Asteraceae
- Thistles
Cirsium - Plumed
Thistles
1. Spear Thistle (Cirsium vulgare)
Spear Thistle or Bull Thistle, Cirsium vulgare (= Cirsium lanceolatum). As the name suggests, this thistle is common, at least in its native range of Europe, western Asia and parts of North Africa. It is also highly invasive in North America and Oceania and some other areas. 'Thistle' is a general term used for a variety of spiny plants, such as the Russian Thistle, Salsola (Prickly Saltwort) in the chenopodium family and Eryngium, Sea Holly in the Apiaceae. Botanically speaking, however, the 'true thistles' belong to the Asteraceae. The Asteraceae itself contains a large-number of thistle-like plants, such as the star-thistles that belong to the genus Centaurea, but the true thistles are usually taken to include a few selected thistle genera, chiefly Cirsium and Carduus, though the Onopords (Onopordum) and Milk Thistles (Silybum) are often considered true thistles. The Asteraceae includes other thistle-like genera, such as Carlina, the Carline Thistles, Atractylis and Chamaeleon (which includes the lethally poisonous Mediterranean stemless thistle, Chamaeleon gummifera = Atractylis gummifera), the Globe Thistles (Echinops), the Cardoon and Artichokes (Cynara), Carthamus (= Kentrophyllum), Xanthium and others.
As a member of the Asteraceae (Daisy Family) the 'flower' of Cirsium is actually a compound flower, a flowering head, capitulum, anthode, or pseudanthium ('false flower') consisting of many tiny flowers or florets. This is generally viewed as a compressed flowering shoot in which the growth of the branches are repressed to bring all the flowers together. However, a review by Zhang and Elomaa, 2021 (available here) present an alternative view in which the compound flower develops much like a single solitary flower. The head of florets is enclosed in a cuplike involucrum of scale-like or spiny floral leaves or bracts (a bract is a modified leaf beneath a flower or inflorescence) called phyllaries. The phyllaries typically undergo hygroscopic movements, closing the capitulum in wet weather, to protect the nectar, and later the fruit, and opening in dry weather to expose the nectar, and later to disperse the fruit. They also provide some protection to the flowers and developing seeds inside from herbivory. The spine-tipped spear-like phyllaries and the spear-like leaves of Cirsium Vulgare are distinct.
Like many thistles, Cirsium vulgare is 'biennial', producing a rosette of large basal or radical leaves in the first year which then bolts up a stem which branches with each apex ending in a capitulum, in the second year. However, as with many biennials this is a somewhat misleading label. It is a monocarpic plant, meaning that once it flowers it sheds seeds and dies, that is it flowers once. In good conditions, the cycle may complete in two years, but under less optimum conditions it may persist as a slowly growing rosette for several years, delaying flowering until the rosette is large enough and has accumulated enough food reserves. This also buffers the population against a large decline in numbers in years where few seeds germinate.
Also diagnostic of Cirsium vulgare, are the bristle-like hairs on the upper surface (adaxial surface) of the leaves, and also on the leaf margins, along with jointed multicellular hairs on the stem and leaf midrib on the leaf undersurface. Detailed examination of the phyllaries and indumentum (hairs) are useful diagnostics in thistles, including certain hybrids.
Thistles of the genus Cirsium are Plume Thistles, so-called because the fruit has a pappus (modified sepals) of feathery hairs (that is the main shift of each hair bears obvious side-branches). In other genera of thistles (Carduus, Onopordum and Silybum) the pappus hairs are simple (that is unbranched, though they may bear short protuberances). The pappus acts as a parachute to reduce the terminal velocity of the falling achene so that wind has longer to act on the fruit and disperse it. Experiments by Sheldon and Burrows (1973) have shown that the terminal velocity of free-falling achenes is dependent largely on the ratio of pappus diameter to achene diameter, such that the higher this ratio the slower the achene descends and the further it is dispersed by gusts of wind. However, additional factors include the efficiency of the pappus: a pappus with branched hairs, a greater number of hairs or with several concentric circles of hairs are more efficient in causing drag and achenes with these types of pappus descend more slowly. For example, the pappus of Sowthistle (Sonchus oleraceus, not a true thistle) is particularly efficient and so is able to disperse well with a small pappus, as is the pappus of Marsh Thistle, Cirsium palustre. The achene of Creeping Thistle, Cirsium arvense, achieves a particularly low terminal velocity (about 20 cm/s).
