Thistle

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.

Thistle is the floral emblem of Scotland, as well as the emblem of the Encyclopædia Britannica.

Genera in the Asteraceae with the word thistle often used in their common names include:

Plants in families other than Asteraceae which are sometimes called thistle include:

Thistles, even if one restricts the term to members of the Asteraceae, are too varied a group for generalisation; many are troublesome weeds, including some invasive species of Cirsium, Carduus, Silybum and Onopordum. Typical adverse effects are competition with crops and interference with grazing in pastures, where dense growths of spiny vegetation suppress forage plants and repel grazing animals from eating either the thistle plants or neighbouring forage. Some species, although not intensely poisonous, do affect the health of animals that swallow more than small amounts of the material.

Conversely however, the genus Cynara includes commercially important species of artichoke and some species regarded as major weeds are commercial sources of vegetable rennet used in commercial cheese making. Similarly, some species of Silybum that occur as weeds, also are cultivated for seeds that yield vegetable oil and pharmaceutical compounds such as Silibinin.

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