''Dryas'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

cushion A cushion is a soft bag of some ornamental material, usually stuffed with wool, hair, feathers, polyester staple fiber, non-woven material, cotton, or even paper torn into fragments. It may be used for sitting or kneeling upon, or to soften th ...

-forming

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...

dwarf

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...

s in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Rosaceae Rosaceae (), the rose family, is a medium-sized family of flowering plants that includes 4,828 known species in 91 genera. The name is derived from the type genus ''Rosa''. Among the most species-rich genera are ''Alchemilla'' (270), ''Sorbus ...

native Native may refer to: People * Jus soli, citizenship by right of birth * Indigenous peoples, peoples with a set of specific rights based on their historical ties to a particular territory ** Native Americans (disambiguation) In arts and entert ...

to the arctic and alpine regions of Europe, Asia and North America. The genus is named after the

dryad A dryad (; el, Δρυάδες, ''sing''.: ) is a tree nymph or tree spirit in Greek mythology. ''Drys'' (δρῦς) signifies "oak" in Greek, and dryads were originally considered the nymphs of oak trees specifically, but the term has evolved to ...

s, the tree nymphs of ancient Greek mythology. The classification of ''Dryas'' within the Rosaceae has been unclear. The genus was formerly placed in the subfamily

Rosoideae The rose subfamily Rosoideae consists of more than 850 species, including many shrubs, perennial herbs, and fruit plants such as strawberries and brambles. Only a few are annual herbs. The circumscription of the Rosoideae is still not wholly cer ...

, but is now placed in subfamily

Dryadoideae The subfamily Dryadoideae consists of four genera in the family Rosaceae,. all of which contain representative species with root nodules that host the nitrogen-fixing bacterium ''Frankia ''Frankia'' is a genus of nitrogen-fixing bacteria ...

. The species are superficially similar to ''

Geum ''Geum'' , (Latinized Greek for "taste" referencing the roots of the plant) commonly called avens, is a genus of about 50 species of rhizomatous perennial herbaceous plants in the rose family and its subfamily Rosoideae, widespread across Euro ...

'' (with which they share the common name avens), ''

Potentilla ''Potentilla'' is a genus containing over 300Guillén, A., et al. (2005)Reproductive biology of the Iberian species of ''Potentilla'' L. (Rosaceae).''Anales del Jardín Botánico de Madrid'' 1(62) 9–21. species of annual, biennial and perenni ...

'', and ''Fragaria'' (strawberry). However, ''Dryas'' are distinct in having

flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...

s with ''eight'' petals (rarely seven or up to ten), instead of the five petals found in most other genera in the Rosaceae. The flowers are erect and white with a yellow centre (''

Dryas integrifolia ''Dryas integrifolia'' is a species of flowering plant in the rose family known by the common names arctic avens,Louis-Marie, P. ND. Flore-Manuel de la Province de Québec, Canada (3e édition). Cooperative Harpell's Press, Gardenvale, PQ. entire ...

'', ''

Dryas octopetala ''Dryas octopetala'', the mountain avens, eightpetal mountain-avens, white dryas or white dryad, is an Arctic–alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies. The specific epithet ...

'') or pendulous and all-yellow ('' Dryas drummondii''), and held conspicuously above the small plants. This makes them very popular in rockeries and

alpine garden An alpine garden (or alpinarium, alpinum) is a domestic or botanical garden, or more often a part of a larger garden, specializing in the collection and cultivation of alpine plants growing naturally at high altitudes around the world, such as in ...

s. The hybrid ''Dryas'' × ''suendermannii'', with cream coloured flowers, has gained the

Royal Horticultural Society The Royal Horticultural Society (RHS), founded in 1804 as the Horticultural Society of London, is the UK's leading gardening charity. The RHS promotes horticulture through its five gardens at Wisley (Surrey), Hyde Hall (Essex), Harlow Carr (Nort ...

’s

Award of Garden Merit The Award of Garden Merit (AGM) is a long-established annual award for plants by the British Royal Horticultural Society (RHS). It is based on assessment of the plants' performance under UK growing conditions. History The Award of Garden Merit ...

