Saxifrage

Saxifraga is the largest genus in the family Saxifragaceae, containing about 465 species of holarctic perennial plants, known as saxifrages^[2] or rockfoils.^[3] The Latin word saxifraga means literally "stone-breaker", from Latin saxum ("rock" or "stone") + frangere ("to break"). It is usually thought to indicate a medicinal use for treatment of urinary calculi (known as kidney or bladder stones), rather than breaking rocks apart.^[2][4]

Description

Most saxifrages are small perennial, biennial (e.g. S. adscendens) or annual (e.g. S. tridactylites) herbaceous plants whose basal or cauline leaves grow close to the ground, often in a rosette. The leaves typically have a more or less incised margin; they may be succulent, needle-like and/or hairy, reducing evaporation.^[5][6][7]

The inflorescence or single flower clusters rise above the main plant body on naked stalks. The small actinomorphic hermaphrodite flowers have five petals and sepals and are usually white, but red to yellow in some species. Stamens, usually 10, rarely 8, insert at the junction of the floral tube and ovary wall, with filaments subulate or clavate. As in other primitive eudicots, some of the 5 or 10 stamens may appear petal-like. and it lives in tundral ecosystems.^[5][8][6]

Taxonomy

A genus of about 465 species. The former monotypic genus Saxifragella has been submersed within Saxifraga, the largest genus in Saxifragaceae, as Saxifraga bicuspidata.^[9][5] Also the genus Saxifragopsis (strawberry saxifrage) was previously included in Saxifraga.^[1]

Subdivision

Based on morphological criteria, up to 15 sections were recognised.^[10] Subsequent molecular phylogenetic studies reduced this to 13 sections with 9 subsections. The former sections Micranthes and Merkianae are more closely related to the Boykinia and Heuchera clades.^[11] Modern floras separate these groups as the genus Micranthes.^[12][6]

The thirteen sections (with subsections) are:^[13]

• Irregulares
• Saxifragella
• Pseudocymbalaria
• Bronchiales
• Ciliatae
• Cymbalaria
• Cotylea
• Gymnopera
• Mesogyne
• Trachyphyllum
• Ligulatae
• Porphyrion
  • Squarrosae
  • Mutatae
  • Oppositifoliae
  • Florulentae
  • Kabschia
• Saxifraga
  • Tridactylites
  • Androsaceae
  • Arachnoideae
  • Saxifraga

Selected species

Formerly placed here

Plants formerly placed in Saxifraga are mainly but not exclusively Saxifragaceae. They include:

• Astilboides tabularis, as S. tabularis
• Bergenia crassifolia, as S. cordifolia, S. crassifolia
• Bergenia pacumbis, as S. ligulata, S. pacumbis
• Bergenia purpurascens, as S. delavayi, S. purpurascens
• Boykinia jamesii, as S. jamesii
• Boykinia occidentalis (Coastal Brookfoam), as S. elata
• Boykinia richardsonii (Richardson's Brookfoam), as S. richardsonii
• Darmera peltata (Indian Rhubarb), as S. peltata
• Leptarrhena pyrolifolia, as S. pyrolifolia
• Luetkea pectinata (Partridgefoot), as S. pectinata
• Micranthes, including:
  • Micranthes integrifolia (wholeleaf saxifrage)
  • Micranthes howellii (Howell's saxifrage),^[14] as S. howellii
  • Micranthes stellaris (Starry saxifrage),^[14] as S. stellaris
• Mukdenia rossii (Mukdenia), as S. rossii

Other "saxifragous" plants

Several plant genera have names referring to saxifrages, although they might not be close relatives of Saxifraga. They include:

• Golden-saxifrages, Chrysosplenium
• Burnet-saxifrages, Pimpinella
• Pepper-saxifrage, Silaum silaus. The name "silaum" comes from the Latin word sil, which means yellow ochre. This refers to the sulphurous yellow colour of the flowers.^[16]