The spear Thistle is a native to most of Europe, western Asia, Pakistan, China and north Africa, but has been introduced and naturalized in the USA, Australia and other parts of Africa. It is the national flower of Scotland. This thistle reaches 1.5 m in height. A single 1 m specimen with 89 capitula (flower heads) produced an estimated 18 800 seeds, based on the assumption of 211 seeds per capitulum which was the measured mean (Michaux, 1989). A study of seed dispersal in this same individual plant showed that 91% of the achenes landed within 1.5 m of the 1 m plant (Michaux, 1989). The pappus hairs of Plume Thistles each have their own pulvinus joint, allowing movement of the pappus, enabling it to open out when the involucre opens and dispersal takes place (see review in Sheldon and Burrows, 1973). Spreading of the pappus possibly loosens the achenes from the capitulum and increases the likelihood of capture by teh wind and escape from the involucre.
As in many Asteraceae, the Spear Thistle presents pollen to pollinators via a stigma/style piston mechanism: the style pushes the stigmas out from between the anthers which surround it. The anthers release their pollen on the inside and as the style and stigma push past they collect pollen from the dehisced anthers. This pollen does not germinate on the flower producing it, favoring cross-pollination. Indeed, the pair of stigma lobes are closed together at this point but when the pollen is dispersed they open to reveal their receptive surfaces to pollen from another plant.
There are several recognized varieties of Cirsium vulgare.
The variety nemorale has deeply pinnatifid leaves (lobed in a
feather-like manner with the incisions between the lobes extending
part-way to the midrib) and less rounded capitula and is possibly a
hybrid between spear and creeping thistles (= Cirsium arvense x
C. vulgare). The var. litoralis is shorter (usually less
than 50 cm) with just a few crowded capitula on short branches and
occurs on beaches. This could be an adaptive response to maritime
conditions or a stunted form growing in harsh conditions. The usual
variety is var. vulgare, which reaches 1.5 to 3 meters in height
and produces many capitula on spreading branches. The specimen above
looks like var. vulgare, even though it is growing on a shingle
beach.
The stems of Spear Thistle have interrupted spiny wings along their length and are leafy to the top. The leaves are deeply divided and armed with spiny teeth, as is characteristic of thistles. The basal leaves form a rosette and the stem leaves are stalkless and have long, spear-like, terminal lobes.
Typical of Asteraceae, each flower head is actually a cluster of tiny flowers called florets. The bracts associated with each floret come together to form a cup-like involucre in thistles, which surrounds and protects the enclosed achene-pappus units. In thistles these bracts, or phyllaries, are armed with spines. These spines probably make it difficult for insects to climb the stem to reach the nutritious achenes. In some Asteraceae these bracts also undergo hygroscopic movements opening and closing the involucre to allow or prevent achene dispersal according to conditions. In Spear Thistle the phyllary spines curve backwards.
Spear Thistle is a biennial/monocrapic plant. Like many biennials it is an opportunist, growing on disturbed ground and setting seed before its population wains as the habitat undergoes succession and other species dominate (De Jong and Klinkhamer, 1998). Such plants rely on long-lived seeds in the soil seed bank to germinate whenever a suitable opportunity arises. It occurs in grassland, waste ground and on cultivated land.
These whole plant pictures illustrate the arrangement of the flowering stems. The capitula are either solitary or aggregated in twos or threes at the ends of branches and stems and are less frequently solitary.
One of the thistles often confused with Cirsium vulgare (above) is Carduus acanthoides (which is also similar to and easily confused with Carduus crispus, the Welted Thistle). The lower leaves are quite different, however. The leaves of Cirsium vulgare have lobes that are acutely tipped and spear-like, whereas those of Cirsium acanthoides and Cirsium crispus are blunter tips and have more way margins and hence more 'three-dimenisonal' and are more similar in shape to those of Cirsium acaulis (see later).
Above: young capitula of Cirsium vulgare before anthesis (flower
opening)
The leaves of Cirsium vulgare also have bristles on top and underneath they are either covered thickly in white hoary or woolly hairs or hairy only on the midribs and veins. In Carduus acanthoides, the underside of the leaves are almost hairless, with a few multicellular hairs on the veins. (The upper surface is either hairless or similarly hairy to the under surface). In Carduus crispus there are, in addition to these multicellular hairs, some arachnoid (cobwebby) hairs and some simple hairs on the lower leaf surface (these latter two hair types do not occur in C. acanthoides). The involucres of Carduus crispus and C. acanthoides are also usually (though not always) smaller than those of Cirsium vulgare. In Carduus acanthoides the involucre is more-or-less spherical or hemispherical and tends to be wider than long when mature, whilst in C. crispus it tends to be longer than wide but also usually occurs in quite tight clusters and the capitula are slightly pendant when mature. In contrast, the capitula of Cirsium vulgare are elongated and somewhat vase-shaped.
