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Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles often occur all over the plant – on surfaces such as those of the stem and flat parts of leaves. These are an adaptation that protects the plant against herbivorous animals, discouraging them from feeding on the plant. Typically, an involucre with a clasping shape of a cup or urn subtends each of a thistle's flowerheads.
The term thistle is sometimes taken to mean exactly those plants in the tribe Cynareae (synonym: Cardueae), especially the genera Carduus, Cirsium, and Onopordum. However, plants outside this tribe are sometimes called thistles, and if this is done thistles would form a polyphyletic group.
Genera in the Asteraceae with the word thistle often used in their common names include:
- Carduus – musk thistle and others
- Carlina – carline thistle
- Carthamus - distaff thistle
- Centaurea – star thistle
- Cicerbita – sow thistle
- Cirsium – common thistle, field thistle and others
- Cnicus – blessed thistle
- Cynara – artichoke, cardoon
- Echinops – globe thistle
- Notobasis – Syrian thistle
- Onopordum – cotton thistle, also known as Scots or Scotch thistle
- Scolymus – golden thistle or oyster thistle
- Silybum – milk thistle
- Sonchus – sow thistle
Plants in families other than Asteraceae which are sometimes called thistle include:
- Kali – Russian thistle, Tartar thistle, or tumbleweed, plants formerly classified in the genus Salsola (family Chenopodiaceae)
- Argemone mexicana - Flowering thistle, Mexican prickly poppy
In the language of flowers, the thistle (like the burr) is an ancient Celtic symbol of nobility of character as well as of birth, for the wounding or provocation of a thistle yields punishment.
The thistle has been the national emblem of Scotland since the reign of Alexander III (1249–1286) and was used on silver coins issued by James III in 1470. It is the symbol of the Order of the Thistle, a high chivalric order of Scotland. It is found in many Scottish symbols and as the name of several Scottish football clubs. The thistle, crowned with the Scottish crown, is the symbol of seven of the eight Scottish Police Forces (the exception being the Northern Constabulary). The thistle is also the emblem of Encyclopædia Britannica, which originated in Edinburgh, Scotland.
It is also used to symbolize connection with Scotland overseas. For example in Canada, it is one of the four floral emblems on the flag of Montreal; in the US, Carnegie Mellon University features the thistle in its crest in honor of the Scottish heritage of its founder, Andrew Carnegie.
Origin as a symbol of Scotland 
According to a legend, an invading Norse army was attempting to sneak up at night upon a Scottish army's encampment. During this operation one barefoot Norseman had the misfortune to step upon a thistle, causing him to cry out in pain, thus alerting Scots to the presence of the Norse invaders. Some sources suggest the specific occasion was the Battle of Largs, which marked the beginning of the departure of King Haakon IV (Haakon the Elder) of Norway who, having control of the Northern Isles and Hebrides, had harried the coast of the Kingdom of Scotland for some years. Which species of thistle is referred to in the original legend is disputed. Popular modern usage favours Cotton Thistle Onopordum acanthium, perhaps because of its more imposing appearance, though it is unlikely to have occurred in Scotland in mediaeval times; the Spear Thistle Cirsium vulgare, an abundant native species in Scotland, is a more likely candidate. Other species, including Dwarf Thistle Cirsium acaule, Musk Thistle Carduus nutans, and Melancholy Thistle Cirsium heterophyllum have also been suggested.
Place names 
Carduus is the Latin term for a thistle (hence cardoon, chardon in French), and Cardonnacum is the Latin word for a place with thistles. This is believed to be the origin of name of the Burgundy village of Chardonnay, Saône-et-Loire, which in turn is thought to be the home of the famous Chardonnay grape variety.
Thistle flowers, along with bugle and brambles flowers, are favourite nectar sources of the Pearl-bordered Fritillary, Small Pearl-bordered Fritillary, High Brown Fritillary, and Dark Green Fritillary butterflies. Thistles (and thistle-seed feeders) also attract goldfinches.
Some thistles (for example Cirsium vulgare, native to Eurasia), have been widely introduced outside their native range. Control measures include Trichosirocalus weevils, but a problem with this approach, at least in North America, is that the introduced weevils may affect native thistles at least as much as the desired targets.
Literary references 
In the Bible, following the fall of man, God says to Adam, "cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life; Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field" (Genesis 3:17-18).
Nicholas Sayre and the Creature in the Case, Garth Nix's novella in Across the Wall: A Tale of the Abhorsen and Other Stories, involves a rare Free Magic creature, a Hrule, being defeated by a thistle.
The thistle also features in the song The Thistle o' Scotland, which uses the plant as a humorous metaphor for the prickly determinations of the Scots.
The poem Thistles by Ted Hughes
In Hadji Murat, the last novel by Leo Tolstoy (published posthumously), thistle is a symbol of the main character and stands for traits of adaptability and adamancy that human beings exhibit because of their lust for life.
Medical uses 
Maud Grieve recorded that Pliny and medieval writers had thought it could return hair to bald heads and that in the early modern period it had been believed to be a remedy for headaches, plague, canker sores, vertigo, and jaundice.
- "Cardueae". Tree of Life webproject. Retrieved 2012-10-30.
- "thistle". Merriam-Webster's online dictionary. Retrieved 2007-11-30.
- Webmaster, John Duncan (2009-04-04). "Scots History Online". UK: Scots History Online. Retrieved 2013-01-14.
- "Flowers of Scotland: Thistle". Twocrows.co.uk. Retrieved 2013-01-14.
- "Why is the THISTLE a Scottish national symbol?". Ormiston.com. Retrieved 2013-01-14.
- "Scotland Fact File: The Thistle". Visitscotland.com. Retrieved 2013-01-14.
- Bracken for Butterflies leaflet c0853 by Butterfly Conservation, January 2005
- Cirsium vulgare (Savi) Ten., Asteraceae , Pacific Island Ecosystems at Risk (PIER)
- Takahashi, Masaru; Louda, SM; Miller, TE; O'Brien, CW (2009). "Occurrence of Trichosirocalus horridus (Coleoptera: Curculionidae) on Native Cirsium altissimum Versus Exotic C. Vulgare in North American Tallgrass Prairie". Environmental Entomology 38 (3): 731–40. doi:10.1603/022.038.0325. PMID 19508782.
- Maud Grieve. "A Modern Herbal". Retrieved 2011-06-03.
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Texts on Wikisource:
- “Lily-Bell and Thistle-down” (a fairy tale) in Flower Fables by Louisa May Alcott (1855)
- Charles Grant Blairfindie Allen (November 1886). "Thistles". Popular Science Monthly 30.
- "Thistle". Encyclopædia Britannica (11th ed.). 1911.
- “Thistle-Down” (a poem) in Flint and Feather by E. Pauline Johnson (1912)