This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The present distribution of this plant is Europe and Siberia, and it is merely an introduction in N. America. There is no evidence as to its occurrence in early deposits. In Great Britain in the Peninsula province it is absent from S. Somerset, in the Channel province from W. Sussex; it occurs throughout the Thames and Anglia provinces; in the Severn province not in Gloucs; whilst in Wales it is found in Carmarthen, Pembroke, Montgomery, Carnarvon, Denbigh, and Flint, and it is found in the Trent province generally, except in Derby, not in Mid Lancs in the Mersey province, in the Humber province not in S.E. or S.W. Yorks, in the Isle of Man, in Lanark, Roxburgh, Berwick, Haddington, Edinburgh, Fife, Stirling, E. Ross. Elsewhere it is probably not native, and is an alien or denizen.
The Cotton Thistle, where it is a native, is a plant of dry places, and elsewhere it is merely a casual found in waste places, in gardens, and where it has been sown by man consciously or unconsciously, like many other plants which now have a sort of temporary home with us but whose native origin is under suspicion.
While not a true thistle, the Cotton Thistle is even taller than the Marsh Thistle, and with its fine heads of bloom and whitish foliage and stems it is far more imposing,
With spreading branches it thus forms quite a magnificent ornament for gardens. The woody stems are freely continuously winged. The leaves are egg-shaped, oblong, stalkless, wavy, decurrent, toothed, covered both sides with woolly down, and very spinous.
The flowerheads are numerous, terminal, purple, upright, in a nearly round involucre, with spreading awl-like phyllaries or whorls of bracts. The receptacle bears scales. The tubular florets are complete. It is 4 to 10 ft. in height. The flowers bloom in July and August. The plant is biennial, reproduced by seeds.
The flowerhead is much like that of Carduus but does not bear chaffy bristles. The tube is 10-12 mm., and honey rises in the cylindrical throat 3-4 mm. This is divided above into 5 linear segments 6-8 mm., which continue to be straight. The length of tube does not prevent honey being reached, and is due to the development of the involucre which protects the buds, and makes the heads conspicuous. They bend outwards more and more. The branches of the style are closely parallel 3-4 mm., and have wart-like knobs on the outer margin. In the second stage (hermaphrodite) they turn outwards and are accessible to insects. There is a ring of short hairs 1 mm. below the point where they fork, pointing upwards, and they sweep the pollen out of the cylinder, which is 8-10 mm. long, 1/3 mm. wide. Pollen lines the cylinder in the first stage, and in the second the style projects with 2 rows of papillae 5-7 mm. above the corolla divisions. The filaments are sensitive, protecting the pollen. When an insect touches them they contract and the anthers do so also, so that pollen is squeezed out upon the stigma, which does not lengthen.
Photo. Chas. Allen - Cotton Thistle (Onopordon Acanthium, L.)
Many flowerheads are pollinated by bees' visits simultaneously. The visitors are Hymenoptera, Megachile, Osmia, Coelioxys, Ste/is, Andrena, Halictus, Bombus, Psammophila; Lepidoptera, Vanessa, Satyrus, Macroglossa; a beetle, Coccinella, and a Hemipterous insect Capsus.
The fruit is provided with several rows of barbed and toothed pappus or hair, which assist in dispersing the fruits by aid of the wind.
This handsome thistle is a sand-loving plant, addicted to a dry sand soil.
A beetle, Apion onopordi, and the flies, Trypeta lappae, Urophora macrura, Eusina sonchi, Acidia heraclei, are to be discovered on it.
Onopordon, Pliny, is Greek from onos, ass, and perdo, break wind; and Acanthium from acanthos, spine.
Cotton Thistle is called Argentine Thistle, Asses' Cotton, Down, Oat, Queen Mary's, Scotch, and Silver Thistle. It is called Queen Mary's Thistle because her attendants brought it to Fotheringay, and Down Thistle because it is covered with wool or down.
Essential Specific Characters:172. Onopordon Acanthium, L. - Stem tall, winged, leaves rough, cottony both sides, oblong, flowerhead large, purple, involucre sub-globose, phyllaries spreading, imbricate, spinose.