Finally, the phyllaries of Cirsium vulgare are narrower and more spear-like, with longer spines, and those of Carduus acanthoides and C. crispus wider, less cylindrical and with shorter spines. See the paper by Verloove, 2014 (available here) for a detailed comparison of Carduus acanthoides with Carduus crispus. Note: detailed examination of the pappus will always distinguish Cirsium from Carduus, the former has long lateral barbs on each pappus bristle, the latter only short protuberances.
Although invasive outside their natural range, Spear Thistles are an important component of their native ecosystems. They are one of the first plants to colonize bare clay, making it more suitable for other plants to follow.
The phyllaries of Spear Thistle usually have a few long woolly hairs along their margins, giving the involucre the appearance of being covered in fine cobwebs, especially at the base (it is slightly 'arachnoid'). The innermost (uppermost) phyllaries often have a few teeth along the margin just beneath the spiny tip.
Spear Thistle is found in waste places, pastures, roadsides and on cultivated ground. Though insect pollinated, it is reportedly self-compatible according to some sources, though it has been reported that non-pollination and self-pollination results in shrunken and hollow capitula (Michaux, 1989). It is native to Europe, N. Africa, W. Asia and Siberia. The flowers, like those of artichoke, are said to have been used to curdle milk.
The fruit of Asteraceae is the achene (or cypsela): a dry fruit with a thin skin tightly enclosing a single seed. Each floret can produce a single achene. The sepals remain attached to the top of the achene as the pappus or 'parachute' of narrow filaments with finer lateral barbs (click on the image below to obtain full size and zoom in to see the side-barbs). Seed barbs have important aerodynamic effects on the pappus.
The seeds have no inherent dormancy and mostly germinate the following spring, with very few remaining in the soil as a seed bank.However, since not all plants flower in the same year, it is not critical to maintain a large seed bank.
The hairs covering the receptacle surface inside the involucrum represent modified bracts accompanying the achenes and possibly serve to keep the achenes dry whilst they are inside the capitulum as they do in Centaurea debeauxii (Skilbeck, 2023).
The radical leaves (basal leaves emerging from the rootstock) form a basal rosette. They are narrowed at the base, almost forming a true leaf-stalk or petiole. The first rosette leaves are flat, but later ones have more undulated margins and are more deeply divided. The margins are spiny and each lobe ends in a longer spine.
The overwintering rosettes of Spear Thistle can reach 1.2 meters across. The larger they are, the higher the probability they will bolt and produce a flowering stem next season. The rosettes cease growth over winter (unless they are damaged when some replacement growth may occur).
The bristles or small spines on the upper surface of the rosette and stem leaves of Cirsium vulgare are diagnostic of this species. These bristles or setae are characteristic of a section of Cirsium that includes Cirsium eriophorum (Woolly Thistle) in the British Isles and other species on the Eurasian continent. Indeed, C. eriophorum has a denser and more uniform and neater-looking covering of such bristles. In both species, the bristles point towards the leaf apex or apex of the leaf lobe they are on.
In Europe, the genus Cirsium is usually divided into three sections: section ($) Cirsium, $Eriolepis ( = $Epitrachys) and $Cephalonoplos. Generally only the section Eriolepis possess the characteristic surface bristles on the adaxial (upper) surface of their leaves (among other differences) and hence has often included C. vulgare, however, genetic analysis sometimes place C. vulgare in section Cirsium. Now, an interesting fact is that C. vulgare is a tetraploid (with twice the usual number of chromosome sets), which is not typical of Cirsium and appears to have evolved from a hybrid between a typical $Cirsium lacking bristles and a $Eriolepis possessing bristles. C. vulgare thus appears to be an allotetraploid, formed by the genetic fusion of two different species. The parental species are unknown and one of them may or may not be C. eriophorum. Based on recent studies, many now move $ Eriolepis into the new and separate genus Lophiolepis, making C. vulgare a species derived from an inter-genus hybrid. Where then should we place C. vulgare? Should it remain in Cirsium or be placed in Lophiolepis along with Lophiolepis eriophorum (formerly Cirsium eriophorum). For the moment, at least, it remains in Cirsium.
