Dryas is a genus of dwarf perennial

Perennial plant

A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...

 herbaceous

Herbaceous

A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...

 plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s in the rose

Rose

A rose is a woody perennial of the genus Rosa, within the family Rosaceae. There are over 100 species. They form a group of erect shrubs, and climbing or trailing plants, with stems that are often armed with sharp prickles. Flowers are large and showy, in colours ranging from white through yellows...

 family Rosaceae

Rosaceae

Rosaceae are a medium-sized family of flowering plants, including about 2830 species in 95 genera. The name is derived from the type genus Rosa. Among the largest genera are Alchemilla , Sorbus , Crataegus , Cotoneaster , and Rubus...

, native to the arctic and alpine

Arctic-alpine

An arctic-alpine taxon is one whose natural distribution includes the Arctic and more southerly mountain ranges, particularly the Alps. The presence of identical or similar taxa in both the tundra of the far north, and high mountain ranges much further south is testament to the similar...

 regions of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, Asia

Asia

Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

 and North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

. The genus is named after the Greek nymph Dryas

Dryas

Dryas is the name of nine characters in Greek mythology1. Dryas was the son of King Lycurgus, king of the Edoni in Thrace. He was killed when Lycurgus went insane and mistook him for a mature trunk of ivy, a plant holy to the god Dionysus, whose cult Lycurgus was attempting to extirpate.Resisting...

. The classification of Dryas within the Rosaceae has been unclear. The genus was formerly placed in the subfamily Rosoideae

Rosoideae

The rose subfamily Rosoideae consists of more than 850 species, including many shrubs, perennial herbs, and fruit plants such as strawberries and brambles...

, but is now placed in subfamily Dryadoideae

Dryadoideae

The Dryadoideae subfamily of the Rosaceae consists of four genera, all of which share root nodules that host the nitrogen-fixing bacterium Frankia. They are sub-shrubs, shrubs, or small trees with a base chromosome number of 9, whose fruits are either an achene or an aggregate of achenes....

.

There are three species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 and one hybrid:

• Dryas drummondii – Drummond's Avens
• Dryas integrifolia – Entire-leaved Avens
• Dryas octopetala
  Dryas octopetala
  Dryas octopetala is an arctic-alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies, and is a popular flower in rock gardens...
  
  – Mountain Avens
• Dryas × suendermannii – D. drummondii × D. octopetala

The species are superficially similar to Geum

Geum

Geum , commonly called avens, is a genus of about 50 species of perennial herbaceous plants in the rose family Rosaceae, native to Europe, Asia, North and South America, Africa and New Zealand...

, Potentilla

Potentilla

Potentilla is the genus of typical cinquefoils, containing about 500 species of annual, biennial and perennial herbs in the rose family Rosaceae. They are generally Holarctic in distribution, though some may even be found in montane biomes of the New Guinea Highlands...

and Fragaria

Strawberry

Fragaria is a genus of flowering plants in the rose family, Rosaceae, commonly known as strawberries for their edible fruits. Although it is commonly thought that strawberries get their name from straw being used as a mulch in cultivating the plants, the etymology of the word is uncertain. There...

, but are distinct in having flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

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 octopetala

Dryas octopetala

Dryas octopetala is an arctic-alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies, and is a popular flower in rock gardens...

) 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

Rock Garden

The Rock Garden or Rock Garden of Chandigarh is a Sculpture garden in Chandigarh, India, also known as Nek Chand's Rock Garden after its founder Nek Chand, a government official who started the garden secretly in his spare time in 1957. Today it is spread over an area of forty-acres , it is...

 and alpine gardens.

Some Dryas plants have root nodule

Root nodule

Root nodules occur on the roots of plants that associate with symbiotic nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia...

s that host the nitrogen-fixing

Nitrogen fixation

Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

 bacterium Frankia

Frankia

Frankia is a genus of nitrogen fixing, filamentous bacteria that live in symbiosis with actinorhizal plants, similar to the Rhizobia bacteria that are found in the root nodules of legumes in the Fabaceae family. Bacteria of this genus also form root nodules.The genus Frankia was originally named by...

.

Dryas is the clan badge

Clan badge

A clan badge, sometimes called a plant badge, is a badge or emblem, usually a sprig of a specific plant, that is used to identify a member of a particular Scottish clan. They are usually worn in a bonnet behind the Scottish crest badge, or attached at the shoulder of a lady's tartan sash...

 of Clan MacNeil

Clan MacNeil

Clan MacNeil, also known in Scotland as Clan Niall, is a highland Scottish clan, particularly associated with the Outer Hebridean island of Barra. The early history of Clan MacNeil is obscure, however despite this the clan claims to descend from the legendary Niall of the nine hostages...

 of Scotland.

The Younger Dryas

Younger Dryas

The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

 and Older Dryas

Older Dryas

The Older Dryas was a stadial period between the Bølling and Allerød oscillations during the Pleistocene glacial period of ~11,700—12,000 uncalibrated years ago...

 stadial

Stadial

A stadial is a period of lower temperatures during an interglacial separating the glacial periods of an ice age. Such periods are of insufficient duration or intensity to be considered glacial periods...

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

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

indicator of those periods.