The genus Cirsium Miller

Remark The words or terms in red (actually dark orange) in the text are defined in a glossary.


sep_18_03.gthmb can't be loaded. The genus Cirsium commonly known as the thistle genus (the Carduus genus is also known as the thistle genus) is native to Eurasia, North America, and North and East Africa; it contains about 250 species. Its occurrence in the southern hemisphere is considered to be non-native. The highest species diversity is concentrated in the mountains of Southern Europe and in the Caucasus mountains. The North American contingent of this genus is represented by at least 96 indigenous, endemic taxa. More than 50 species have been reported from China. Four naturalized species are found in Australia. The species are perennial or biennial or rarely annual.

Most species, about 70 %, are diploid with a chromosome number 2n of 34; about 10 % of the species are tetraploid (2n = 68), other counts being less frequent. Cirsium is a typical example of a genus with a high affinity to form natural interspecific hybrids. Several species are serious weeds. The genus Cirsium comprises both gynodioecious and dioecious species.

The species of the Cirsium genus are at times quite similar to the species of the Carduus genus. In fact, at one time botanists considered the species of Cirsium to be Carduus taxa. The only significant difference between these two genera is the nature of the pappus. The Cirsium species can be distinguished from the Carduus species by the plumose pappus of their seeds. The members of the Carduus genus are known as plumeless thistles because of their simple, not plumose, pappus. In the genus Carduus the pappus is composed of capillary bristles that are naked, single, and silky; whereas in the genus Cirsium the pappus is composed of featherlike bristles. The pappus must always be examined to determine the genus of an unidentified thistle.

The genus belongs to the Asteraceae family and to the Cardueae tribe of this family.

Name

In Greek κιρσιον (kirsion) is the name of a Thistle that was used to heal varicose veins, and κιρσοσ (kirsos) means varicose vein, kirsion being then the (bygone) remedy of kirsos. As for Thistle it is the common vernacular of most Cirsium species (and of some Carduus species too).

Description

Plants of the Cirsium genus are spiny herbs; some species reach 4 m in height, some species are not higher than a few centimeters. They usually have erect, prickly stems and prickly leaves. The leaves are alternate, entire or pinnatisect, with spinulose margins and usually spiny teeth or lobes.

The involucral bracts are in many series, imbricate, usually with a simple apical spine. The capitula. do not have ray florets. The disk florets are usually hermaphrodite, purple or yellowish, and rarely white. The seeds are oblong achenes with a pappus of several rows of plumose setae united in a ring at the base, the inner setae somewhat longer than the outer and simple.

Morphological intermediates, quite often hybryds, are frequent in the genus. Devising an identification key to the Cirsium species is quite difficult. Extensive and overlapping ranges of variation in morphologic characteristics often require that a species be keyed two or more times.

Notes

Cirsium plants are used as food plants by the larvae of some Lepidoptera species. Most species are considered weeds, but some are cultivated in gardens for their aesthetic value.

The reputation of the genus has suffered greatly as a result of the introduction to North America of a few invasive weedy species from Eurasia. Cirsium vulgare and Cirsium arvense (picture above) have long been despised as noxious weeds. In recent years Cirsium palustre has joined their ranks. Additionally, weedy Eurasian species of Carduus, Centaurea, etc., add to the public perception that all thistles are bad. Most North American native Cirsium are not at all weedy, and many are strikingly attractive plants. All are spiny plants that command respect, but they deserve a better reputation.

In an old Russian fairy tale, fairies in a wildflower meadow protect themselves from a greedy merchant by turning all the lovely blossoms into prickly Cirsium.

The Cirsium of Québec

Cirsium species are common throughout Canada. In Québec one can find the following species from the genus:

Home