. ''Dryas'' tolerates a wide variety of unshaded habitats, including alpine situations with sand or gravel substrate, similar substrates in flat

tundra In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless moun ...

lowlands, and also

fen A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. T ...

habitats upon organic substrate where some shading from adjacent

sedge The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" genus ''Carex'' wit ...

s or shrubs may occur. The

Younger Dryas The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stage ...

and

Older Dryas The Older Dryas was a stadial (cold) period between the Bølling and Allerød interstadials (warmer phases), about 14,000 years Before Present, towards the end of the Pleistocene. Its date is not well defined, with estimates varying by 400 years, ...

stadial Stadials and interstadials are phases dividing the Quaternary period, or the last 2.6 million years. Stadials are periods of colder climate while interstadials are periods of warmer climate. Each Quaternary climate phase is associated with a Ma ...

s are geological periods of cold temperature that are named after ''Dryas octopetala'', which flourished during that time and is used as a

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

indicator of those periods.

Gallery

Image:Yellow Dryad (3815981059).jpg, '' Dryas drummondii'' – Drummond's avens Image:Dryas integrifolia (7833269336).jpg, ''

'' – entire-leaved avens Image:Weiße Silberwurz (Dryas octopetala) 1.jpg, ''

'' – mountain avens

Taxonomy

Species

''Dryas'' comprises three

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

, but the genus is in need of

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

revision: *'' Dryas drummondii'' Richardson ex Hook. – Drummond's avens ** var. ''drummondii'' Richardson ex Hook. – Yellow avens ** var. ''tomentosa'' (Farr) L.O. Williams – Tomentose avens *''

'' Vahl – Entire-leaved avens ** subsp. ''chamissonis'' (Spreng.) Scoggan – White avens ** subsp. ''crenulata'' (Juz.) J. Kozhevn – Crenulate avens ** subsp. ''integrifolia'' Vahl – Entire-leaved avens ** subsp. ''sylvatica'' (Hultén) Hultén – Forest avens *''

'' L. – Mountain avens ** subsp. ''alaskensis'' (A.E. Porsild) Hultén – Alaskan avens ** subsp. ''hookeriana'' (Juz.) Hultén – Hooker's avens ** subsp. ''octopetala'' L. – Eight-petal avens *** var. ''angustifolia'' C.L. Hitchc. – Narrow-leaved avens *** var. ''argentea'' Blytt – Silvery avens *** var. ''kamtschatica'' (Juz.) Hultén – Kamtschatca avens *** var. ''octopetala'' L. – Eight-petal avens ** subsp. ''punctata'' (Juz.) Hultén – Pointed avens

Species names with uncertain taxonomic status

The status of the following species is unresolved: * ''Dryas camschatica'' Juz. * ''Dryas dasypetala'' Juz. * ''Dryas grandis'' Juz. * ''Dryas henricae'' Juz. * ''Dryas incisa'' Juz. * ''Dryas integrifolia'' C.A.Mey. ex Juz. * ''Dryas integrifolia'' Ledeb. * ''Dryas kamtschatica'' Juz. * ''Dryas longifolia'' C.A.Mey. ex Juz. * ''Dryas octopetala'' J.G.Gmel. * ''Dryas octopetala'' M.Bieb. * ''Dryas octopetala'' Makino * ''Dryas oxyodonta'' Juz. * ''Dryas sumneviczii'' Serg. * ''Dryas viscosa'' Juz.

Hybrids

Two hybrids have been described: * ''Dryas'' × ''suendermannii'' Kellerer ex Sundermann—(''D. drummondii'' × ''D. octopetala'') * ''Dryas'' × ''wyssiana'' Beauverd—(''D. drummondii'' × ''D. integrifolia'')

Species names with uncertain taxonomic status

The status of the following hybrids is unresolved: * ''Dryas'' × ''chamissonis'' Jurtzev * ''Dryas'' × ''grandiformis'' Jurtzev * ''Dryas'' × ''intermedia'' Juz. * ''Dryas'' × ''lewinii'' Rouleau

Nitrogen Fixation

Some ''Dryas'' plants have

root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known a ...

s that host the

nitrogen-fixing Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmos ...

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

Frankia ''Frankia'' is a genus of nitrogen-fixing bacteria that live in symbiosis with actinorhizal plants, similar to the ''Rhizobium'' bacteria found in the root nodules of legumes in the family Fabaceae. ''Frankia'' also initiate the forming of root ...

''. * ''Dryas drummondii'' forms root nodules and fixes nitrogen with ''Frankia''. * ''Dryas integrifolia'' does not form root nodules or fix nitrogen with ''Frankia''. * ''Dryas octopetala'' does not form root nodules or fix nitrogen with ''Frankia''. * ''Dryas'' × ''suendermannii'' (''D. drummondii'' × ''D. octopetala'') does not form root nodules or fix nitrogen with ''Frankia''.(Reported as ''Dryas drummondii'' var. ''eglandulosa''.)

References

External links

{{Taxonbar, from=Q123279 Dryadoideae Rosaceae genera