The stem leaves are pinnatifid (incisions dividing the leaf into pairs of lobes almost to the midrib) with each lobe consisting of two main sub-lobes, an upper and a lower. The basal lobes of the leaf runs down the stem as a spiny wing. The spiny bristles on the upper surface gives the leaves a rough appearance and texture, whilst the lower surface is typically grey or whitish depending on the density of the woolly or arachnoid (cobweb-like) hairs.
2. Creeping Thistle - Cirsium arvense
Above: Creeping Thistle, Cirsium arvense. Creeping Thistle is a native of southeastern Europe the eastern Mediterranean but has been introduced and naturalized in parts of North and South America, parts of Africa, Japan, the Middle East, Australia and New Zealand. This thistle is a creeping plant with a perennial root system with horizontal roots bearing 'adventitious shoots' that produce upright flowering stems lacking wings. These adventitious shoots produce scale leaves and are essentially short rhizomes. The leaves clasp the stem. The florets are mauve or white.
This species reproduces vegetatively by means of the spreading perennial root system which can rapidly form an extensive subterranean system. Following seed germination, vertical roots develop that branch off lateral horizontal roots 30 to 60 cm beneath the soil surface. These lateral roots can grow as much as 6 meters in a single season, or up to 12 m in a single year, growing horizontally for 60 to 120 cm then bending downwards to grow to a depth of up to 2 meters. The resulting bend or knee puts out a new shoot from an adventitious bud that can produce a flowering shoot where it emerges above and also a new lateral root. Remarkably expansive root networks can be formed in this way, supporting multiple flowering stems across a considerable area.
The roots store food reserves and even fairly small root fragments are capable of regenerating into a whole plant. A root fragment 5 to 10 cm in length can produce a new plant in three weeks, bearing a few adventious roots on the basal internodes of the new stem. This creeping root system gives the plant its common name of 'Creeping Thistle'.
These flowers have long stamens and are presenting pollen on their everted styles, so must be male flowers (or hermaphrodite). Note that the florets are functionally male, but need the female parts to provide the piston mechanism that pushes out the pollen for the pollinator, even though the female parts are usually sterile. Male plants also have longer and broader petal lobes. At the fruiting stage, any hermaphrodite florets that set seed will produce elongated pappi, making the capitula look more feminine.
Stem fragments can also regenerate. The stems contain a loose pith along the central axis which later forms a hollow central shaft. The roots contain obvious air spaces and a central pith that is later replaced by vascular xylem tissue.
The capitulum above is evidently female. The styles end in well-developed stigmas that present no pollen and emerge from short (and sterile) stamen tubes. Each clone keeps its gender when repoducing vegettaively, resulting in patches of female plants and patches of male plants.
Cirsium arvense is another invasive alien thistle in North America, where it has become naturalized. Like C. vulgare, it causes considerable economic damage and is a potential threat to native ecosystems.
The leaves of Cirsium arvense differ from those of Cirsium vulgare in several key ways: the upper surface of leaves of C. arvense lacks the distinctive spinelike bristles found in C. vulgare and is more-or-less glabrous (though there are multicellular hairs on the midrib beneath). This gives the leaves of C. arvense a shinier, smoother and brighter green appearance. The lobes, espevcially the apical lobe, is more obtuse (blunter and more rounded) in C. arvense and the margin more undulate, giving the leaves a very three-dimensional form.
The stems of C. arvense are not spiny and lack wings, except sometimes right beneath the leaf bases. However, there are several varieties of C. arvense found in the British Isles and Europe, which differ in leaf shape. A large herbarium will contain useful examples and these varieties have also been described by Sell and Murrell as summarized in Tiley, 2010.
Fruit Dispersal
Below: the phyllaries of Cirsium arvense possessed what has been described as strips of 'glandular tissue' seen colored purple here. However, dissection of this tissue produces no visible secretion and the tissue is better described as corky tissue. This tissue loses its coloration upon drying. If indeed this is not glandular tissue, then what else might its function be? Corky tissue is often a flotation device, but the tissue is not present on the achenes in this case, however, sometimes the achenes remain inside the capitula after their deciduous pappi have been shed. It would be interesting to note whether or not the capitula are shed with these achenes still inside when the flowering stems die away. Indeed the capitulum sometimes does not open sufficiently to permit separate achene dispersal by wind (see reviews by Donald, 1994 and Moore, 1975). This raises the possibility that the achenes could be dispersed in detached capitula with the corky tissue aiding dispersal by flotation in run-off from rain. Creeping Thistle itself prefers well-draining soils and is not particularly a wetland plant, but there is a coastal variety.
