dryas (plant)
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 dryads, 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, but is now placed in subfamily Dryadoideae.The species are superficially similar to Geum (with which they share the common name avens), Potentilla, and Fragaria (strawberry). However, Dryas 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. This makes them very popular in rockeries and alpine gardens. The hybrid Dryas × suendermannii, with cream coloured flowers, has gained the Royal Horticultural Society’s 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 lowlands, and also fen habitats upon organic substrate where some shading from adjacent sedges or shrubs may occur.
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.

Gallery
Taxonomy
Species
Dryas comprises three species, but the genus is in need of taxonomic revision:
Dryas drummondii Richardson ex Hook. – Drummond's avens
var. drummondii Richardson ex Hook. – Yellow avens
var. tomentosa (Farr) L.O. Williams – Tomentose avens
Dryas integrifolia 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
Dryas octopetala 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:

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 nodules that host the nitrogen-fixing bacterium Frankia.
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.

References


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