][4][6]
100%maximum likelihoodbootstrap 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
Numerous attempts have been made to subdivide Saxifragoideae (Saxifragaceaes.s.). These have included dividing the family by the placentation of the ovules, as either parietal (e.g.Heuchera) or axile (e.g.Saxifraga). None of these has been supported by molecular data.[1][7]
Molecular data indicate that the family can be considered as a number of informalclades, with two main lineages, saxifragoids and heucheroids and further subdivision of heucheroids into nine subclades or groups:[7][8][1]
Saxifragaceaes.s. has about 33–35 genera and about 640 species. About half of the genera (18 of 33) aremonotypic, butSaxifraga has about 400 species, and has generally been divided into sections.[7][11][2][8]
Thecrown group of Saxifragaceaediversified at about 38 Mya (Mid–LateEocene), with the two main lineages diversifying arising at about 30 Mya (Late Miocene/EarlyOligocene). The present day heucheroid diversity dates later than the Miocene (see sublabels in Cladogram II). Ancestral Saxafragaceae emerged in either East Asia or Western North America, with subsequent dispersal West to Europe and south to South America. From the Eocene to the late Miocene these ancestral land masses were joined by theBering Land Bridge facilitating plant migration.[8]
The family and type genus name are derived from the two Latin wordssaxum (rock), andfrango (to break), but the exact origin is unknown, although surmised to refer to either growing in crevices in rocks or medicinal use forkidney stones.[2]
Primarily Northern hemisphere temperate and arctic regions, and also tropical montane, including the Americas, Europe, North Africa (including montane Ethiopia) and temperate and subtropical Asia toLuzon and New Guinea. In the Americas it extends south to central Mexico and the Andes toTierra del Fuego.Centres of diversity are western N America, East Asia and the Himalayas. The largest concentration of genera are in the Pacific North West. SomeSaxifraga andChrysosplenium arecircumboreal. Others havedisjunct distributions between E Asia and N America, while other taxa have separate narrow distributions in southern S America. Some species are found in wet woodlands, swamplike conditions and wet cliff edges.[2][8][1]
Schulze-Menz, G K (1964). "Saxifragaceae". InMelchior, Hans; Werdermann, E (eds.).A. Engler's Syllabus der Pflanzenfamilien: mit besonderer Berücksichtigung der Nutzpflanzen nebst einer Übersicht über die Florenreiche und Florengebiete der Erde. (2 vols.) (12th ed.). Berlin:Gebrüder Borntraeger Verlag. pp. 201–206.(see alsoSyllabus der Pflanzenfamilien)
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