Thistle, the common name for plants of the genus cnicus (Gr.κνίζειν, to prick), of the composite family. In most works the American species are placed under Cirsium, a genus mainly differing from cnicus by the character of the pappus, and some European authors unite all the thistles under Carduus. Gray in a late revision of the North American thistles ("Proceedings of the American Academy") restores them to the Linnrean genus cnicus. The name is sometimes used in combination for plants not closely related; thus the teasle is called fuller's thistle. The thistles are herbs, often with perennial roots, with sessile alternate leaves which are often much divided and prickly; the branches of the stem terminated by heads of flowers (often very large), with an ovoid or spherical involucre, the scales to which are imbricated in many rows, and tipped with a point or prickle; the flowers in the head are all tubular and similar, usually perfect, but sometimes dioecious; their usual color is purple, but in some species they are yellowish or cream-colored; the receptacle on which the flowers are placed is furnished with numerous soft bristles; the one-seeded akenes bear at the top a pappus, or tuft of numerous hairs, which are united into a ring at the base and are feathery with smaller hairs, forming the well known thistle down.
About 30 species are found in the United States; two of them are introduced, and are among the most common and most annoying weeds. The common thistle (C. lanccolatus), often called in this country bull thistle, is one of these, and the most frequent of all the species. The large leaves are decurrent, i. e., their bases are prolonged downward upon the stem as a spiny, lobed wing; they are prickly on the upper surface and covered below with cobwebby hairs; the heads, about an inch in diameter, have all the scales tipped with prickles, the outer ones spreading; flowers purple. This is a large showy species; its stems in rich soil are 3 to 4 ft. high, and its robust spreading leaves give it a formidable appearance, while in reality it is very easily destroyed; its root being biennial, there is no danger that it will retain possession of the soil. In Great Britain this is called spear thistle; it is the Scotch national emblem. The other introduced species is C. arvensis, known as the creeping thistle in Europe, and in this country by the misnomer of the Canada thistle; it is a perennial, spreading rapidly and extensively by its long creeping rootstocks, which send to the surface numerous stems 18 in. to 3 ft. high; the handsomely cut leaves are smooth, or somewhat woolly below, and very prickly on the margins with slender spines; the heads about half an inch in diameter, on short pedicels and forming a loose terminal corymb; the outer scales of the involucre with minute prickly points; flowers pale lilac.
In this species the flowers are dioecious, the male heads nearly globular, with more conspicuous flowers than the female heads, which are longer; plants of each sex form separate patches. This plant, which has followed cultivation to nearly all parts of the world, is supposed to have received the name by which it is exclusively known in this country from its having been introduced in the fleeces of sheep brought from Canada; it is justly regarded by our farmers, as it was in Lapland in the days of Linnseus, as "the greatest pest of our fields;" its deep roots, below the reach of the plough, and its abundant seeds, furnish it with ample means for spreading; the creeping rootstock is exceedingly tenacious of life, and when broken every fragment is capable of forming a new plant. Many states have a law which makes it obligatory on each owner to destroy it upon his land, under penalty of its being done by the authorities and the cost charged as a tax. Like other perennial weeds, it soon yields to frequent mowing; but to be effective this must be persistent. None of our native thistles can be regarded as troublesome weeds.
The yellow thistle (C. horridulus) is found near the coast; the pasture thistle (C. pumi-lus) is a low species with very large heads of fragrant purple (or white) flowers; the tall thistle (C. altissimus), a more southern species, is often 10 ft. high and a rather showy plant. The blessed thistle (C. benedictiis), so called on account of its former use in medicine, is barely naturalized southward. - The cotton thistle, of a closely related European genus, is Onopordon acanthium; it is a stately plant covered with whitish cottony hairs, and is occasionally met with in the older states. This is said to be cultivated in Scotland as the Scotch thistle, but the best authorities give the common species already described as the emblematic thistle. The milk thistle is silyoum Marianum, related to the true thistles, and sometimes cultivated in old gardens; it has purple flowers, and leaves blotched with white. Torch thistle is a name given in tropical countries to the tall species of Cereus of the Cactacea. Sow thistles are coarse composite weeds of the genus Sonchus.
Common Thistle (Cnicus lanccolatus).
Canada Thistle (Cnicus arvensis).