saxifrage

Saxifraga
Saxifraga cochlearis
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Eudicotyledoneae (unranked): Core eudicots Order: Saxifragales Family: Saxifragaceae Genus: Saxifraga L.
Sections
Ciliatae Cotylea Cymbalaria Gymnopera (London Pride saxifrages) Heterisia Irregulares Ligulatae (Silver saxifrages) Merkianae Mesogyne Micranthes Odontophyllae Porphyrion Saxifraga (Mossy saxifrages) Trachyphyllum Xanthizoon
Synonyms
Boecherarctica Á.Löve Cascadia A.M.Johnson Micranthes Haw. Zahlbrucknera Rchb. and see text[1]

Saxifraga is a genus containing about 440 known species of Holarctic perennial plants, making it the largest genus in the family Saxifragaceae. They are commonly called saxifrages or stone-breakers. The Latin word Saxifraga means literally "stone-breaker", from Latin saxum "rock" or "stone" + frangere "to break". This refers to certain saxifrages' ability to settle in the cracks of rocks, which they may in fact wear down by bioerosion to the point of splitting.

The genera Saxifragopsis (saxifragopsises), Saxifragella and the monotypic Chondrosea (Pyramidal Saxifrage) are sometimes included in Saxifraga.^[1]

Description and ecology

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

Most saxifrages are smallish plants whose 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. 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,^[2] but the matter has apparently not been studied since his time.

The inflorescences or single flowers are raised above the main plant body at flowering time on stalks devoid of true leaves. The small actinomorphic hermaphrodite flowers have five petals and sepals and are usually white, but red to yellow in some species. As in other primitive eudicots, some of the 5 or 10 stamens may appear petal-like.

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 habitat, such as S. biflora which can be found some 4,000 meters ASL 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 habitat and are larger (though still rather delicate) plants found on wet meadows.

Among those saxifrages found outside mountaineous, subarctic or glacial regions, Starry Saxifrage (S. stellaris) is often seen alongside Fountain Apple-moss (Philonotis fontana) at springs in Ireland, Britain, and other parts of Western Europe. In the same regions, Yellow Mountain Saxifrage (S. aizoides) is typically found in calcifugous grassland of the northern uplands together with Smooth Lady's-mantle (Alchemilla glabra), as well as (less abundantly) on upland calcicolous pastures characterized by Common Bent (Agrostis capillaris), Bendy Ditrichum moss (Ditrichum flexicaule), Sheep's Fescue (Festuca ovina) and Mother-of-Thyme (Thymus praecox). But it is also a common sight in acidic mires with rich growth of the sedge Carex demissa.

Various Saxifraga species are used as food plants by the caterpillars of some butterflies and moths, such as the Grey Chi (Antitype chi), Hebrew Character (Orthosia gothica) and Phoebus Apollo (Parnassius phoebus).

Use by humans

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

S. × urbium, the hybrid between Pyrenean Saxifrage (S. umbrosa) and Saint Patrick's Cabbage (S. spathularis), is known as London Pride and commonly grown as an ornamental plant. 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. Saxifrages are popular cushion plants for rock gardens in temperate and cold climates; a splendid example of their use in landscape gardening can be seen in the Arctic-alpine Botanic Garden of Tromsø (Norway). Cambridge University Botanic Garden hosts the United Kingdom's national collection of saxifrages.

Purple Saxifrage (S. oppositifolia) is a popular floral emblem. It is the territorial flower of Nunavut (Canada) and the county flower of County Londonderry in the UK. Known as rødsildre ("red saxifrage") in Norway, it also it is the county flower of Nordland. Tsukuba in Japan has as its city flower hoshizaki-yukinoshita (Katakana: ホシザキユキノシタ), the aptera form of Creeping Saxifrage (S. stolonifera).

In literature, saxifrages do not figure prominently – that is, outside scientific writing such as the studies of Adolf Engler or the landmark The structure and biology of Arctic flowering plants. White Mountain Saxifrage (S. paniculata) is discussed in Nicholas Culpeper's 1652 herbal The English Physitian. Well-known references to saxifrages in literature are:

• In William Carlos Williams' poem "A Sort of a Song", Williams refers to his idea of perception (to see through the metaphorical rock, see into the essence of the object, "no ideas but in things") when he writes:

"Invent! Saxifrage is my flower that splits the rocks."

• In Kim Stanley Robinson's Mars Trilogy, the character Sax Russell – an atomic physicist sent to Mars as part of Earth's first colony attempt on that planet – is named after this plant. There are several references to the saxifrage genus, and Robinson uses the plant's common name "stonebreaker" and descriptions of the flower to describe aspects of Russell's personality.

Selected species

Saxifraga caesia

Lesser London Pride (Saxifraga cuneifolia)

Saxifraga decipiens

Northwestern Saxifrage (Saxifraga integrifolia)

Irish Saxifrage (Saxifraga rosacea)

Tolmie's Saxifrage (Saxifraga tolmiei)

Prickly Saxifrage (Saxifraga tricuspidata) flowers

Formerly placed here

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

Bergenia crassifolia, like many of its congeners, was originally believed to be a saxifrage

• 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
• Chondrosea cotyledon, as S. cotyledon
• Darmera peltata (Indian Rhubarb), as S. peltata
• Leptarrhena pyrolifolia, as S. pyrolifolia
• Luetkea pectinata (Partridgefoot), as S. pectinata
• Mukdenia rossii (Mukdenia), as S. rossii

Opposite-leaved Golden-saxifrage (Chrysosplenium oppositifolium

Other "saxifragous" plants

Tunicflower (Petrorhagia saxifraga) resembles a saxifrage in habitus

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

• Pyramidal Saxifrage, Chondrosea cotyledon
• Golden-saxifrages, Chrysosplenium
• Burnet-saxifrages, Pimpinella
• Pepper-saxifrage, Silaum silaus

Some plants refer Saxifraga in their generic or specific names, 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 – saxifragopsises

Footnotes

References

• Darwin, Charles (1875): Chapter XV (Drosophyllum – Roridula – Byblis – Glandular hairs of other plants – Concluding remarks on the Droseraceae). In: Insectivorous Plants (1st ed.): 332-367. J. Murray, London. Fulltext (various formats) at the Internet Archive
• United States Department of Agriculture (USDA) (2005): Germplasm Resources Information Network – Saxifraga. Version of 2005-FEB-09. Retrieved 2009-JAN-30.

External links

Wikimedia Commons has media related to: Saxifraga

• saxifraga.org: Saxifrage society
•  "Saxifrage". Encyclopædia Britannica (11th ed.). 1911.