Asteraceae - Thistles

Above: Spear Thistle, Cirsium vulgare. 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 naturalised 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, favouring cross-pollination.

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.

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.

Spear Thistle is a biennial 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 ling-lived seeds in the soil seed bank to germinate whenever a suitable opportunity arises. It occurs in grassland, waste ground and on cultivated land.

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 naturalised 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 thistle is dioecious, with male and female flowers being borne on separate plants, at least in Canadian populations (Moore, 1975). However, the staminate (male) flowers have vestigial ovaries that sometimes produce seed. Female flowers produce abortive stamens. Even fairly small root fragments are capable of regenerating into a whole plant. The main pollinators are honey bees (wind may affect occasional pollination) and the plant is a good producer of sugary nectar. The average capitulum contains about 100 florets and a shoot may produce as many as 100 capitula. One plant may produce over 5000 seeds, but the average is reported as 1530.

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. 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 colonise new sites, however, a more common strategy is to colonise 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. In the case of Creeping Thistle it has also been reported that the achenes often remain in the capitulum when the pappus has broken off. 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.

The amount of corky tissue on the phyllaries varies considerably and it would be interesting to see whether or not this depends on a population's proximity to water or on whether the plant is male or female and thus shed some possible light on their possible function in dispersal.

Marsh Thistle - Cirsium palustre

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. 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.

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.

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).

Nodding (Musk) Thistle - Carduus nutans

As a member of the genus Carduus, the Nodding Thistle is not a Plume Thistle since the hairs of its pappus are simple (unbranched). There are more than 100 species of Carduus worldwide. The large capitula (4 to 6 cm in diameter) are solitary and pendant (hence Nodding Thistle) and very fragrant (hence Musk Thistle). This species is generally biennial, but sometimes perennial. It prefers calcareous soils but is also found on sandy and shingly soils and grows on grassy or bare places, roadsides and rough ground. The purple-red phyllaries narrow just above their base and are lanceolate and spine-tipped and tend to curve strongly backwards (they are strongly reflexed). The phyllaries often bear arachnoid (cobweb-like) hairs.

The arachnoid hairs perhaps acts as a physical barrier to insects attempting to reach the flowers and steal the pollen without effecting pollination.

Carline Thistle

References

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.

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.