The amount of corky tissue on the phyllaries varies considerably. It possibly serves a protective function against insect herbivores trying to eat through the phyllaries to reach the nectar or achenes inside. In any case, the phyllaries of thistles, like those of Centaurea, undergo hygroscopic movements: opening up in dry conditions to expose nectar or to disperse fruit under ideal conditions and close in damp to protect the nectar and pappi.
The achenes can disperse on the wind aided by their pappus and are particularly airborne, readily dispersing up to 4 m from the parent (Sheldon and Burrows, 1973). However, it is also reported that occasional fruit may disperse up to 1 km (see review in Moore, 1975). Intuitively one may expect plants to evolve to scatter their seeds as wide as possible to colonize new sites, however, a more common strategy is to colonize near to the parent where conditions are likely to be suitable, perhaps building up a soil seed bank waiting for an available niche and hence many plants disperse much of their seed within a few meters of the parent.
Some seeds have an inherent dormancy programmed into them, compared to C. vulgare the seeds of which have no innate dormancy. Some seeds may persist in a viable state in the soil for up to about 20 years before germinating. This thistle prefers well-drained and fine-textured soils.
The extensive pappus consists of barbed filaments connected together into a cylindrical collar at their base, which attaches to the achene. The pappus is, however, readily deciduous and the whole detaches as a unit, often before the achenes have been shed from the capitulum, as can be readily seen in the field. The plant edges its bets between allowing some achenes to disperse a variable distance on the wind to possibly colonize new sites, and maintaining a population on the current site which is known to be suitable.
Cirisum arvense belongs to Cirsium $ Cephalonoplos and is evidently quite different from other Cirsium thistles belonging to the other two sections.
3. Marsh Thistle - Cirsium palustre
Marsh Thistle, Cirsium palustre, is a monocarpic wintergreen, overwintering as a rosette which may reach 2.2 meters in diameter. It is self-compatible.
Above: Marsh Thistle, Cirsium palustre. Marsh Thistle is native to Europe and west Asia, but has been introduced into the USA, Canada and Australia. This is a biennial or perennial growing to 2 m or more in height. In unfavourable conditions the plants may live up to 6 years, allowing sufficient time for the rosette to mature before flowering (Falinska, 1997). The stems are leafy to the top and very spiny. This plant occurs on damp ground in woods, marshes and beside ditches. It occurs in woodland clearings, meadows, seacliffs and moorland. The stems have continuous spiny wings. In the first year of growth it forms a rosette of basal leaves and in the subsequent year the tall flowering stem is produced, which branches to form a 'candelabra' of flowers. The dense clustering of the flower-heads and the obvious glandular ridges on the phyllaries make this species easy to identify, but beware its hybrids!
Marsh Thistle is noted for its purple and white flower color variants. The purple form is the more common and intermediates occur. The flowers are self-compatible and can self-pollinate, but are visited by a variety of insects and have sensitive anthers which contract to expel pollen when touched by a potential pollinator. The white form (var. alba or var. albiflora or var. ferox) is homozygous (Mogford, 1974a) and so will only produce white offspring when self-pollinated. At high altitudes, the white variant is more common on less exposed areas.
The achenes of Marsh Thistle disperse on the wind fairly well, though not as well as those of Creeping Thistle (Sheldon and Burrows, 1973). Note that as in Cirsium arvense, the Marsh Thistle has prominent corky/glandular tissue on its phyllaries. The lower (outer) phyllaries have a few woolly/arachnoid hairs on their margins and outer surface and have short spiny tips. The upper (inner) phyllaries have a reddish-purple appendage at their tips instead of spines.
Another useful diagnostic feature is the presence of articulated multicellular hairs on the stem and leaf midribs, though its is not the only thistle species to possess these. There are also some very fine woolly hairs on the stem. Multicellular hairs also occur on the upper leaf surface and some woolly/arachnoid hairs occur on the leaf underside. The wings on the stems are thickly covered in long spines.