Some plants refer to Saxifraga in their generic names or specific epithets, either because they are also "rock-breaking" or because they resemble members of the saxifrage genus:

• Campanula saxifraga
• Celmisia saxifraga (Benth.) W.M.Curtis
• Cineraria saxifraga DC.
• Dryopteris saxifraga
• Petrorhagia saxifraga – Tunicflower
• Pimpinella saxifraga – Burnet saxifrage
• Ptychotis saxifraga
• Saxifragella
• Saxifragodes
• Saxifragopsis Small

Ecology

Round-leaved saxifrage (S. rotundifolia), whose sticky leaves seem to catch small invertebrates

Saxifrages are typical inhabitants of Arctic–alpine ecosystems, and are hardly ever found outside the temperate parts of the Northern Hemisphere; most members of this genus are found in subarctic climates. A good number of species grow in glacial habitats, such as S. biflora which can be found some 4,000 m (13,000 ft) above sea level in the Alps, or the East Greenland saxifrage (S. nathorstii). The genus is also abundant in the Eastern and Western Himalayan alpine shrub and meadows. Though the archetypal saxifrage is a small plant huddling between rocks high up on a mountain, many species do not occur in such a habitat and are larger (though still rather delicate) plants found on wet meadows.

Various Saxifraga species are used as food plants by the caterpillars of some butterflies and moths, such as the Phoebus Apollo (Parnassius phoebus).^[17]

Charles Darwin – erroneously believing Saxifraga to be allied to the sundew family (Droseraceae) – suspected the sticky-leaved round-leaved saxifrage (S. rotundifolia), rue-leaved saxifrage (S. tridactylites) and Pyrenean saxifrage (S. umbrosa) to be protocarnivorous plants, and conducted some experiments whose results supported his observations,^[18] but the matter has apparently not been studied since his time.

Cultivation

Saxifraga urumoffii at the Royal Botanic Garden Edinburgh.

Numerous species and cultivars of saxifrage are cultivated as ornamental garden plants, valued particularly as groundcover or as cushion plants in rock gardens and alpine gardens. Many require alkaline or neutral soil to thrive.^[7]

S. × urbium (London pride), a hybrid between Pyrenean saxifrage (S. umbrosa) and St. Patrick's cabbage (S. spathularis), is commonly grown as an ornamental plant.^[2] Another horticultural hybrid is Robertsoniana saxifrage (S. × geum), derived from kidney saxifrage (S. hirsuta) and Pyrenean saxifrage. Some wild species are also used in gardening. Cambridge University Botanic Garden hosts the United Kingdom's national collection of saxifrages.^[2]

Award of Garden Merit

The following species and cultivars have gained the Royal Horticultural Society's Award of Garden Merit:-^[19]

Uses

The leaves of some saxifrage species, such as creeping saxifrage (S. stolonifera) and S. pensylvanica,^[50] are edible. The former is a food in Korea^[51] and Japan. The flowers of purple saxifrage (S. oppositifolia) are eaten in Nunavut, Canada and the leaves and stems brewed as a tea.^[52]

Species are also used in traditional medicine, such as creeping saxifrage in East Asia^[53] and round-leaved saxifrage (S. rotundifolia) in Europe.^[54]

Two species—purple saxifrage and creeping saxifrage—are popular floral emblems. They are official flowers for:

• Nunavut, Canada - purple saxifrage^[55]
• County Londonderry, Northern Ireland - purple saxifrage^[56]
• Tsukuba, Japan - creeping saxifrage, "hoshizaki" form (S. stolonifera Curtis f. aptera)^[57]

• 

  Pyrenean saxifrage (S. umbrosa), ancestor to horticultural hybrid saxifrages

• 

  Saxifraga caesia

• 

  Lesser London pride (Saxifraga cuneifolia)

• 

  Saxifraga decipiens

• 

  Irish saxifrage (Saxifraga rosacea)

• 

  Prickly saxifrage (Saxifraga tricuspidata) flowers

References