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''Dryas'' is a genus of perennial cushion-forming evergreen dwarf shrubs in the family Rosaceae, native to the arctic and alpine regions of Europe, Asia and North America. The genus is named after the Greek nymph Dryad. The classification of ''Dryas'' within the Rosaceae has been unclear.〔Morgan, D.R., et al. (1994). Systematic and evolutionary implications of rbcL sequence variation in Rosaceae. ''American Journal of Botany''. 81(7): 890–903.〕 The genus was formerly placed in the subfamily Rosoideae, but is now placed in subfamily Dryadoideae.〔Potter, D., et al. (2007). Phylogeny and classification of Rosaceae. ''Plant Systematics and Evolution''. 266(1–2): 5–43.〕 There are three species and one hybrid: *''Dryas drummondii'' – Drummond's Avens *''Dryas integrifolia'' – Entire-leaved Avens *''Dryas octopetala'' – Mountain Avens *''Dryas × suendermannii'' – ''D. drummondii'' × ''D. octopetala'' The species are superficially similar to ''Geum'', ''Potentilla'' and ''Fragaria'', but are distinct in having flowers 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 octopetala'') or pendulous and all-yellow (''Dryas drummondii''), and held conspicuously above the small plants. The hybrid has pale yellow flowers. This makes them very popular in rockeries and alpine gardens. ''Dryas'' tolerates a wide variety of unshaded habitats, including alpine situations with sand or gravel substrate, similar substrates in flat tundra lowlands, and also fen habitats upon organic substrate where some shading from adjacent sedges or shrubs may occur. Some ''Dryas'' plants have root nodules that host the nitrogen-fixing bacterium ''Frankia''.〔Swensen, S.M.; Mullin, B.C. (1997). The impact of molecular systematics on hypotheses for the evolution of root nodule symbioses and implications for expanding symbioses to new host plant genera. ''Plant and Soil''. 194: 185–192.〕〔Markham, J. H. (2009). Does Dryas integrifolia fix nitrogen? ''Botany''. 87: 1106-1109.〕 ''Dryas'' is the clan badge of Clan MacNeil of Scotland. The Younger Dryas and Older Dryas stadials are geological periods of cold temperature that are named after ''Dryas octopetala'', which flourished during that time and is used as a fossil indicator of those periods. ==References== 〔 *(Germplasm Resources Information Network: ''Dryas'' ) *Huxley, A. (1992). ''New RHS Dictionary of Gardening''. Macmillan (× suendermannii'' ). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dryas (plant)」の詳細全文を読む スポンサード リンク
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