4. Dwarf Thistle
Cirsium acaule is a perennial with a basal rosette and one to a few capitula in the center or borne on short stems up to 10 or occasionally 30 cm tall. The leaves are deeply lobed. The capitula are quite large at 2 to 3 cm in diameter. The Dwarf Thistle occurs on calcareous grassland and is native to Europe. The Dwarf Thistle is gynodioecious, meaning that some plants are female whilst other are hermaphrodite. In this species the capitula of hermaphrodite plants are noticeably larger than female heads (Proctor and Yeo, 1972).
5. Hybrid Thistles
Many Cirsium species hybrid with one-another (though hybrids with the genus Carduus, the plumeless thistles, are not confirmed and are likely to be very rare if they occur at all).
Cirsium vulgare x C. palustre = C. x subspinuligerum
This species is not common, but has been recorded at the edge of damp places where both parent populations overlap. Typically, the stem contains many articulated hairs (as in C. palustre) and some arachnoid hairs and some clustering of the capitula (as in C. palustre) but has the bristly-spines on the upper leaves as in C. vulgare and the phyllaries most closely resemble those of C. vulgare.
Cirsium arvense x C. palustre = C. x celakovskianum
This hybrid is locally common where both parents occur, there has been some historical dispute over whether it is a hybrid or a form of C. palustre lacking the usual dense packing of the capitula. The capitula are usually single (as in C. arvense) or in loose groups and often have phyllaries resembling those of C. palustre (in which the lower phyllaries having more prominent spines) and in flower color (pale pink). The stem is slightly spiny or spiny at the base (from C. palustre). Some could perhaps be C. palustre forms with pale pink flowers. It might help to note that the corolla (petal) tubes of both species are dilated at their upper ends, but for about half their length in C. palustre and about one third their length in C. arvense (this is obvious when the two are compared side-by-side).
6. Other Cirsium species
The European species covered on this page are invasive in N. America, but N. America has its own very wide diversity of Cirsium species. Many are endemic to small regions and habitats, some are widespread, but the highest species diversity occurs in the western USA and Mexico, with species found in montane coniferous forests, juniper woodlands, desert scrub, grasslands etc. in locations such as the Rocky mountains, the Great basin and the Sierra Nevada. Faced with such a complex variety of forms, botanists tended to lump different lineages as varieties of single species but genetic analysis is beginning to disentangle this taxonomic complexity (Ackerfield et al., 2020).
References
Ackerfield, J.R., Keil, D.J., Hodgson, W.C., Simmons, M.P., Fehlberg, S.D. and Funk, V.A., 2020. Thistle be a mess: Untangling the taxonomy of Cirsium (Cardueae: Compositae) in North America. Journal of Systematics and Evolution, 58(6), pp.881-912. (Available here).
De Jong, T.J. and Klinkhamer, P.G.L. 1998. population ecology of the biennials Cirsium vulgare and Cynoglossum officinale in a coastal sand-dune area. J. Ecol. 76: 366-382.
Donald, W.W. 1994. The biology of Canada thistle (Cirsium arvense). Rev. Weed Sci. 6: 77-101.
Fali?ska, K. 1997. Life history variation in Cirsium palustre and its consequences for the population demography in vegetation succession. Acta Societatis Botanicorum Poloniae 66(2): 207-220.
Michaux, B. 1989. Reproductive and vegetative biology of Cirsium vulgare (Savi) Ten. (Compositae: Cynareae). New Zealand Journal of Botany 27: 401-414. (Available here).
Mogford, D.J. 1974a. Flower colour polymorphism in Cirsium palustre 1. Heredity 33(2): 241-256.
Mogford, D.J. 1974b. Flower colour polymorphism in Cirsium palustre 2. Pollination. Heredity 33(2): 257-263.
Moore, R.J. 1975. The biology of Canadian weeds 13. Cirsium arvense (L.) Scop. Can. J. Plant Sci. 55: 1033-1048.
Proctor, M. and Yeo, P. 1972. The Pollination of Flowers. The New Naturalist vol. 54. Taplinger Pub. Co. New York.
Sheldon, J.C. and Burrows, F.M. 1973. The dispersal effectiveness of the achene-pappus units of selected compositae in steady winds with convection. New Phytol. 72: 665-675.
Theis, N. and Raguso, R.A., 2005. The effect of pollination on floral fragrance in thistles. Journal of Chemical Ecology, 31, pp.2581-2600. (Available here).